Weather - Nature of the Atmosphere Flashcards
What are general characteristics in regard to the flow of air around high and low pressure systems in the Northern Hemisphere?
LOW PRESSURE - inward, upward, and counterclockwise HIGH PRESSURE - outward, downward, and clockwise
What is a TROUGH?
An elongated area of relatively low atmospheric pressure. Low pressure is bad weather.
What is a RIDGE?
High Atmospheric Pressure.
High pressures is good weather.
What are the standard temperature and pressure values for sea level?
15 deg Celcius and 29.92” Hg
What are ISOBARS?
line on a weather chart which connects areas of equal or constant barometric pressure.
If isobars are relatively close together on a surface weather chart or a constant weather chart, what information will this provide?
The spacing defines how steep or shallow the pressure gradient is.
When isbars are close together, a steem pressure gradient exists, which indicates HIGHER WIND SPEEDS.
A shallow pressure gradient (isobars far away from each other), usually means LOWER WIND SPEEDS.
What causes the winds aloft to flow parallel to the isobars?
The Coriolis Force
Why do surface winds generally flow across the isbars at an angle?
Surface Friction.
At what rate does atmospheric pressure decrease with an incrase in altitude?
1” Hg per 1,000 feet
What does DEW POINT mean?
It is the temperature at which a sample of air must be cooled to attain a state of saturation.
When temperature and dew point are close together (within 5 degrees), what type of weather is likely?
Clouds, dew or fog.
Ideal conditions for carburetor icing.
What factor primarily determines the type of verticle extent of clouds?
The stability of the atmosphere.
How do you determine the stability of the atmosphere?
Unstable air - when temp decreases uniformly and rapidly as you climb (approching 3 deg C per 1,000 feet)
Stable Air - temp remains unchanged, or decreases only slightly with altitude.
List the effects of stable and unstable air on clouds, turbulence, precipitation and visibility.
STABLE = Stratiform Clouds, Smooth Air
UNSTABLE = Cumuliform Clouds, Rough Turbulence
At what altitude above the surface would the pilot expect the bases of CUMULIFORM CLOUDS if the surface temperature is 82 deg and the dew point is 62 deg?
Temp - Dewpoint / 4 x 1,000 = base of clouds
82 - 62 = 20
20 / 4 = 5
5 x 1,000 = 5,000 feet AGL
What is the definition of FREEZING LEVEL? and how can you determine where that level is?
Freezing level is at the lowest altitude in the atmosphere over a given location at which the air temperature reaches 0 deg Celcius.
It is possible to have mulitple freezing levels when a temperature inversion occurs above the definied freezing level.
Sources to find freezing level:
- Current Icing Products (CIP)
- Forecase Icing Products (FIP)
- Area Forecasts
- PIREPS
- AIRMETS
- SIGMETS
- Surface Analysis Charts
- Low-Level Significant Weather Charts
- Winds and Temperature Aloft
Name three types of structural icing that may occur in flight.
1) CLEAR ICE - forms after initial impact when the remaining liquid portion of the drop flows out over the aircraft surface, gradually freezing as a smooth sheet of solid ice.
2) RIME ICE - forms are when drops are small, such as those in stratified clouds or light drizzle.
3) MIXED ICE - forms when drops vary in size or when liquid drops are intermingled with snow or ice particles. The ice particles become embedded in clear ice, building a very rough accumulation.
What factors must be present for a thunderstorm to form?
- Sufficient water vapor
- Unstable lapse rate
- Initial upward boost (lifting) to start the storm process in motion.
What are three stages of a thunderstorm?
CUMULUS STAGE - updrafts cause raindrops to increase in size
MATURE STAGE - rain at earth’s surface; it falls through or immediately beside the updrafts; lightening; perhaps roll clouds.
DISSIPATING STAGE - downdrafts and rain begin to dissipate.
What is a TEMPERATURE INVERSION?
Temperature Inversion is an increase in temperature in height - a reversal of normal decrease with height.
Temperature in cold air can be critical to icing.
A ground-based inversion favors poor visibility by trapping fog, smoke and other restrictions into low levels of the atmosphere.
The air is stable, with little or no turbulence.
Two ways that fog may form:
1) Cooling air to the dew point
2) Adding moisture to the air near the ground
What causes RADIATION FOG to form?
The ground cools the adjacent air to the dew point on calm, clear nights.
This type of fog forms at night under clear skies with calm winds when heat absorbed by the earth’s surface during the day is radiated into space.
What is ADVECTION FOG? and where is it most likely to form?
Advection fog results from the transport of warm humid air over a cold surface. A pilot can expect advection fog to primarily form along coastal areas during the winter.
Unlike radiation fog, it may occur with winds, cloudy skies, over a wide geographic area, and at any time of the day or night.
What is UPSLOPE FOG?
Upslope fog forms as a result of moist, stable air being cooled adiabatically as it moves up sloping terrain. Once the upslope wind ceases, the fog dissipates.
Upslope fog is quite often dense, and extends to high altitudes.
What is a ‘Wind Shear’? and when is it most likely to occur?
Wind shear is the rate of change of wind velocity (direction and/or speed) per unit distance; conventionally expressed as vertical or horizontal wind shear.
It may occur at any level in the atmosphere, but three areas of special concern:
a) Wind shear with low-level temperature inversion
b) Wind shear in a frontal or thunderstorm
c) Clear Air Turbulence (CAT) at high levels associated with jet stream or strong circulation.
Why is wind shear an operational concern to pilots?
Because wind shear can cause unexpected changes in wind speed and direction, which can potentially be very hazardous to aircraft operations at low altitudes on approach to and departing from airports.
What types of weather information will you examine to determine if wind shear conditions might affect your flight?
- TERMINAL FORECASTS - any mention of low level wind shear (LLWS) - possibility of severe thunderstorms, heavy rain showers, hail, and wind gusts suggest the potential for LLWS and microbursts.
- METARS - inspect for any indication of thunderstorms, rain showers, or blowing dust. Additional signs of warming trends, gusty winds, cumulonimbus clouds,…
- SEVERE WEATHER REPORTS, SIGMETS AND CONVECTIVE SIGMETS - severe weather is a prime source for wind shear and microbursts
- LLWAS - (Low Level Windshear Alert System) - installed at 110 airports across the US, designed to detect wind shifts between outlying stations and a reference centerfield station.
- PIREPs - reports of sudden airspeed changes on departure or approach, and landing corridors provide a real-time indication of the presence of wind shear.
