Chapters 6-11 + Midterm questions Flashcards
What is an adiabatic process?
When an air parcel is forced to rise or sink, it adjusts to the pressure level of the environment by expanding or compressing (i.e. by doing work to the parcel). This changes the temperature of the air parcel but does not exchange heat with the environment. When it rises, it cools (does work on the environment), and when it sinks, it warms (the environment does work on it).
Lapse Rate
The rate of decrease or increase in temperature with increasing or decreasing altitude.
Specific humidity does not change, but relative humidity does. The RH of a rising parcel increases since colder air parcels are able to hold less water.
Dry adiabatic lapse rate
The parcel is unsaturated (RH<100%)
10C/km
Saturated adiabatic lapse rate
The parcel is saturated (RH = 100%) 6C/km
In this case, condensation or evaporation of water is occurring in rising or sinking air parcels
When is air more dense?
In a fixed air pressure, it is when it is colder.
Buoyancy force
This relates the difference between the density of the environment and the density of the air parcel. If the air is warmer/less dense than the environment, it floats. If it is colder/more dense then it sinks.
Absolutely stable environment
This is when the environmental lapse rate is less than the saturated lapse rate - meaning also less than the dry lapse rate (less than 6C/km). This can also happen at inversion lapse rates where the temperature rises with altitude.
- This stability usually occurs over a cool surface where the denser air is below the warmer air
How can stratus or stratiform clouds form?
When air parcels are forced to rise in a absolutely stable environment, causing it to spread horizontally and possible form clouds.
Absolutely unstable environment
This is when the environmental lapse rate is greater than both the dry and saturated lapse rates (greater than 10C/km). In this case, if the air is displaced upwards, it will simply continue to rise.
- It is usually over a warm surface causing the surface air to heat up due to radiation and advection.
- Associated with convection, thunderstorms and severe weather
What type of clouds does an absolutely unstable environment form?
When an air parcel in an unstable environment is saturated, clouds with vertical development are formed.
Conditionally unstable environment
This is when the environmental lapse rate is in between the saturated lapse rate and the dry lapse rate (6<here<10).
- It is stable for unsaturated air parcels but unstable for saturated air parcels
- Troposphere is usually in this state
What affects the environmental lapse rate (how to make larger or smaller)?
- When there is warmer air below and colder air aloft it leads to a less stable environment and a larger environmental lapse rate
- When there is colder air below and warmer air aloft it leads to a more stable environment and a smaller environmental lapse rate
2 factors that affect the air parcel lapse rate
- Higher RH. The parcel is able to saturate more easily and at a lower altitude so then it follows the saturated adiabatic lapse rate. Environmental instability is more likely since the air parcel now has a lower lapse rate.
- Higher temperature. If the parcel is saturated then its lapse rate is reduced causing a highly likelihood for instability.
2 causes of instability
- Cooling air aloft (advection, radiative cooling of cloud tops)
- Warming of surface (insolation, advection, warm surface) The warm air parcel would rise and be warmer than the air surrounding it
Vertical mixing of a stable air layer
This brings the layer closer to a dry adiabatic process causing it to destabilize with respect to saturated parcels. This means that the lapse rate is increased.
- Mixing means that the warmer air (that usually is rising) is brought down and the cooler air is pushed up
What is subsidence of air?
Instead of looking at how small air parcels behave in the environment, subsidence is looking at movements of whole layers of air. More specifically, it is the downward motion of air over a large area as it cools and becomes more dense.
Why does subsidence cause layers to become more stable?
The sinking air aloft causes a warmer middle troposphere which means that warm air lies above cold air. This is associated with temperature inversions at lower levels proving that inversions are very stable.
- If pollution is found below the inversion it is then trapped as inversion suppresses vertical motions
What do we want for layer stability?
Warm air aloft with cold air underneath. This is because warm air rises and cold air sinks.
What layer subsidence movements cause stability?
Decent and squashing of the air. As the air shrinks, it becomes more dense causing it to sink and descend. The shrinking also causes the upper area of the layer to warm faster than the lower area ultimately causing stability.
What layer subsidence movements cause instability?
Lifting and stretching. As the air ascends, its expands. To add to that, the rising causes the upper area of the layer to cool faster than the lower area which means instability.
Convective potential instability
When an air parcel is lifted, certain parts may cool faster than others. This happens when the bottom of the layer is more saturated than the upper layer and so the bottom layer cools at a slower saturated rate while the upper layer cools at a faster dry lapse rate. This causes layer instability as the colder air is found aloft. It also causes the environmental lapse rate of the layer to increase.
Radiative cooling by clouds
There is a strong imbalance on cloud tops where clouds are constantly emitting long wave radiation faster than it absorbs it. This means that the top layers of the clouds are cooled causing steep lapse rates below and instability. This instability causes downdrafts enabling stratus clouds to turn into stratocumulus clouds.
When do cumulus clouds develop?
These develop when air parcels are brought to saturation by some sort of lifting mechanism. The environmental profile needs to be conditionally stable or convectively unstable. (this means that it happens when there is cooling aloft due to large scale lifting, when there is warming below or when there is more humid low-level air so parcels cool at a reduced rate.
4 types of lifting to form clouds
- Convection (surface heating)
- Lifting along topography (like mountains)
- Convergence of air (cause of snow in MTL)
- Lifting along weather fronts (like cold fronts)
How are cumulus clouds formed?
These are formed when bubbles of warm air are able to detach from the warm surface and rise to saturation. As this happens, cooling and sinking air surrounds the cloud formation.
How is cloud stability determined?
It is determined by the layer stability that surrounds it. This shows how cumulus clouds can only become vertically large thunderstorm clouds within conditionally or convectively unstable layers.
Orographic lifting (mountains)
This is a mechanical forcing technique of air lifting. On the side of the mountain with wind, cold saturated air is being pushed upwards following the shape of the mountain forming clouds. On the other side of the mountain, warm dry air is being pushed down.
Mountain wave clouds/lenticular clouds
These are caused over and downwind (leeward) hilly terrain. The hilly air created little waves in the layers causing clouds formation in the moist air on the lifting part of the wave. Those that are formed downwind of the mountain are called leeward clouds.
How are stratocumulus clouds formed?
These are formed where the upper portion air is cool and moist after vertical mixing (due to radiative cooling). The dry air aloft keeps clouds shallow and the often ends up being a inversion at the top of the cloud layer.
- These are common off the west coasts of large continents where warm air flows over cold oceans
Do all clouds make precipitation?
No, cumulonimbus and nimbostratus clouds do.
Why do only some clouds succeed in creating precipitation?
- Some clouds evaporate right after forming and do not have time to make precipitation
- Cloud droplets need to be able to grow big enough for gravity to pull them down, and this is done by certain mechanisms
- This will depend on many factors like temperature, cloud width, cloud thickness, internal cloud motions
How does the temperature of the cloud affect the precipitation?
Warm clouds will form liquid precipitation while cold clouds form ice or mixed phase contents.
- Cold clouds can have the presence of supercooled liquids
How do cloud droplets compare to rain droplets?
Cloud droplets are much smaller than rain droplets meaning that cloud droplets are too small to overcome drag and updrafts.
- Typical cloud droplets are 20 micrometers in diameter while typical rain droplets are 2000 micrometers in diameter
How does saturation vapour pressure work on droplets?
Saturation vapour pressure over the curved surface of a droplet is greater than over a flat surface (on flat surfaces there are less attractive forces making it easier to evaporate). But saturation is defined relative to flat surfaces so for cloud droplets to exist, air must be supersaturated.
- The smaller the droplet, the larger the supersaturation needed to keep it in equilibrium
How much humidity/saturation is needed for the growth of droplets?
The amount of relative humidity needed will depend on the droplet size. If the relative humidity is less than the saturation given by the droplet, then they droplet will simply evaporate. This means that the flatter the surface, the less saturation needed for cloud droplets to grow and so larger droplets need less saturation than smaller droplets. (I.e cloud droplets are more likely to precipitate).
- Droplets grow when the humidity is greater than the saturation given by the surface
What is the solute effect?
It is when hygroscopic nuclei (like salts) reduce the equilibrium vapour pressure by forming an aqueous solution.
- Aqueous solution droplets contain less water molecules at its surface causing it to have a lower evaporation rate and a lower saturation vapour pressure
Do you need the relative humidity to be over 100% for condensation of water?
No, hygroscopic nuclei allow for condensation of nuclei when the the RH is less then 100%. This means that by adding things like salt that dissolve in water, it reduces the RH needed for equilibrium allowing for easier condensation.
What is the difference between the concentration of CCN on land vs in the sea?
- Over land, there are more CCN such as aerosols from dust, vegetation and human activities than there is over the sea, meaning the air is cleaner over the sea.
- Additionally, the liquid water over land is distributed over more droplets, so the droplets end up being smaller than the sea water droplets. This means that the CCN over the land stay suspended for longer and it is more likely for precipitation to be formed over the sea (the cloud lifetime is shorter)
What are ship tracks?
Ship tracks are a visible effect of enhanced CCN concentration over the sea due to human activities. This changes the cloud droplets sizes to more land like conditions where the droplets are smaller and the clouds are brighter.
Can rain be formed simply by condensation?
No, it cannot as this process by itself is too slow to be able to produce droplets large enough to be rain. Droplet growth is best when there is collisions and coalescence of droplets.
What does the droplet fall speed depend on?
It depends on the size of the droplet. Air drag will have a bigger effect on droplets with larger diameters, but gravity will have an even more significant effect on heavier droplets. This means that terminal velocity will increase with the droplet size.
How does the terminal velocity of the droplet affect the growth of the droplet?
As the larger droplets fall faster than the smaller droplets, they will collide with the smaller droplets and merge causing them to fall even faster. This follows the idea that the rich get even richer.
What can effect the efficiency of rain droplet formation?
- The airflow around bigger droplets can push the smaller ones out of the way
- Sometimes droplets can collide but not merge
What is the volume of a raindrop?
It is equivalent to about 1 million cloud droplets
Why do not all warm clouds make rain?
This is because the process of growth of the droplets requires substantial lifting of the cloud (i.e while the droplets rise and fall through the cloud, they will collide and grow large enough to precipitate).
- This lifting is common is deeper cumulus clouds and uncommon in warm stratus clouds
What are 5 processes that enhance droplet growth?
- Number of hygroscopicity of CCN
- Larger vertical displacements of clouds
- Broad droplet size spectrum (various terminal velocities)
- Deep clouds (more area for collisions)
- Electrical charge of droplets
What are supercooled liquid droplets?
Cloud droplets do not freeze immediately at temperatures below 0C. This means that liquid droplets can still exists in temperature down to -40C which are called supercooled liquid droplets. Mixed phase clouds include these droplets in addition to ice particles.
What are the two types of ice nucleation?
- Homogenous: pure liquid droplets simply freeze (this does not happen on our planet)
- Heterogenous: freezing if CCN when in contact with IN (IN are less common than CCN) - catalytic process
What are three heterogenous ice nucleation processes?
- Deposition freezing: the deposition of water vapour directly into IN
- Immersion freezing: IN is embedded directly into supercooled liquid droplet
- Contact freezing: supercooled liquid droplet freeze upon contact with IN
Ice crystal Bergeron-Findeisen process
A process that shows how mixed phase clouds go from consisting of mostly liquid droplets to mostly ice droplets.
- The water saturation vapour pressure is higher over supercooled liquids than it is over ice, so in the process of ice crystal formation, the air is saturated with respect to the liquid droplet but super saturated with respect to the ice nuclei
- This means that the ice nuclei present will begin to grow rapidly in size reducing the vapour pressure which then causes the liquid droplets to either evaporate or collide with the IN and freeze
What is required for pure condensed liquid to form in clouds?
The actual water vapour pressure has to exceed the saturation vapour pressure of the molecule at the temperature. If not, the liquid would evaporate
Cloud seeding
- IN is injected into the supercooled liquid clouds and then heterogenous ice formation occurs via the Bergeron process
- Occurs naturally when winds cause upper ice clouds (cirrus) to seed the lower supercooled liquid clouds
- Occurs artificially using dry ice or silver iodide particles
Accretion
Falling ice crystals freeze supercooled liquid droplets upon contact producing larger ice particles
What is the difference between nimbostratus clouds and cumulonimbus clouds?
Cumulonimbus: high liquid water content - rain and hail can be formed from graupel after accretion
Nimbostratus: low liquid water content- rain and snow can be formed after aggregation
Aggregation
Falling ice crystals collide and stick with other falling ice crystals
Virga
Rain that evaporates before it reaches the ground
What size is rain usually?
It is usually over 0.5 mm in diameter but less than 6 mm
What type of clouds does rain originate from?
Convective: Intense and short lasting (cumulonimbus)
Stratiform: lighter and steadier/longer lasting (nimbostratus)
What clouds does snow come from?
Most precipitation from nimbostratus clouds starts off as snow but it can melt before it hits the ground. In images, snow is darker than rain since snow can scatter sunlight more efficiently
How does snow grow?
When snow falls in above melting temperatures, the edges of the snowflakes melt first creating a glue for when they come into contact with other snowflakes. (this is better for snowmen)
- Snowflakes are smaller and more powdery in colder and drier air conditions since there is less water vapour pressure (better for skiing)
Why is snow a hexagonal shape?
This comes from how water molecules are arranged for optimum stability. Plus, it is better for packing and there is less energy loss.
What determines the shape of the snow?
The temperature of the environment. Dendrite (pretty ones) are the most common since they form at temperatures around -12C.
How does the vertical temperature profile affect falling precipitation?
- If the temperature profile stays below 0 than snow will fall
- If the temperature profile starts in below melting, goes briefly into above melting and then returns long enough than ice pellets will fall
- If it starts in the negatives and then dips into the positives for too long before freezing than freezing rain will fall
- If it starts below melting and then stays above 0 than rain will fall
Riming
When supercooled liquid freezes upon contact with frozen/cold objects. This can happen inside clouds to form graupel or outside of clouds on metal
Hail
Rimed ice particles that are over 5mm in diameter. Caused by heavy riming due to multiple cycles of uplift and descent in cumulonimbus clouds.
How is precipitation most commonly measured?
By radars that send pulses in all directions. The precipitation will scatter the energy sending it back to the receiver. The precipitation rate is related to the intensity of the return signal.
What is the normal annual snow fall in montreal?
2.3m
What are the 2 reasons montreal has so much snow?
- The upwards movement of air (it expands and cools as it lifts causing condensations) - provided by the cyclonic low pressure storm system
- It has a good source of moisture
Where does the cyclonic storm system come from?
In the winter, the polar front moves south into the US. To add to that, storm systems providing moisture begin in the south of the US and move in the direction of montreal
How to remove snow?
- Salt (damages air quality that shows effects in the ice nuclei that form)
What is pressure?
It is the Force/Area. P2-P1 is the pressure gradient. In other words, it is the weight of the overlying air column per unit area. Lower pressure means that there is less mass above that level in the vertical column.
How does temperature affect air pressure?
Colder air is more dense and therefore takes up less volume. This means that at a certain point, the air pressure will be lower than that same point in a warmer column of air. Warm air is less dense and takes up more volume. At the very bottom of both of these columns, the pressure will be the same.
Which way does air flow?
Air goes from high pressure areas to low pressure areas.
Is there a big range in air pressure?
No, the range of air pressure on Earth is only a few % from the standard sea level pressure. However, small changes in pressure can cause bigger changes elsewhere.
How to measure pressure?
Using barometers. When the pressure goes up, the mercury levels go up as well.
Station pressure vs Sea-level pressure
Station pressure is the pressure at a certain location/elevation (can be affected by temperature).
Sea-level pressure is the pressure at the fixed level of the sea, barometers are standardized to this level.
What do isobars show?
The variations in horizontal pressure at a certain altitude
How do surface level and upper level isobar maps differ?
Surface level maps show wind moving across the isobars where the upper level maps show the wind moving parallel to the isobars. The ridges and troughs show the pole to equator pressure gradients that affect the day to day weather.
Pressure gradient force (PGF)
This is a force that comes from the difference in pressure over a certain distance. (change in pressure/change in distance). This force goes from high pressure areas to low pressure areas (perpendicular to isobars) and is what causes the wind to blow.
What does the distance between isobars indicate?
if the distance between isobars is small, that means that the PGF is stronger as the same pressure changes over a smaller distance. Meaning the smaller the distance, the larger the PGF over a consistent change in pressure. Closer isobars also indicated stronger winds.
Coriolis force (COR)
This is the pressure felt only by rotating objects or on a rotating reference frame. This force increases with latitude and wind speed and influence the wind direction. It is directed at 90 degrees to the left in SH and 90 to the right in NH. It has effects on deviation of wind path at the larger scale (like for planes). On the equator, the Coriolis force has no effect and planes can travel in a straight line with no deviation, at the poles it is the strongest.
3 dominant wind balances
Geostrophic wind (straight flows across isobars): PGF+COR=0
Gradient (flows that exhibit curvature): PGF+COR-CEN=0
Surface wind: PGF+COR-CEN+FRIC=0
Geostrophic winds
At upper levels where friction is negligeable the PGF and COR balance each other. The PGF pushes air from high to low pressure, and as the winds strengthen the COR strengthens pushing the flow to the right or left. Once the winds flow parallel to the isobars the PGF and COR come into balance with the COR opposing the PGF. The winds parallel to the isobars are geostrophic winds.
Centripetal force (CEN)
The inward force required to keep an object rotating on a circular path, used for gradient wind balance.
Gradient wind balance
PGF+COR=CEN
The air circulating around the L or H have curvatures that require a CEN to maintain that circular orbit. When adding the opposing force of PGF and COR together must equal a net force to the centre CEN.
- For winds circulating around L, the COR must be weaker than the PGF causing the winds to be weaker (sub-geostrophic winds).
- For winds circulating around H the COR must be stronger than the PGF causing stronger winds (super-geostrophic)
Cyclonic vs anticyclonic flows
Cyclonic flows are when the wind moves around a low pressure area. In the NH this is counter clockwise as the winds move to the right of PGF. (bad weather days)
Anticyclonic flows are when winds move around a high pressure area.
Zonal vs meridional air flows
Zonal means east-west
Meridional means north-south
Surface wind balance
(PGF+COR+FRIC=0)
The surface friction weakens the Coriolis force in a way that it can no longer oppose the PGF force. The winds then turn slightly towards like low pressure area. The friction then helps to balance the PGF.
How does horizontal air motions affect vertical air motions?
Circulations around L bring in air (convergence) and the air then rises to create clouds.
Air circulating around H are pulled away (divergence) and so air from above is being pulled in. This causes nice days.
Viscosity
The fluids resistance to flow (water has a lower viscosity than peanut butter)
Eddy viscosity
This is the bulk effect of eddies (current running contrary to the flow). Eddies slow down the main flow and can be seen as a form of internal friction
Mechanical vs thermal generation of eddies/instability (micro-scale winds)
Mechanical generation: regions of strong wind shear (wind gradient) when winds flow past big bodies (buildings or mountains)
Thermal generation: This is absolute/conditional instability where there is an interface between rising updrafts and sinking downdrafts with zero vertical speed
The planetary boundary layer (PBL)
A shallow atmospheric layer directly affected by the surface. This is the layer where the turbulence/surface friction influences the air flow substantially. It is deeper when eddies cause stronger instabilities (strongly heated surface or stronger lower winds). Winds are slowed down due to friction but eddies cause wind gusts by transporting winds down.
-Micro-scale winds
What deepens PBL
Afternoon heating creates thermals that deepen the PBL which means more transport of fast winds down to the surface. The surface winds then flow past obstacles creating small eddies.
Mountain lee dynamics
Lee vortices: these are little vortices that form over small-scale obstacles, they slowly dissipate downwind
Lee waves: Alternating up and down motions in lee of a ridge. These cause linear cloud formations parallel to the ridge and small rotors underneath the crests
Thermal circulations at a local scale
These are caused by horizontal temperature gradients. A high pressure area develop at an upper level on the warm side, PGF pushes wind to the colder lower pressure area. This causes more pressure on the cold side and since the warm side looses air aloft, the lower areas become lower pressure. Therefore the PGF pushed winds from the cold side to the warm side. The fact that cold air sinks and warm air rises completes the circle.
Sea breeze vs land breeze
These are both forms of thermal circulation at a local scale that are caused by the temperature gradient between the land and sea. During the day, there is a sea breeze as the water warms slower than the land so low pressure (as the air sinks) areas aloft are found over water while high pressure areas aloft (as the air rises) are formed over land. At night there is a land breeze since the land cools faster than the water creating the low pressure area aloft over the land.
Convergence of sea-breeze fronts
When sea breeze form over peninsulas, that means two currents of winds are coming from two opposing directions. When they meet, there is a convergence of air and warm air is pushed upwards to form clouds. Perfect for initiating summer thunderstorms.
Valley vs mountain breezes
These are thermally driven and are similar to sea and land breezes but with a vertical component due to the topography.
- During the day, the air is heated by the surface causing it to rise against the mountain and create cumulus clouds at the top. This is a valley breeze.
- At night, air cools quickly due to the proximity of the mountain, it sinks creating a high pressure area below. This is mountain breeze, a form of katabatic wind (when cold dense air rushes down elevated slopes with strength)
Chinook winds
Dry, warm descending air on the leeward side of an orographic barrier (mountain).
On the windward side, saturated air rises creating clouds, releasing heat and creating precipitation. The air is now dry as it descends over the leeward side.
Monsoons
A large-scale wind system that varies seasonally. These are very large sea and land breezes that are affected by the Coriolis force. This is the cause for wet and dry seasons as large amounts of warm moist air comes in during the summer and leaves during the winter.
How are winds shown on a weather map?
Wind barbs show the wind speed and direction. There are different shapes and sizes that correlate to different magnitudes of wind.
What is the general circulation of the atmosphere driven by?
The unequal heating of the Earth’s surface. As a whole, the Earth is in radiative energy balance but some areas experience a net gain/loss. Circulation acts to maintain the the energy budget but not everywhere is treated equally. It is a time averaged picture of the global wind system, so local winds and weather can vary greatly from it.
Idealized 3 cell model of the Earth
I the Earth were not rotating and uniformly covered in water, there would be a simple thermally direct circulation. This would mean a low pressure, warm area near the equator and high pressure colder areas at the poles
Idealized 3 cell model of Earth
This model consider the Earths rotation dividing both of the hemispheres into three sections with layers of alternating pressures.
Real world pressure and wind patterns
It is a more complicated system with semi-permanent highs and lows due to land/ocean contrasts, topography and seasonality.
How do global patterns show where is most likely to get precipitation?
Precipitation occurs in areas of organized rising motion which are found in low pressure areas. Following the 3 cell model, this means along the equator (tropics) and at the midlatitudes.
Can we predict regional precipitation by following the global winds flow patterns?
We could but this would not give an accurate result. If you look at both sides of north america, there are generally similar areas of high pressure circulation that are thought to bring in similar weather. However, due to the conditions that they are coming from this is not true. On the western side you get dry summers while on the eastern side it is more humid.
How do the flow patterns change with the seasons?
The difference between the winter and summer months can be seen in the latitudes of the flows. For example, in the winter, the high pressure areas tend to be lower than in the summer.
Jet streams
Narrow bands of strong winds in the upper levels of the troposphere. They encircle the globe in wavy currents and can vary seasonally or on a day to day basis. They regulate large scale weather patterns.
Error in the three cell model
The three cell model shows strong westerlies at low levels in the midlatitudes that weaken aloft, but really they are very strong in the area.
Why do jets exists?
There is a large N-S temperature gradient along the polar front that separates the cold area (shorter column) from the warm area (taller column). This creates a strong PGF and strong westerly flow that is the polar jet. But these flows can vary depending on the latitude and this is to conserve the angular momentum. The flows that are higher up have to be stronger since they have a smaller radius to the axis of rotation.
How do ocean current compare to airflow?
They move much slower.
Ekman spiral
Spirals in the water caused by wind on the surface. The water is deflected to the right in NH due to the Coriolis force. The surface water moved by the drag of the wind moves the water a layer below it that is deflected even more to the right and so on. As the effects go deeper, a spiral is created with diminishing strength. This leads to coastal upwelling of cold water along the west coasts (water rising from beneath)
El Nino vs La Nina
This is an important atmospheric and oceanic interaction.
- El Nino is when warm water is pushed to the western side of the americas stopping the cold upwelling at the coasts and changing the location of the jet stream.
- La Nina is the opposite of El Nino. Trade winds strengthen pushing the warm pacific water from the west coast of south America towards Asia. This means colder waters and lots of upwelling on the west coast of the americas.
How do El Nino and La Nina patterns change throughout the year?
There is a variable 3-7 year return period on the water trends but they are very irregular and very hard to predict. It should be noted that El Nino and La Nina do not happen in the same years, and one of them could last for years in a row.
El Nino teleconnections
This is the idea that El Nino causes effects worldwide and well outside of the equatorial pacific, mainly through the precipitation levels.
Three stages of thunderstorms
- An unstable atmosphere with vertical updrafts keep the precipitation suspended. Horizontal winds cause anvil top clouds.
- The entrainment of dry air keeps the cooler air from evaporation which causes downdrafts, precipitations and gusts (the storm)
- After 15-30 minutes, the updrafts weaken and the loss of fuel causes the storm to die. Lower level cloud particles evaporate leaving cirrus clouds up top
How do dying storms trigger new storms?
The dying storms leave cool downdrafts that force the surrounding low-level moist air upwards, creating new thunderstorms.
What can we observe from severe thunderstorm clouds?
These clouds are usually dark in colour with a huge vertical profile. The layers in the clouds can be used to predict the short term weather.
How to kill storm?
Go to its source of fuel and cut it off. This means go to where the updraft is. If you weaken the updraft, the storms weakens as-well.
- In normal storms, the downdraft and precipitation cut off the updrafts but in severe storms, winds aloft push the rain ahead as to not interfere with the updrafts
Gust front
Turbulent air forms along the leading edge where the warm moist air rises to create the updraft, also forming shelf clouds These are associated with the cold dense air.
- Little eddies that spin about a horizontal axis can form underneath the shelf clouds due to the turbulent air
Microbursts
Dry air that is brought into the thunderstorm will evaporate and cool the precipitation. This creates dry or wet microbursts of strong winds.
- When flying into a microburst, the plane is initially lifted by updrafts and then pushed down by downdrafts.
How to find areas of thunderstorms on graphs?
These are the areas with colours, usually red being the most severe storms.
Mesoscale convective complex
An organized or collection of thunderstorms that extend across a large region. These are formed beneath a ridge of high pressure and can continuously regenerate storms.
Flash floods
These are caused by jet streams due to thunderstorms and can be very devastating to the area.
Lightning and thunder
Charge differences between the storm and the ground can cause lightning strikes that heat the air rapidly and cause explosive shock waves we know as thunder.
- To be safe, do not stay on the ground during thunderstorms. rather find something that will transfer the lightning into the ground, like a car.
Tornados
A rapidly rotating column of air that goes through stages of strength. The winds will vary with the local winds found along its path. There are multi-vortex tornados with multiple smaller whirls
- They forms after spinning horizontal vortex tubes are tilted and forced to a vertical path by updrafts
- There are land spouts and water spouts that are much smaller but follow the same concepts
Directions of tornados NH vs SH
NH: counterclockwise
SH: clockwise
How are tornados shown on radars?
The area of precipitation and winds in the tornado are undetectable and are displayed as a black dot on the radar screen.
How do tropical winds create storms?
These winds blow from the east, they sometimes encounter low pressure areas that are slower moving (tropical waves). These winds converge and lift to create showers and thunderstorms
Hurricane
These form from and organized mass of storms that were formed along a tropical wave. A spiral of rain and clouds form around an eye which is an area of broken clouds.
Hurricane wind profile
The low pressure core of the hurricane (eye) is surrounded by several thunderstorms each with their updraft and downdraft systems.
Hurricane paths
Hurricane can move throughout several areas, usually at a speed from 10-50 knots. But they can also stall and cause destructive flooding. Their paths are erratic and unpredictable but there are some observable patterns.
