Workbook Flashcards
What percent of the atmosphere is nitrogen?
78%
What percent of the atmosphere is oxygen?
21%
What is the percent of other (argon etc) gases in the atmosphere?
1%
What percent of water vapour is in the atmosphere that gives us ‘weather’?
4%
What are the atmospheric constituents that vary?
Water vapour, CO2 and solid particles like dust and ash
What role does CO2 play in the atmosphere
CO2 (greenhouse gas) plays a large role in absorbing Earth emitted heat and so warming the atmosphere. This is known as the greenhouse effect
What role does ozone play in the atmosphere
Ozone (O3) provides a protective layer in the lower stratosphere. As ozone absorbs the suns UV radiation it warms and heats the atmosphere above it (Stratospheric inversion)
What role do solid particles play in the atmosphere
Solid particles suspended in the atmosphere play a major role in the formation of clouds
Describe the troposphere
The troposphere is the lowest layer. It extends from the surface upwards and where it ends is called the tropopause. The troposphere is defined as being an area where there is a reduction of approx 2°C in temp with every 1,000 foot gain in altitude. Where this temperature reduction stops is where the end of the troposphere is – the tropopause
How is the atmosphere divided
The atmosphere is divided into layers based on change of temperature with a gain in height
Describe the tropopause
At the tropopause, the temperature remains the same with a gain in height. This is an example of an isothermal layer
Where does most of the weather occur
As the troposphere is the layer closest to the Earth’s surface, the troposphere contains almost all atmospheric moisture and this is where most significant weather occurs
Describe the stratosphere
Above the tropopause is the stratosphere where temperature increases with a gain in height. This is due to the ozone layer being located in the stratosphere
The tropopause
The tropopause is where temperature stops decreasing with a gain in height and instead remains the same with a gain in height. The altitude and temperature at where this occurs varies around the Earth
Is the tropopause hotter or colder over the equator?
As the tropopause is highest over the Equator, it is therefore coldest over the Equator
Is the tropopause hotter or colder over the poles
As the tropopause is lowest over the Poles, it is therefore warmest over the Poles
Why is the tropopause higher over the equator
The reason the tropopause is higher over the Equator compared to the Poles is because of differences in air density
What is air density
Air density is a measure of the amount of air molecules within a given volume of air. Air density is primarily affected by two variables, temperature and pressure
Why is a column of air taller in warm air and shorter in cold air
Temperature is a measure of heat energy (kinetic energy) contained within a body. If air has a high temperature, the air molecules will have high kinetic energy, bounce off one another and take up space. This will therefore reduce the amount of molecules within a given volume and therefore reduce air density. If air has a low temperature, the air molecules will have low kinetic energy, will not bounce off one another as much and take up less space. This will therefore increase the amount of molecules within a given volume and therefore increase air density. If the volume is not fixed, warm air will cause a column of air to expand and cold air will cause it to contract
What is pressure
Pressure is a measure of the ‘weight’ of a column of air above a point. If air pressure is high, the force exerted by the atmosphere on a given volume will be high so more air molecules will be packed into that given volume therefore increasing density. If air pressure is low, the force exerted by the atmosphere on a given volume will be low so less air molecules will be packed into that given volume therefore decreasing density
What density and temperature will increase and decrease aircraft performance
For the highest air density a combination of low temperature and high pressure is best. This will improve aircraft performance. If temperature is high and air pressure is low air density will be low and this will reduce aircraft performance
Why is the tropopause different heights at the poles and equator
The difference in temperature between the Poles and Equator is the reason for different tropopause heights. At the Poles the air is cold so contracts. This reduces the height of the tropopause. At the Equator the air is warm so expands. This increases the height of the tropopause
Describe temperature
Temperature is simply a measure of heat contained in a body. When the temperature rises it is due to the “hot plate effect” where the sun heats the earth and the warm air then begins to rise
Heat radiation
All bodies emit heat radiation. This energy is in waves. The wavelength is affected by the amount of energy. High energy, has a short wavelength, low energy has a long wavelength. The sun emits short wavelength, high energy solar insolation. The Earth emits long wavelength, low energy terrestrial radiation
Solar insolation
The Earth only receives solar insolation during the day however it is constantly emitting terrestrial radiation. When there is more solar insolation than terrestrial radiation there is a net energy gain so temperatures increase. When there is more terrestrial radiation than solar insolation there is a net energy loss so temperatures decrease. This provides the basis for the ‘energy budget’ which gives an average global temperature of 15ºC at sea level
What percent of solar radiation reaches the earths surface
Not all incoming solar insolation reaches the Earth’s surface. Only around 45% reaches Earth with the rest being reflected, absorbed or scattered by the Earth’s atmosphere
The greenhouse effect
The Sun’s energy heats the Earth from below by conduction. Warm air then rises heating the rest of the atmosphere through convection. Carbon Dioxide and water vapour absorb outgoing terrestrial radiation and contribute to atmospheric heating. This is known as the ‘greenhouse effect’. If the amount of carbon dioxide in the atmosphere is increased this will enhance the greenhouse effect and increase global temperatures
The suns angle of inclination
Near the Equator the sun’s energy is spread out over a small area and so is concentrated and this gives higher temperatures. Near the poles, due to the greater angle of inclination of the sun, the energy is spread over a larger area and so is not as concentrated
What is albedo
Albedo is a measure of an object’s reflectivity. If reflectivity is high, Albedo will be high. Objects with high albedo reflect incoming solar insolation rather than absorbing it
What is specific heat
Specific heat is a measure of the energy required to heat a substance by 1ºC. If a substance has high specific heat then it requires a lot of energy to increase its temperature. It will therefore not heat up as fast
What is specific heat capacity
Specific heat capacity is a measure of how well a substance retains its heat. If it has a high specific heat capacity it will retain its energy well and therefore not cool down as fast
Maximum temperatures-diurnal variation
Maximum temperature will occur just after incoming solar insolation is at its greatest as it takes time for this energy to heat the Earth. The maximum temperature therefore occurs in the early afternoon
Minimum temperature-diurnal variation
The minimum temperature occurs at the time where there has been the longest amount of time where outgoing terrestrial radiation is greater than incoming solar insolation. This occurs just after sunrise
What affects diurnal variation
- Specific heat of the surface
- Specific heat capacity of the surface
- Wind
- Cloud cover
How does wind affect diurnal variation
Wind increases the mixing between warm and cold air. This reduces the maximum temperature and means the minimum temperature will not be as cold. This therefore reduces the diurnal variation
How does cloud cover affect diurnal variation
Cloud cover reflects back incoming solar insolation therefore reducing the amount that reaches the surface. It also creates a ‘blanketing effect’ trapping outgoing terrestrial radiation therefore meaning the minimum temperature will not be as cold. Cloud cover therefore reduces diurnal variation
What are inversions?
Inversions are where the temperature increases with a gain in height. Dust and pollutants are trapped below an inversion giving poor visibility below and inversion and good visibility above
What is isothermal
Isothermal are where the temperature stays the same with a gain in height. They have similar characteristics to an inversion. The tropopause is an example of an isothermal
What is atmospheric pressure
Atmospheric Pressure is a measure of force per unit area exerted by the weight of the column of air above the area. In nature, air travels from areas of high pressure to areas of low pressure. Variations in pressure over a horizontal distance results in surface winds and types of weather experienced at the surface
At what rate does pressure decrease as height increases
As height increases, pressure decreases. Pressure decreases at an exponential rate
What is atmospheric pressure measured in
This force is measured in hectopascals (hPa), millibars or inches of mercury (inHg)
Pressure variations with height
To understand how pressure changes with height, imagine an infinitely high column of air extending from sea level. The closer you are to the surface the more air molecules there are above you and therefore there is a greater mass of air acting down upon you. The higher you go the ‘thinner’ the air becomes
The aneroid barometer (measures air pressure)
An aneroid barometer works on the principle of sealed capsules that expand and contract based on the air pressure. This contraction drives a needle which is calibrated to indicate air pressure. An altimeter works on this principle and uses 3 capsules for its 3 needles. This method gives air pressure in millibars or hectopascals (hPa)
The mercury barometer (measures atmospheric pressure)
A mercury barometer works on the principle of air pressure exerting a force on a reservoir of mercury. If the air pressure increases, this causes the mercury in a tube to rise. The amount that it rises or falls is calibrated to give air pressure. Using this method, air pressure is given in inches of mercury (inHg)
Does cold or warm air have a greater pressure lapse rate
because pressure reduces much more rapidly in a cold column of air, cold air has a greater pressure lapse rate than warm air.
Surface variation with time and position
Uneven heat distribution throughout the Earth’s surface creates uneven densities and therefore uneven surface pressure. These differences in surface pressure over a horizontal distance trigger the transport of air over a horizontal plane – this called wind
What is pressure gradient
In nature air will always flow from high pressure to low pressure, an example is a balloon being blown up and then let go. The rate at which the air flow moves (the wind speed), will depend upon the pressure gradient which is dependent on:
- The difference in pressures between the pressure systems (pressure differential)
- The distance between the pressure systems
What are isobars
Isobars are lines that join areas of equal barometric pressure. The closer the isobars are, the greater the pressure gradient and therefore the stronger the winds. Closer isobars are created when either the pressure differential increases or the distance between the high and low pressure is reduced
Explain a depression/low pressure system
A depression is another name for a low pressure system where in the Southern Hemisphere the winds move around in a clockwise direction. A low has the lowest pressure in the centre of the system and increasing pressure outwards. Winds associated with a depression are generally strong and thus a strong pressure gradient will exist in a low
What is a trough
Is a system that extends from the centre of a low pressure system. Pressure increases either side of the axis while weather inside the trough is similar to that of the parent low. The shape of the trough is usually V shaped and maybe associated with a front
Explain an anti-cyclone/high pressure system
An anti-cyclone is another name for a high pressure system where in the Southern Hemisphere the winds move in an anti-clockwise direction. A high is generally associated with fine weather and light winds, indicating a weak pressure gradient
What is a ridge
As a trough is an extension of a low, a ridge is the extension of a high. Once again, similar weather can be expected with a ridge to that of the parent high. Thus the weather is often fine and clear. A ridge often forms as an egg shaped wing with fairly rounded curves. Pressures are always lower either side of the axis of the ridge
What is a col
A col is a system that exists between two lows and two highs. The pressure gradient within a col is almost non-existent and it is difficult to predict whether the high or the low will be the predominant system. Often hazy and poor visibility conditions can be expected and due to the weak pressure gradient, light and variable winds exist
What is the international standard atmosphere? (ISA)
The atmosphere is a constantly changing and dynamic body with many variables. It was therefore necessary to establish a global average. The International Standard Atmosphere is the solution (ISA) ISA is based on measurements at Mean Sea Level (MSL)
What is the ISA average sea level temperature
+15 degrees
What is the ISA average temperature lapse rate
2 degrees/1,000ft (1.98)
What is ISA average sea level pressure
1013.25hPa
What is ISA average pressure lapse rate
1hPa per 30ft
What is the height of the tropopause
36,090ft
What is the temperature of the tropopause
-56.5 degrees
Define condensation nuclei
Small particle such as salt, carbon, soot or dust etc for water vapor to condense onto
Define super saturation
Where there is a lack of Condensation Nuclei, water vapour cannot condense.
What is latent heat
Latent heat is the extra energy required or released when a substance changes state. It does not affect the temperature of the substance however it will warm or cool the surrounding environment depending on whether latent heat is released or absorbed. The measurable change in temperature of the surrounding environment is called ‘sensible heat’
What is relative humidity
Relative Humidity is the ratio of the weight of water vapour in a given volume of air (absolute humidity) to the maximum weight of water vapour that the same volume can actually hold at the same temperature (saturation content), expressed as a percentage
Dew point
A dew point is a temperature at which a parcel of air saturates under constant pressure. It is important to note that dew point is a temperature not a height
What happens when a parcel of air reaches its dew point
When a parcel of air reaches its Dew Point, relative humidity is 100%. Cooling beyond this will result in the air no longer being able to hold additional moisture so it will condense forming visible moisture – cloud
What happens when relative humidity changes temperature
The relative humidity will change with a change in temperature. The warmer the air the more water vapour can be held before the air saturates, so with an increase in temperate RH decreases. This is the reason why the air feels so wet near the equator and so dry at the poles
What is the only thing that affects dew point
The dew point temperature is only affected by the actual water content of the air (absolute humidity).
What factors affect the rate of evaporation
The four main factors that affect the rate of evaporation are temperature, relative humidity, air pressure and wind
How does temperature increase the rate of evaporation
An increase in temperature will increase the air’s ability to hold moisture therefore increasing evaporation
How does relative humidity affect rate of evaporation
If relative humidity is low, then the air is not holding what it can potentially hold. This will increase the rate of evaporation
How does air pressure affect rate of evaporation
If air pressure is low then there is more room within a given volume of air to contain the vapour. Low air pressure increases the rate of evaporation
How does wind affect rate of evaporation
Wind increases a moisture sources exposure to dry air, therefore increasing the rate of evaporation
What factors will cause maximum evaporation
for maximum evaporation the air will be warm, dry, windy and have a low air pressure
Calculating relative humidity
R elative Humidity (%) = 100-5
Difference between Temperature and Dew Point
Wet bulb/dry bulb (measuring atmospheric moisture content)
One thermometer is wrapped in a muslin rag which is soaking in a reservoir of water. If the air is dry, the rate of evaporation will increase and so water will evaporate from the reservoir. As latent heat is absorbed during evaporation, temperature drop will be shown on the thermometer wrapped in the muslin rag
Hair hydrometer (measuring atmospheric moisture content)
This works on the principle of measuring the length of hair. If the atmosphere is moist, then the hair will absorb moisture and expand. The length of the hair is then calibrated to give atmospheric moisture levels
Hydrometer (measures atmospheric moisture content)
This measures the density of air to give atmospheric moisture. Water vapour is less dense than dry air. The greater the moisture content, the lower its density. This is important to note as this means moist air will reduce aircraft performance
Describe stability of air
Stability of air is its ability to return to its original position after being disturbed by an external force. Stability in weather refers to the vertical movement of air and whether this movement is rising upwards or subsiding downward. It is by this stability that we can determine what type of cloud we will get, whether it will rain or not and even the temperature we might expect
What are airs tendencies if there stable, unstable or neutrally stable
Air that is stable will return to its original position after being disturbed by an external force. If air is unstable it will continue to move away from its original position. If the air stays in its new position and neither moves towards nor away from its original position, it is neutrally stable
What is a lapse rate
A lapse rate is the temperature decrease with a gain in height
What is adiabatic cooling
When air rises it’s pressure drops causing it’s volume to expand. This expansion requires energy so the temperature of the parcel of air decreases. This is called adiabatic cooling
What is adiabatic warming
When air subsides it’s pressure increases causing it’s volume to contract. This contraction releases energy so the temperature increases. This is called adiabatic warming
What is the environmental lapse rate (ELR)
Temperature decreases with a gain in altitude. The rate at which the temperature decreases varies day to day but for a standard set of conditions the temperature lapse rate is given as 2°C/1000 ft. So this tells us that on a “standard day” for every 1000ft we gain in altitude it will get colder by 2°C. The ELR is the temperature change with height of the ambient surrounding environment. The ELR can change
What is the dry adiabatic lapse rate (DALR)
Air that is less than 100% water is known as Dry Air. When forced to rise, dry air will cool. If forced to descend it will warm. This is known as the dry adiabatic lapse rate or the DALR. Dry air cools/warms at 3°C/1000ft. The DALR is constant and does not change
What is the saturated adiabatic lapse rate (SALR)
Air that is 100% relative humidity is known as Saturated Air. When forced to rise or descend, saturated air cools/warms at a different rate to dry air. This is known as the saturated adiabatic lapse rate or the SALR. Saturated air cools/warms at 1.5°C/1000ft. The SALR is constant and does not change
What are the two factors that determine whether air is stable or unstable?
- The temperature lapse rate of the surrounding ambient air (the ELR)
- The temperature lapse rate of the rising air (DALR for dry air or SALR for saturated air)
When the ELR is less than the DALR is air stable or unstable?
Stable
When the ELR is more than the DALR is air stable or unstable?
Unstable
When the ELR is less than the SALR is the air stable or unstable?
Stable
When the ELR is more than the SALR is the air stable or unstable?
Unstable
For an ELR that is less than 1.5 degrees/1000ft
- Both Dry and Saturated air is stable
* ABSOLUTE STABILITY
For an ELR which is between 1.5/1,000ft and 3/1,000ft
- Dry air is stable, Saturated air is unstable
- Stability therefore depends on whether or not the parcel of air is dry or saturated
- CONDITIONAL STABILITY
For an ELR greater than 3/1,000ft
- Both Dry and Saturated air is unstable
* ABSOLUTE INSTABILITY
What are clouds
In simple terms cloud is defined as suspended water in liquid droplet or solid ice crystal form. So when ever cloud is present, at some stage, water vapour has condensed (i.e. changed from a gas to a liquid state). For this condensation process to occur, air must contain Condensation Nuclei. These can be small hydroscopic particles of dust, salt or smoke which have a suitable surface for water to form on
What characteristics are clouds determined by
Shape
Vertical extent
Altitude
Formation of clouds
For clouds to form, air must rise. By whatever method air is lifted, the type of cloud that forms will depend on whether that air is STABLE or UNSTABLE
What methods can clouds form
Convective rising - heating of the earth’s surface or of the upper air
Orographic rising - due to significant terrain (i.e. mountain ranges) Mechanical lifting - due to small hills and buildings
Frontal lifting - cold dense air undercutting less dense warm air.
If rising air is stable, what type of clouds will form
If the rising air is stable this air will tend to form Stratiform type cloud. This is a layer cloud with small water droplets. Turbulence is generally light in these clouds as the air is stable and not rising
If rising air is unstable, what clouds will form?
If the rising air is unstable it will most likely form Cumuliform cloud. This is a heaped cloud with large water droplets. Turbulence can be moderate to severe in these clouds due to air rising from its instability
What are the three types of high clouds (17,000ft+)
Cirrus (Ci)
Cirrostratus (Cs)
Cirrocumulus (Cc)
What are the two middle clouds (7,000-17,000ft)
Altostratus (As)
Altocumulus (Ac)
What are the six low clouds (7,000ft and below)
Stratus (St) Cumulus (Cu) Stratocumulus (Sc) Towering Cumulus (Tcu) Cumulonimbus (Cb) Nimbostratus (Ns)
Cirrus (CI)
The most ethereal looking of all the main types, Cirrus clouds are also the highest, and are composed entirely of ice crystals. These typically fall through the high winds of the upper troposphere to appear as delicate, celestial brush strokes, called fallstreaks. This species of Cirrus, known as uncinus, has a hooked appearance, which indicates that the winds up at the cloud level are particularly strong. These clouds can indicate the presence of a Jet Stream or the approach of a warm front
Icing: no risk
Turbulence: light to moderate
Precipitation: none
Cirrostratus (CS)
Cirrostratus is composed of ice crystals. As it is so high it is very cold so its moisture content is very low. For this reason it is thin meaning the sun can still be seen through it. Often sunlight can be refracted by the ice crystals creating a halo around the sun.
Icing: nil
Turbulence: none
Precipitation: none
Cirrocumulus (CC)
Cirrocumulus is also composed of ice crystals. Its cold temperature means moisture content and therefore icing is not a risk. It often appears to resemble sheep’s wool
Icing: no risk
Turbulence: light to moderate
Precipitation: none
Altostratus (AS)
Altostratus is a mid-level, generally featureless, grey, overcast layer. It is a ‘Tupperware sky’ that often extends over several thousand square miles. Known by some as the boring cloud, Altostratus produces little more than a lingering drizzle or light snow. Once it is thick enough to produce more significant precipitation, it has generally developed into Nimbostratus
Icing: moderate to severe 4-6,000ft above freezing level
Turbulence: light to moderate
Precipitation: light
Altocumulus (AC)
This is a mid-level cloud (with bases between 7000ft and 17,000ft), which is typically a layer of joined or separated ‘cloudlets’. As it is lower than the high level clouds it is warmer and will therefore have a higher moisture content. It can be composed of water droplets and ice crystals. Altocumulus is one of the most dramatic and variable of the ten main cloud types, showing more possible species and varieties than any other.
Icing: moderate to severe 6-8,000ft above freezing level
Turbulence: moderate
Precipitation: light- in form of Virga
Stratus (ST)
This featureless, grey, overcast layer is the lowest-forming of all the cloud types, having a base that is typically no higher than 1,500ft. Stratus cloud like this that is down at ground level is called fog or mist.
Icing: moderate if above freezing level
Turbulence: nil significant
Precipitation: light drizzle
Cumulus (CU)
Cumulus are often described as ‘fair-weather clouds’ because they tend to form on sunny days. When they are small like these, they are the species known as ‘humilis’ (which means ‘humble’ in Latin). Cumulus generally have flat bases and bumpy, cauliflower-shaped tops
Icing: nil significant
Turbulence: light to moderate
Precipitation: none to light
Stratocumulus (SC)
In many parts of the world, Stratocumulus is a very common cloud type. It is a low layer, composed of individual or joined-up cloud clumps, known as ‘cloudlets’. Stratocumulus is generally less regular looking than the higher Altocumulus and Cirrocumulus layers of cloudlets. A high Stratocumulus cloud layer like this can be hard to distinguish from a low Altocumulus one. It is Stratocumulus when the base of the cloud layer is lower than 7000ft.
Icing: light to moderate above freezing level
Turbulence: light to moderate
Precipitation: none to light showers
Towering cumulus (TCU)
When the fair-weather clouds, known as Cumulus, grow taller than they are wide, they have developed into the largest species, known as ‘congestus’. These tall Cumulus often produce showers and can, if conditions are right, continue to grow into Cumulonimbus storm clouds. These clouds formed from large amounts of unstable air. Up draughts can be strong enough to ‘suck up’ a plane.
Icing: moderate to severe
Turbulence: moderate to severe
Precipitation: moderate showers to heavy showers
Cumulonimbus (CB)
This enormous storm cloud, which is often in the shape of a blacksmith’s anvil, can form individually or co-ordinate with neighbours to form ‘multicell’ and ‘supercell’ storms. The cloud is defined as a Cumulonimbus once its summit has changed from droplets to ice crystals, which give the top softer edges. Below a Cumulonimbus, you will see just its dark, ragged underside, which (being so low) appears to cover the whole sky. Cumulonimbus can form thunderstorms and have high moisture content and vast amounts of unstable air.
Icing: severe
Turbulence: severe
Precipitation: heavy showers
Nimbostratus (NS)
When people claim clouds are depressing, they’re often thinking of Nimbostratus. This thick, grey, featureless rain cloud gives all the other ones a bad name. Not only does it block much of the Sun’s rays, casting everything in a dim, miserable light, it also produces widespread rain – and lots of it. Nimbostratus generally results from the thickening and lowering of Altostratus clouds.
Icing: moderate to severe
Turbulence: none to light
Precipitation: moderate widespread rain to heavy widespread rain
How is cloud assessed
Cloud is assessed in how many eighths of the sky it covers. The term OKTAs is used to describe how many eighths
Sky’s clear - SKC
0 oktas
Few - FEW
1-2 oktas
Scattered - SCT
3-4 oktas
Broken - BKN
5-7 oktas
Overcast - OVC
8 oktas
NSC
Nil significant cloud. (Oktas)
Does not provide info on sky coverage
Cloud ceiling
A cloud “ceiling” is the height of the lowest level of cloud obscuring more than half of the sky (BKN or OVC)
Cloud base
A cloud “Base” is the height of any amount of cloud (FEW, SCT, BKN or OVC)
How is cloud base measured
Cloud searchlight
Ceilometer
Vaisala laser ceilometer
How can cloud be dispersed
- Adiabatic warming of subsiding air – warming causes relative humidity to decrease
- Warming of the air through surface conduction and convection
- Warming of air through absorption of solar or terrestrial radiation
- Reduction in buoyancy of air causing subsidence of air
- Mixing of saturated and unsaturated air reducing relative humidity -Precipitation resulting in a reduction in the relative humidity
Define wind
Wind is defined as the horizontal movement of air due to the forces acting upon it. Wind speed and direction is expressed as a velocity. Direction is given to the nearest 10° and is expressed as where the wind is coming from. For example a wind from the south is a southerly wind. If, however, the wind is expressed as where it is going, a southerly wind would be a northwards wind
Velocity and variation with height of wind
The principle reason for the movement of air is that variations of temperature and pressure cause air to flow. These pressure gradients have their origins in solar insolation. It is affected by cloud, surface (land or water), latitude and season. The sun’s energy is the prime cause of atmospheric motion
Backing and veering
If the wind change occurs in a clockwise direction then the wind is said to VEER. If the wind change occurs in an anti-clockwise direction then it is said to BACK. In the southern hemisphere the surface wind backs and increases after sunrise and it veers and decreases after sunset.
What is coriolis force
Due to the tilt and rotation of the earth the wind is subject to an apparent deflection force called Geostrophic Force or Coriolis Force (here in NZ).
This Coriolis force causes a deflection to the RIGHT in the NORTHRN HEMISPHERE and to the LEFT in the SOUTHERN HEMISPHERE
What is coriolis force proportional too
- The wind speed, the stronger the wind the stronger the turning force
- Latitude, the higher the latitude, the greater the Coriolis force
What are the two forces acting on coriolis wind
- Pressure gradient force acting from high to low
- Coriolis force acting at 90° to the wind towards the high
What happens when coriolis force is equal to pressure gradient
When the Coriolis force is equal to the pressure gradient force, isobars will be straight
What happens when coriolis force is greater than the pressure gradient force
When the Coriolis force is greater, isobars will curve around the high.
What happens when the pressure gradient force is greater than coriolis force
When the pressure gradient force is greater, the isobars will curve around the low
What is the friction layer
Due to drag from the Earth’s surface, the wind at the surface is slowed down. The layer of air affected by this drag is known as the friction layer.
What is the depth of the friction layer?
The depth of the friction layer is approximately 2000 feet however this varies depending on the amount of surface drag. Over the land, where there is significant terrain, buildings and trees, there is a lot of drag so the friction layer is deep. Over the sea, there is a relatively flat surface so the friction layer is not very deep
Will wind speed decrease more over the sea or land
Greater surface drag will slow the wind speed to a greater extent. This means wind speed is decreased to a greater extent in the friction layer over the land compared to the sea.
What’s the effect of the wind speed slowing down in the friction layer
The effect of the wind speed slowing down in the friction layer is that it causes a reduction in Coriolis force meaning the pressure gradient force is dominant and wind travels across the isobars.
Descending into the friction layer
D-descending
V- veers (wind direction)
D-decreases (wind speed)
Ascending out of the friction layer
A-ascending out
B-backing (wind direction)
I-increases (wind speed)
Amount of wind direction change due to the friction layer
Because the type of surface affects the friction layer, the amount that the wind direction will change will also be different
Land - wind speed decreases by 2/3
- wind direction change 30 degrees
Sea- wind speed decrease by 1/3
- wind direction change 10 degree
Changes in the friction layer due to density
The affect of the friction layer will also be changed by the density of the air. During the day, air is warm so the air within the friction layer will be less dense. This will mean that it will not slow down the wind by as much and therefore the wind will travel more parallel to the isobars. At night when temperatures are colder, the air becomes denser. This slows the wind speed down to a greater extent and results in wind travelling across the isobars
What’s the effect of the friction layer on pressure systems
As wind speed is slowed down at the surface it results in a decrease in the coriolis force. This causes wind to travel across the isobars from high to low. This has the effect of decreasing the pressure in a high pressure system and increasing the pressure in a low pressure system.
Effects of the friction layer on a HIGH pressure system
As air moves away from the high at the surface, in a high pressure system there is low level divergence of air. Air subsides within the high to replace the air and there is high level convergence at altitude. As air is subsiding within a high, it warms adiabatically resulting in a decreased relative humidity and therefore clear, generally cloud free, skies.
Effects of the friction layer on a LOW pressure system
In a low pressure system, air moves towards the centre of a low at the surface, it then rises and cools adiabatically. This increases the relative humidity and when the air reaches saturation it condenses giving cloudy skies. At altitude the air diverges
Buy ballots law
Buy Ballot’s law states that if an observer were to stand with their back to the wind, in the Southern Hemisphere, the area of low pressure would be on one’s right
Applying buy ballots law to see if your flying toward or away from a high or low pressure system
Applying Buy Ballot’s law, if you were to stand with your back to the wind, the area of low pressure is to your right. This means that, from the aircraft’s perspective, the area of low pressure is ahead of the aircraft. The aircraft is flying from high to low so if the altimeter is not adjusted, the aircraft’s true altitude would be lower than indicated. As it is flying towards a low pressure system, cloudy skies and increased wind speeds could be anticipated
Windsock 30 degrees
8kts
Windsock 45 degrees
15kts
Windsock 75 degrees
22kts
Windsock 90 degrees
25kts +
Determining windspeed and direction from smoke
Smoke blows in the direction of the wind. The degree to which the smoke is ‘bent over’ can be used to assess wind speed. In nil wind the smoke will rise directly upwards, the smoke will blow horizontally in strong wind
Determining wind speed and direction from dust
Dust can come from cars travelling on a dusty road or cultivation. Dust acts similar to smoke however it can be confusing as it can show the path of the car rather than the wind direction
Determining wind speed and direction from aircraft drift and ground speed
The direction in which the aircraft is holding drift can show wind direction and the amount of drift can show strength. Changes to groundspeed can also be used. This must be used with caution as this is the wind speed and direction at the aircraft’s altitude, not what is necessarily on the ground
Determining windspeed and direction from crops/trees
Wind can form a wave pattern on crops and trees. The direction and strength can be assessed from this
Determining windspeed and direction from water
Ripples form on water at 90° to the wind. A ‘shadow’ area will form where the bank of the lake shields the water from the wind. This is used to assess wind direction. If foamy ‘whitecaps’ can be seen, this indicates a wind strength greater than 15 knots
Determining windspeed and direction from wind lanes
Wind lanes are streaks of disturbed water caused by very strong winds. Unlike ripples, these form in line with the wind direction
Determining wind speed and direction from cloud shadow
Tracking the path of a cloud’s shadow can indicate wind direction and its speed shows the strength of the wind. Caution must be taken however as this indicates the wind at the altitude of the cloud and not necessarily what is on the ground.
Determining wind speed and direction from local knowledge
If in close proximity to an aerodrome, the duty runway and/or ATIS can be used to assess wind speed and direction
What is wind shear
Wind shear is a change in wind velocity over a vertical or horizontal distance. This could mean a change in wind speed or direction. A change in the wind will result in a change in the headwind component the aircraft is experiencing. The boundary between layers of air with different winds is known as the shear zone where the winds mix and tumble creating significant turbulence. The narrower the shear zone is, the more sudden the change in wind velocity will be and therefore the greater the winds shear
What happens to lift when wind shear is encountered
Due to inertia, the aircraft’s groundspeed will initially be slow to react. Therefore, wind shear causes changes in the aircraft’s indicated airspeed resulting in changes to lift production.
- If the aircraft gains lift, it is known as an energy gain or positive wind shear
- If the aircraft loses lift, it is known as an energy loss or negative wind shear.
What should you do if you encounter wind shear
Vacate the area or;
Increase power and approach speed
What happens if you encounter wind shear when approaching an aerodrome
If an aircraft is approaching an aerodrome, positive wind shear can result in overshooting the runway and negative wind shear could result in undershooting the runway.
This is an important consideration as downdraughts from a Cumulonimbus cloud that could be situated between the aircraft and the runway could cause this wind shear.
What is vertical wind shear
This is a change in wind velocity over a vertical distance and would be experienced by an aircraft climbing or descending
What is horizontal wind shear
Horizontal wind shear is a change in wind velocity over a horizontal distance and would be experienced by an aircraft in level flight
What is a sea breeze
During the day the land heats up more quickly than the adjacent sea. The air over the land rises creating a comparatively lower pressure compared to the air over the sea. Air then travels from the area of comparatively higher pressure over the sea to the area of comparatively lower pressure over the land. The air over the land is therefore replaced by the cooler moist air from the sea.
What are the strengths of a sea breeze
The strength of a sea breeze is greatest when the temperature difference between the land and the sea is at its greatest. This is during Spring and Summer as the sea is still cold from the previous winter and land temperatures are at their hottest. It is at it’s greatest strength in the afternoon when land temperatures are at their hottest. The strength of a sea breeze is, on average, around 10 to 15 knots
What are the times for sea breezes
The time a sea breeze can be expected is that it starts around 10am and peaks in intensity around 3pm. It stops as the sun sets.
What are the extents of a sea breeze
Sea breezes can penetrate inland around 25 to 40 kilometres and reach a height of around 2000 to 3000 feet above ground level.
What cloud is likely to develop with a sea breeze
As moist air is being transported inland and then heated, it is forced to rise. Cumulus cloud can develop and is called Fair Weather Cumulus.
Can wind shear be present during a sea breeze
If the sea breeze opposes prevailing winds, wind shear can be present
What is a land breeze
This is the opposite of the sea breeze and is due to the land being cooler than the sea at night, (the sea retains its heat a lot more easily than the land does). Therefore air from over the sea is relatively warm and rises. The cooler air from the land moves in an offshore direction to replace the rising air. Once again the cycle is set up but this time in the opposite direction.
When do land breezes occur
The land breeze is most common in Autumn as this is when the sea is still relatively warm from the previous summer but land temperatures are decreasing. It occurs at night as this is when the land will be at its coldest
What’s the strength of the land breeze
It is the temperature difference between land and sea which determines the strength of a land breeze. Unlike a sea breeze where the land is much warmer than the sea, the sea is never too much warmer than the land. For this reason, the temperature differential for a land breeze is a lot less creating a much less wind speed of around 3 to 4 knots.
Is there any cloud associated with a land breeze
As air from the land is not likely to contain much moisture, there is no cloud associated with a land breeze
What is an anabatic wind
These are mountain breezes caused by the sun heating up the surface of the mountains during the day causing the air to rise. This results in a gentle upslope breeze allowing air to cool adiabatically. These winds are generally weak as gravity opposes.
What is a katabatic wind
As an anabatic wind flows up a valley the katabatic does the opposite and flows down and out. This is due to the cooling of the mountain surface causing the air to subside down the slopes. It is a night phenomenon however it can persist into early morning
What is a fohn wind
This may be experienced when a mountain range forms a barrier to the wind flow. The orientation of the mountains determines the direction from which the Fohn wind blows.
The fohn wind is the wind that descends down the Lee side. It is a warm, dry and turbulent wind
(Calculating cloud bases)
What does a fohn wind require to begin
- A substantial mountain range
- A wind blowing at more or less right angles to the range
- A high moisture content of the approaching air
What happens if the air doesn’t reach its dew point in a fohn wind
If the air does not reach its dew point then will be no temperature difference between the Windward side and the Lee side. For this reason the approaching air must have a high moisture content
What are mountain waves
Mountain waves are often dramatic weather phenomenon which can affect hundreds of miles of sky and cause severe turbulence under certain conditions
What are the requirements for mountain waves to occur
- A substantial mountain range
- A wind blowing at right angles to the mountain range
- Wind speed a minimum of 10 to 15 knots at the surface increasing in strength with height
- An unstable atmosphere at low levels to assist air rising, a stable layer at or just above the mountain tops and a slightly unstable atmosphere above the mountain tops
What are the dangers of mountain waves
Flying in the Lee side is potentially very dangerous as the turbulence in the ‘rotor zone’ can be sufficient to flip an aircraft. Up and downdraughts in the wave can be greater than an aircraft’s ability to climb or descend
What clouds are associated with mountain waves
In the wave air rises and cools. Once it reaches it’s dew point it condenses forming cloud. It then descends and warms and when it passes through its dew point it ceases being cloud. The cloud therefore forms in the crest of each wave and this cloud is known as lenticular cloud. Lenticular clouds indicate mountain wave conditions
What is rotor streaming
Rotor streaming is similar to Mountain Waves however the wind speed decreases in strength from the top of the crest. This then acts like a ‘lid’ keeping the wind speed strong and turbulent on the lee side. As there is no wave, lenticular clouds are not associated with rotor streaming however swirling cloud can form from the turbulence experienced in the rotor zone. This cloud is known as rotor cloud. Turbulence will be severe to extreme beneath the mountain peak on the Lee side but above the crest of the mountain the flight conditions will be much less turbulent.
What is a gust
A gust is a momentary increase or decrease in wind speed and/or direction, which lasts for a few seconds and is due to mechanical turbulence
What is a squall
This is a rapid increase in wind speed and/or direction, which lasts for a number of minutes and then dies away due to frontal activity. They may be pronounced by the passage of individual cumulus clouds, thunderstorms or by the passage of a frontal system. A squall is defined as an increase in wind speed by 16kts to a minimum of 22kts for greater than 1 minute
What is a lull
A lull is a DECREASE in wind speed lasting less than a few minutes. Gusts include lulls.
What is a gale
Average surface wind is greater than 34 knots or gusts are greater than 43 knots.
What is a hurricane
Average surface wind is greater than 63 knots
What is a wind gradient
This is the gradual change in wind velocity between the surface and the top of the friction layer
What is a funnel effect
Wind flow that is forced into a confined space such as Cook Strait or Lyttelton Harbour will speed up, causing strong gusty winds
What are airmasses
Air masses are large blocks of air which possess similar characteristics in temperature, moisture and temperature lapse rate. Air masses can be classified by describing its temperature and moisture
Arctic continental (Ac)
Very cold and dry
