Weather Information

Clouds: PHAK 12-15

High clouds (above 20,000’ AGL)

Cirrus Clouds: Stable air at high altitudes of 30,000 feet. Thin, white and wispy. Sign of approaching bad weather.
Cirrostratus Clouds: Thin, white and long. Low moisture poses no icing hazard.
Cirrocumulus Clouds: White and patchy. Shallow convective currents at high altitudes produce light turbulence.

Middle clouds (above 6,500 ' AGL)

Altostratus Clouds
Altocumulus Clouds

Low clouds (below 6,500' AGL)

Stratus
Stratocumulus
Nimbostratus

Vertical Development

Cumulus Clouds: Turbulent but little icing and precipitation. Convective currents from the heating of the earth’s surface. Shallow layer of instability.
Towering Cumulus: Indicate deep area of unstable air. Moderate to heavy turbulence with icing and thunderstorms.
Cumulonimbus Clouds: Thunderstorms, large amounts of moisture.

Pressure Systems: PHAK 12-7

High-pressure systems are generally areas of dry, descending air. Good weather is typically associated with high-pressure systems for this reason.

Conversely, air flows into a low-pressure area to replace rising air. This air usually brings increasing cloudiness and precipitation. Thus, bad weather is commonly associated with areas of low pressure.

Flight Categories: AIM 7-1-7

Special VFR: FAR 91.157

Only at controlled airports
Visibility: 1 statute mile
Cloud Clearance: clear of clouds
ATC Clearance: Required to obtain. Must be requested and granted.
Night Operations: Must hold an instrument rating and be in an IFR equipped aircraft.

Frontal Weather

Air Mass: A large body of air with similar characteristics of the surrounding area.
Front: The boundary layer between two types of air masses.

Approaching Front: a change in weather.
Cold Front: Fast moving (25-30 mph), displaces warm air rapidly. Cumuliform clouds, showery precipitation, unstable air, good visibility.
Warm Front: Slow moving (15-20 mph), overtakes cool air gradually. Stratiform clouds, steady precipitation, stable air, poor to fair visibility.
Stationary Front: Little to no movement between opposing air masses of similar characteristics. Extended precipitation, low clouds, fog.
Occluded Front: Cold front overtakes a warm front. Combination of warm and cold front weather, often severe.
Trough: Elongated area of low pressure, typically slow moving. Cloud development, showers or storms depending on moisture gradient.
Ridge: Elongated are of high pressure, typically slow moving. Clear skies, calm conditions, stable air, light winds.
Squall Line: Line of thunderstorms ahead of a cold front, rapidly moving. Severe thunderstorms, wind shear, heavy rain.
Tropical System: Cyclonic system (hurricane or tropical storm. Heavy rain, wind, thunderstorms, low ceilings, poor visibility.
Dry Line: Boundary between moist and dry air, sharp moisture gradient. Thunderstorms due to instability, especially in dry regions.

Thunderstorms

Three conditions to form:

Moisture
Rising unstable air
Lifting action

Three stages:

Cumulus
Mature (the most dangerous because of a combination of rising and falling air, you know it is in this phase when rain hits the ground)
Dissipating

Avoidance: Stay clear of thunderstorms by 20 nautical miles because of hail/precipitation discharge.
Dangers: Turbulence, possible icing, wind shear, or hail.

Air Mass Stability

Wind Shear: PHAK 12-8, 11

Wind Gusts: brief increases in wind speed, typically lasting a few seconds, and are reported as a range (e.g., 12 gusting 18 knots). They can cause rapid changes in lift during critical phases of flight, requiring careful control adjustments, especially during takeoff and landing.

Wind Shear: a sudden change in wind speed and/or direction over a short distance, either vertically or horizontally. It can occur at any altitude and is especially hazardous during takeoff and landing because it can cause abrupt airspeed and performance changes.

Microbursts: the most severe type of low-level wind shear, associated with convective precipitation into dry air at cloud base. Typically 1-2 miles in diameter and a nominal depth of 1,000', usually lasting 5-15 minutes and can produce downdrafts up to 6,000' per minute and headwind losses of 30-90 kts that can seriously degrade performance.

Temperature Inversions: PHAK 12-13

When the temperature of the air rises with altitude. Commonly shallow layers of smooth, stable air close to the ground. Ice pellets are an indication of a temperature inversion and that freezing rain exists at a higher altitude.

Fog: PHAK 12-15

Dew point: the temperature at which air becomes saturated and cannot hold more moisture making the moisture condense and become visible in the form of fog.
Six types of fog:

Radiation Fog: Clear, calm nights
Advection Fog: Warm and moist air moves over a cold surface. Wind up to 15 kts. Common in coastal areas.
Upslope Fog: Moist stable air forced up sloping land features.
Steam Fog: Cold and dry air moves over warm water. Common over bodies of water during coldest times of the year. Low level turbulence and icing are common.
Ice Fog: Comprised of supercooled water droplets which normally deposit rime ice on contact with an object with a surface temperature of less than 0°C.
Frontal Fog: Forms during the transition of warm and cold fronts.

Ice

Induction, Structural, Instrument
Visible moisture is necessary for structural icing.
Freezing rain gives the highest rate of accumulation.
Temperature of aircraft surface must be 0ºC or less.
Effects: Reduces thrust and lift. Drag and weight increased.
Three types of ice:

Rime:
1. Stratus clouds
2. Tiny, supercooled water droplets
3. Trapped air gives an opaque appearance
4. Changes shape of airfoil destroying lift
5. Develops on the leading edge
6. -15ºC to -20ºC
Clear
1. Cumulus clouds or freezing rain under warm font inversions
2. Large, supercooled water droplets
3. Flow over the surface freezing slowly
4. Glaze the surface
5. Most hazardous
6. 0ºC to -10ºC
Mixed
1. Combination of Rime and Clear
2. -10ºC to -15ºC

Weather Briefings: AIM 7-1-4

Standard
- Adverse conditions
- VFR flight is/not recommended
- Synopsis
- Current conditions
- En route forecast
- Destination forecast
- Forecast winds and temperature aloft
- NOTAMS
- ATC delays
- Other information
Abbreviated: Shortened version of the standard briefing.
- Requested when the departure is delayed
- When weather information is needed to update the previous briefing
Outlook: Requested when flight is six or more hours away.
- Provides initial forecast information limited in scope due to the time frame of the planned flight
Inflight: Given within the FSS area of responsibility.

Weather Charts

Winds and Temperatures Aloft: Forecast of wind direction, wind speed, and temperature. No winds are reported within 1500' AGL and no temperatures reported with 2,500' of the station.
- Light and variable winds are represented as '9900'.
- If the wind speed is forecasted to be from 100 to 199 kt., the forecaster will add 50 to the wind direction and subtract 100 from the wind speed. To decode, you must subtract 50 from the wind direction and add 100 to the wind speed.
- Above FL240, temperatures are assumed to be negative.
  - Example: STL 2113 2325+07 2332+02 2339---04 2356---16 2373---27 239440 730649 731960
    - 730649 = 230º @ 106 Kts -49ºC
    - 731960 =230º @ 119 Kts -60ºC
Low-Level SigWx: Forecasts significant weather including turbulence, IFR conditions, and freezing levels from the surface to FL240.
Radar Summary Chart: Displays the location, type, intensity, and movement of precipitation based on radar returns. Does not show cloud cover or ceilings.
Surface Analysis Chart: Analyzes surface pressure systems, frontal boundaries, isobars, temperature, dew point, and wind. Used to identify pressure trends and frontal systems.

Weather Reports

METAR: Routine weather report, issued hourly, valid at time of issuance.
TAF: Terminal aerodrome forecast, issued four times daily, valid for 24-30 hours. Types include TEMPO, FM. PROB, and BECMG.
- TEMPO: a temporary change (>50% chance)
- FM: rapid change lasting at least one hour
- BECMG: gradual change lasting at least two hours
- PROB: <50% chance of happening

Note: TAF's are valid for 24 hours at most airports and 30 hours at large international domiciles because of the departure and arrival times for long haul flights.

PIREP: Pilot report, issued as received, valid as observed.
AIRMET: Airmen's Meteorological Report
- AIRMET Sierra: IFR conditions and mountain obscuration, issued every six hours, valid for six hours.
- AIRMET Tango: Moderate turbulence and low level wind shear or surface winds grater than 30 kts, issued every six hours, valid for six hours.
- AIRMET Zulu: Moderate icing and freezing levels, issued every six hours, valid for six hours.
SIGMET: Significant Meteorological Information. For severe turbulence or icing not related to convective activity, volcanic ash, dust storms, or tropical cyclones, issued as needed, valid for four hours (six for hurricanes).
- - A.C.I.D.

Volcanic Ash

Clear Air Turbulence

Severe Icing

Dust/Sandstorm (lowering visibility to less than 3 sm)

Convective SIGMET: Severe or embedded thunderstorms, lines of thunderstorms or significant convective activity greater than 50 miles, hail greater than 3/4", issued hourly, valid for two hours.
- E.A.T.S.H.T.

Embedded Thunderstorms

A Line of Thunderstorms

Thunderstorms with 40% precipitation of 3,000 square miles

Surface Winds > 50 Kts

Hail 3/4"

Tornado