Meteorology Flashcards
How is wind described?
Wind direction- the direction FROM WHICH the wind is blowing, expressed in terms of degrees clockwise from north.
Wind strength- expressed in knots (kts).
Direction and strength combined give VELOCITY.
If the wind direction INCREASES it is said to be VEERING, if the wind direction DECREASES it is said to be BACKING.
What causes the wind to blow?
Pressure Gradient Force- movement of air from a high pressure area to a low pressure area.
Coriolis Force- (not actually a force!), apparent change in direction caused by the earth’s rotation. Alters the direction of movement to the RIGHT in the northern hemisphere.
What is a Geostrophic wind?
The wind produced by the effect of the Coriolis force on a Pressure Gradient wind. Wind is bent to the right until it follows the line of the pressure isobars.
Therefore Gradient winds blow anti-clockwise around low pressure areas and clockwise around high pressure areas.
Buys Ballot’s Law- If you stand with your back to the wind in the Northern Hemisphere, the lower pressure is always toner left.
What are the effects of friction on Surface Wind over the land and sea?
Over Sea- Wind will BACK by approximately 10° and its speed will be reduced to 80% of Geostrophic wind speed.
Over Land- Wind will BACK by approximately 20° and its speed will be reduced to 40% of Geostrophic wind speed.
What effect does surface wind have on aircraft operations at aerodromes?
- Aircraft take off and land into the wind (more lift, slower for landing). This is the greatest influence for runway selection.
- Strong winds, sudden changes in wind velocity are hazardous to aircraft.
What is a ‘Gust’ of wind?
A temporary increase in the mean wind speed lasting a few seconds (associated with turbulent eddies in the friction layer).
What is a Mesoscale System?
Mesoscale meteorology is the study of weather systems with horizontal dimensions generally ranging from 2-2000km. Eg. sea and land breezes, mountain and valley winds.
What are Sea and Land Breezes?
Sea breeze- a surface wind which blows from the sea to the land as a result of differential heating of the surface (during the day the land warms faster). They are strongest on a sunny day with a weak pressure gradient.
Land breeze- at night the process may be reversed as the land cools quicker.
Describe the 3 types of valley wind.
Katabatic wind- at night the land cools quickly, which cools the air making it less dense and causing it to ‘fall’ down the hill.
Anabatic wind- by day the land warms quickly, as the air is warmed it becomes less dense and rises up the hill.
Föhn wind- a dry wind that blows down the lee side of a mountain. Due to the difference in value of the Dry Adiabatic Lapse Rate (DALR) 3°/1000ft, and the Saturated Adiabatic Lapse Rate (SALR) 1.5°/1000ft.
What is a tornado?
A violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud.
Most tornados have wind speeds less than 110 mph, are approximately 80m across and travel only a few miles before dissipating.
What is a waterspout and why might it be dangerous to aviation?
Waterspouts are characterised by a spiralling funnel-shaped wind current, connecting to a large cumulus or cumulonimbus cloud. They are generally classified as non-super cellular tornados that develop over bodies of water.
Aircraft flying over the sea or large bodies of water would have to be careful not to get close to one as they are very dangerous.
What is wind?
The flow of air over the earths surface. Almost completely horizontal- only 1/1000th flow is vertical. This flow is very important to weather and aviation.
What is Air Mass?
An air mass is a body or mass of air in which the horizontal gradients or changes in temperature and humidity are relatively slight. May cover several million square kms.
What are the main weather ‘bands’ of the Earth?
InterTropical Convergence Zone (ITCZ):
Permanent Low Pressure belt approximately 10° wide, located near the equator but moves with the seasons between 20°N and 20°S between July and January.
Hadley Cell:
Between the Equator and 30°N/S. Warm air rising near the equator moves pole-ward in the upper troposphere, while cooling air around 30° moves equator-ward at lower levels.
Mid-Latitude Cell (Ferrel Cell):
As air descends at 30° it becomes compressed and denser and creates a High Pressure Belt in the subtropics.
Polar Front:
The boundary between polar air and warmer air is a polar front. The movement of warm air pole-ward and cold air equator-ward is achieved by air moving horizontally. There is a sharp temperature gradient.
Polar Cell:
Within 30 degrees of the Polar cap the air circulation is primarily away from the poles at low levels and towards the poles at upper levels.
The area between the cells has the greatest temperature changes. As the temperature difference increase the wind also increases in the upper atmosphere. Jetstreams can vary in level from 4 to 8 miles and have wind speeds of up to 275mph.
What characteristics of the earths surface modify air masses, and how?
Air mass moves over: Ocean surface- becomes more moist at low levels from oceanic evaporation. Land/continental area- remains dry. Warm surface- warms Cold surface- cools.
How do we classify air masses?
By region of origin:
- Arctic A. - Antarctic AA. - Polar P. - Tropical T. - Equatorial E.
And by route followed by the air mass:
- Continental c. - Maritime m
What are the main Isobaric Features?
An area of high pressure is known as an anticyclone.
An area of low pressure is know as a cyclone.
A front is a distinct division between adjacent air masses.
Frontal activity describes the interaction between the air masses as one replaces another.
What is a Front?
A boundary between two air masses. Shown on a weather map as a line. Has a sloping surface.
Fronts always lie in troughs of low pressure, where pressure decreases and then increases. This is often accompanied by a significant change in the wind direction.
What are the characteristics of cold and warm fronts?
Cold Front:
- Steep slope (1:50)
- Pressure increases as front moves through
- Temperature decreases as front moves through
- Wind veers.
- Often causes Cb or TCu as warm air is forced up, cools and water condenses.
- High visibility
Warm Front:
- Shallow slope (1:200)
- Temperature increases and pressure decreases as front moves through.
- Wind backs slightly and then veers.
- Cirrostratus, alto stratus and stratocumulus clouds formed over long distance as warm air creeps slowly higher.
What is an Occluded Front?
An occluded front occurs when a cold front catches a warm front.
Either the warm air will be forced very high as the colder air then catches the cool air of the preceding high pressure (cold occluded front), or if the preceding cold air is colder than the cold front, both the warm and cold front will be forced higher (warm occluded front).
What is a warm sector?
The area between a fast moving cold front and preceding warm front, before the warm front is caught.
What shall a pilot do on meeting hazardous conditions?
On meeting hazardous conditions in the course of a flight the commander shall inform the appropriate ATCU as soon as possible by the quickest means possible, giving particulars of the hazard as may be pertinent to the safety of other aircraft.
What conditions are necessary for the formation of thunderstorms?
- Deep instability, eg warm air in lower levels and cold air in upper levels.
- High moisture content.
- A trigger action to start the air rising, such as a front forcing the air aloft, mountainous terrain, or strong surface heating.
What are the hazards of a thunderstorm for aircraft?
- Severe turbulence
- Severe icing (large supercooled droplets striking a sub-zero surface)
- Hail (structural damage)
- Lightning Strikes
- Interference to radio communication and radio navigation instruments.
- Severe wind shear
- Low cloud base and reduced visibility.
What is a Line Squall?
A cold front forcing warm air up to create towering Cb over a long lateral distance.
How can temperature affect aeronautical activities? (3)
High temperature= Low density: reduces aircraft performance so longer take off run needed.
Temperature= Dew Point= Fog: probable holding and low vis procedures.
Very low temperature= Icing: affects control surfaces, de-icing needed before take off.
What are the hazards of icing?
- Adverse aerodynamic effects= loss of lift, increase in drag, unbalance of propeller.
- Piston engine degradation from carburettor icing.
- Faulty readings from instruments caused by ice blocking pitot tube/static vent.
- Windscreens frozen.
- Landing gear accretion- unable to deploy.
What should an ATCO he briefed on prior to taking over watch?
Controllers shall obtain full information regarding the weather expected during the period of their watch by the study of forecasts, charts and, if clarification is required, a briefing from the designated forecast unit.
What is a Barometer?
A barometer is used to pressure gas pressure. 2 types are the Mercury and Aneroid Barometer (the altimeter is an aneroid barometer). A barograph transfers readings onto a graph.
What is a Thermometer?
Used to measure temperature (dry bulb) or humidity (wet bulb).
If the 2 measurements are close together, this means humidity is high and the air is near saturation. In these conditions fog and low visibility are probable.
Measured by Mercury in glass- expands as temperature increases.
Housed in Stevenson Screen (shelter) 1.5m above the ground.
What are the characteristics of the Troposphere?
- Depth of 16km (55,000ft) at the equator to 8km (25,000ft) at the poles.
- Air temperature and pressure decrease with height.
- Upper limit know as the tropopause.
- Generates and contains 99% of earths weather.
