- boundary between two air masses referred to as front:
- warm front
- cold front
- stationary front
- occluded front
- Passage of a front is noted by a change in temperature and wind direction
- isobars connect points of equal pressure
- when isobars are close together (steep pressure) expect strong winds
- Common scenario with cold fronts: cloudy hazy in advance, thunderstorm along the front and clear air behind
- Frontal air masses move from high pressure to low pressure
- When flying across a front, pilots should expect a change in the wind direction
- The most obvious sign of the passage of a front is a change in temperature
- Dew point the temperature at which a volume of air becomes saturated
- Unsaturated air in a convective current cool at roughly 5.4F per 1,000 feet
- The dew point decreases at roughly 1F per 1,000 feet
- Thus, in a convective current, temperature and dew point converge at roughly 4.4F per 1,000 feet
Ex: calc the base of clouds if the surface temp is 82F and the dew point is 38F at what altitude is the base if the clouds?
82-38=44 (there is a 44 deg spread in the temp and dew point)
44/4.4 (tem/dew point convergence factor) = 10.
The base of the clouds is at 10,000 feet above the surface
- Fog is a stratus cloud that is formed at the surface
- Fog forms when the temperature and dew point are equal (the air is saturated and relative humidity is 100%)
- Frost forms when the temperature of the collecting surface us below the dew point and the dew point is below freezing
- Frost significantly reduce lift and increase drag
- Expect fog in the morning and evening
- 5 types of fog (memory aid: Fog usually seems present after rain)
- Upslope (require wind to exist)
- Steam (low level turbulence and icing can become hazardous)
- Precipitation-induced
- Advection (require wind to exist and is most likely to form when an air mass moves inland from the coast in winter)
- Radiation (frequently forms when warm, moist air exists over low areas on clear, calm nights)
- Convection = Vertical Movement of Air
- Mammatus Clouds sometimes form under weakening cumulonimbus
- Thunderstorm develop in 3 stages:
- Cumulus stage: moist air & instability presented to a lift force, it goes higher warm air spiral into the icy vortex
- Mature stage: within 15min it has powered up to the troposphere, dense air above cannot be supported by warm air, precipitation begins as cold air descent back to the atmosphere
- Dissipating stage: the air below the storm is cooled, destroying the warm air that was feeding the storm with precipitation
- Pilots must be alert for wind shear when flying in the vicinity of a storm and is especially dangerous when the aircraft is taking off or landing
- Wind shear is a significant change in wind direction or speed over a small area; the change can be vertical or horizontal
- Wind shear can occur at any altitude and in any direction
- Requirement for development of thunderstorm:
- high humidity
- lifting force
- unstable air
- Precipitation signals the beginning of the mature stage of a thunderstorm. This is when the greatest intensity can be expected
- Lightning is always associated with a thunderstorm
- Wind shear is associated with passing fronts, thunderstorms, and temperature inversions with strong upper level winds
- A nonfrontal band of thunderstorms that often develop ahead of a cold front is known as a squall line. These are generally some of the most intense storms.
- The stability of the air depends of its ability to resist vertical motion
- Instability can lead to extensive vertical clouds, significant turbulence & severe weather
- The combination of moisture and temperature determine the stability of the air
- Cool dry air is very stable and resist vertical movement lead to good & clear weather
- Warm moist air is very instable in tropical regions in summer
- Stable Air:
- Stratiform clouds
- Smooth air
- Steady precipitation
- Poor, smoggy visibility
- Unstable Air:
- Cumuliform clouds
- Turbulent air
- Showury precipitation
- Gud visibility
- Visible moisture is necessary for structural icing
- Freezing rain generally causes the highest accumulation rate of structural icing
- Ice pellets at the surface indicates a temperature inversion with freezing rain at a higher altitude
- Temperature inversions are associated with an increase in temperature with altitude (normally temperature decreases with altitude)
- The most frequent type of surface-based temperature inversion is produced by terrestrial radiation on a clear cool night
- Stable conditions usually exist under temperature inversions resulting in smooth air poor visibility fog haze or low clouds
- Pilots should expect a wind shear zone in a temperature inversion whenever the wind speed at 2,000-4,000 feet AGL is 25knot or greater