Weather Flashcards
What is Windshear?
Windshear is a sudden change in either wind speed or direction, or both, over a relatively short distance
Classifications of clouds
Cirrus - high clouds
Nimbus - clouds giving off precipitation
Alto - medium height (7,000 - 25,000)
Lows vs Highs (Depressions vs Anticyclones)
Lows:
- air circulates counter-clockwise around area of low pressure in the N. Hemisphere (opposite in S. Hemisphere)
- move quickly
- large cloud cover
- precipitation
Highs:
- air circulates clockwise around areas of high pressure in the N. Hemisphere (opposite in S. Hemisphere)
- moves slowly
- little cloud cover
- no precipitation
- generally good weather
Ridge vs Trough
Ridge = elongated area of low pressure
Trough = elongated area of high pressure
Types of Fog
- Radiation Fog
- Advection Fog
- Steam Fog
- Frontal Fog
- Upslope/Hill Fog
Types of Frontal systems
- Fronts are boundary’s between two or more air masses
- all front are different
- an approach of front of any type means a change in weather is imminent
- classified by the air mass that is overtaking
- cold front (typically move faster than warm fronts; 25 - 30+ mph)
- warm front (typically slow moving at 10 - 25 mph)
- stationary front
- occluded front (typically travel at the speed of the cold front)
Conditions necessary for a Thunderstorm
- unstable air
- relatively high moisture content
- lifting action
Thunderstorm Life Cycle
- Growth stage (cumulus stage): updrafts 3000fpm
- mature stage: downdrafts 2500fpm plus downdrafts 6000fpm
- dissipating stage: downdrafts only
Types of Icing
- Rime Ice
- Clear Ice
- Mixed Ice
- Freezing Rain
- Snow
What is required for ice formation on an aircraft surface?
- moisture must be present
- aircraft surface must be below 0°C
Effects of icing
- changes aerodynamics of an airfoil
- adds weight
- reduces lift (up to 30%)
- increases drag (up to 40%)
- loss of performance
- Radio interference
Types of Turbulence
- Low-Level (below 15,000 MSL)
- mechanical turbulence (created by buildings and rough terrain)
- Convective/Thermal turbulence (created by surface heating)
- Frontal turbulence (occur ahead of fast moving cold fronts)
- wake turbulence (caused by wingtip vortices)
- Clear Air Turbulence (usually above 15,000’ MSL, but can take place at any altitude)
- caused by air masses moving over each other at different speeds
- Mountain Wave Turbulence (stable air crossing mountain barrier)
- Wake Turbulence
Contaminated runway
Contaminated Runway:
More than 25% of the runway surface is contaminated with:
> 3mm. surface water, slush or loose snow
Snow that is compressed into a solid mass
Ice or wet snow
What defines Severe Windshear
Airspeed changes greater than 15 knts and vertical speed changes greater than 500 fpm
Common sources if windshear
Thunderstorms, temperature inversions, jet stream winds
Define a microburst
Small downburst about 1 mile diameter with outbursts of damaging winds extending 2.5 miles or less
- downdrafts as strong as 6,000 fpm and horizontal winds near the ground as strong as 45 knts
Cold front vs warm front
Cold Front: faster moving than a warm front with increasing pressure. Colder air overtaking and replacing warmer air. Fast moving cold front has showers and thunderstorms. Slow moving cold front has embedded CBs and TSs
Warm Front: tend to move at half the speed of a cold front with a decrease in pressure. Warmer air replacing colder air. Stratiform clouds and widespread precipitation
Stable vs unstable air
Stable: stratiform clouds and fog, continuous precipitation, fair to poor visibility
Unstable: cumuliform clouds, showery precipitation, good visibility
Low-level WINDSHEAR
Identified by wind shear events within 1000’ of the ground
Jet stream
- A narrow band of high speed wind that meanders west to East around the globe at altitudes near the tropopause
- speeds from 50 to over 150 knts
- lower and weaker in the summer
- higher and stronger in the winter
Causes of CAT
CAT = clear air turbulence
- jet stream…especially flying perpendicular; and in areas where the jet stream curves around surface lows
- mountain waves … caused by stable air being moved up the side of a mountain (frequently found east of the Rockies
CAVOK meaning
At least 10km of visibility and no clouds below 5000’
Weather associated with a Low pressure system
- air flows counter clockwise and toward Low center (northern hemisphere)
- rising air
- cloud formation
- continuous precipitation with possibilities of Thunderstorms
- milt temps
- moderate to strong winds
- low vis under precipitation…good vis outside of precipitation
Weather associated with High pressure system
- air flows clockwise and outward from High center (Northern Hemisphere)
- descending air
- generally good weather
- low winds
- no clouds or precipitation
- hazy conditions in the summer
- warm temp in the summer and cold temps in the winter
Forecast area of a TAF
5NM radius of the airport
VC = vicinity = 5-10 NM from airport
When are Mid and High Level Prog Charts issued
4 times daily:
0200Z
0800Z
1400Z
2000Z
Weather Radar Tilt Guidlines
- 4º up for take off
- aim for 0º at FL250
- +/- 1º per 5,000’ above or below FL250
