National Weather Service
1. Convective Outlooks (AC) Use connect-the-dots method to define area.
2. Severe Weather Outlook Charts (graphic of AC) also called Convective Outlook charts use slgt, mdt, high to describe area weather. The charts connect the dots.
3. Radar Summary charts (snapshots of past weather and rain clouds can extend much farther.)
4. SIGMETs (WS) Severe turbulence, icing, wind shear when first detected and then 55 minutes after every hour. Uses connect-the-dots method to define area.
5. Convective SIGMETs (WSTs) Significant meteorological information of severe, embedded, and lines or 40% coverage of Level 4
6. Center Weather Advisories (CWAs) ATC frequency will give advance warning of severe weather.
7. Radar Reports (Rareps)locates weather by direction and distance 060/075; NC =no change; MT=maximum tops; +/+= strong storms increasing intensity; LN=line; A=area; C=cell;
8. Terminal Aerodrome Forecasts (TAFs) Airport forecasts every 8-hours mention adverse weather and thunderstorms
9. Area forecasts (FAs)
10. Hourly Surface Observations (METARs) from airports with weather observers who mention storm activity.
11. Thunderstorm Forecasts
12. Lifted Index is measure of temperature at altitude; K index is measure of moisture and saturation. As a fraction the lower the fraction the greater likelihood of severe weather.
13. Composite Moisture and Stability Chart
14. Severe Weather Watch Bulletins (
15. PIREPs
Item on Weather
New weather site identifier codes are now in effect. Many remain the same.
Checking the AWC Web site now counts as a legal weather briefing according to FAA
39 U.S. Towers are are getting what it takes to predict local weather
Getting the Weather
As a pilot you want first of all to get an overview of the big picture. Begin with the upper-air forecast charts such as the 500-millibar chart, which is about halfway up the inch-square air column or 18,000 feet. This chart shows the various pressure centers, troughs, ridges and winds. By becoming familiar with these charts and the accompanying weather the pilot can begin to anticipate and interpret weather.
Once you see the relationship of the 500-millibar chart and surface weather you are better able to understand and orient yourself to weather hazards. Additional information comes available through the infrared and water vapor. These visual images show the details of visible weather systems. Infrared works at night as well as day. Interpretation requires practice. Only by knowing where a storm is expected to go can you even make preliminary flight plans. Doppler systems (Nextrad) is giving us a forward step in this weather process.
Becoming Weather Wise
For a beginning a pilot should make a practice of watching the evening TV weather and the early morning weather channel. Next, by using DUAT or calling a Flight Service Station (FSS) available nation wide at 1-800-WX-BRIEF you can obtain a briefing. These can help you meet the FAR 91.l03 preflight requirement of having all available information as part of the preflight. When below freezing level temperatures are encountered an important part of the preflight is to assure that the oil breather tube is not ice-clogged. Additionally, when cold weather arrives, the grease in the throttle wire cable can become so congealed that movement is restricted. Do not takeoff in an aircraft where throttle movement is restricted in this manner. Climbing into still colder weather may prevent any movement.
As a minimum a pilot needs to know, for local flights, airport conditions, fog prediction, freezing level, turbulence, local notams, and winds at 3000. Weather briefings are rapidly becoming less available except through computerized access by the pilot.
Radar returns are based only on water. Wet is not always convective. Water is not a thunderstorm until it includes vertical development and Vertically developed clouds can contain severe turbulence and icing. Low clouds with flat tops usually do not have turbulence or icing. Virga is indicative of evaporation cooling of the air, which accelerates downward as microbursts. Virga is most dangerous when surface temperature is warm, winds are light, and dew point spread exceeds 35 degrees.
Static Electricity
Static electricity is a common phenomenon. It is not dangerous unless it builds enough to produce a spark. It is invisible and silent medium that causes many aircraft fires. Susceptibility to static electricity exists when fuel is being
poured between plane and any ungrounded to plane fueling source. Plastic fuel cans and plastic funnels are the most common sources. Even carrying plastic fuel cans in plastic truck beds is a potential five hazard. Do not drain fuel into plastic buckets.
Making Weather
Atmosphere has three levels. Troposphere extends from sea level up to 35,000 feet around the poles and to 65,000 feet at the equator. The standard temperature lapse rate of the troposphere is 2-degrees C per 1000 feet of altitude. On top of the troposphere is a thin layer called the tropopause, which acts as a moisture barrier. The top of the troposphere is the bottom of the atmosphere. The 850-millibar air pressure level will be lower in true altitude than warmer air. All air pressure is the measure of the weight of air above. The stratosphere is next extending to 22 miles high and because of the tropopause barrier has no moisture and a constant temperature of -55-degrees C. 50% of the world's atmospheric weight is below 18,000 feet.
Consider a rotating spherical envelope. A mixture of gases-occasionally murky and always somewhat viscous. Place it around an astronomical object nearly 8000 miles in diameter. Tilt the whole system back and forth with respect to its source of heat and light. Freeze it at the poles of its axis of rotation and intensely heat it in the middle. Cover most of the surface of the sphere with a liquid that continually feeds moisture into the atmosphere. Subject the whole to tidal forces induced by the sun and a captive satellite. Then try to predict the conditions of one small portion of that atmosphere for a period of one to several days in advance.
A pilot must learn to be skeptical about what the weather is for now, what it is forecast to be, and what he hopes it to become. The entire process is so capricious that even in 1997 the gigabite computers can't keep up.
All of the factors in the foregoing paragraph are factors in making weather but only three main factors, heat, water, and wind are of major importance. There are three kinds of heat transfer in the atmosphere:
--Convection is that which causes thunderstorms.
--Radiation is that which causes ground fog
--Conduction is that which causes you to drop a nail held by the fingers when you are heating the other end.
Weather Stability:
The presence of moisture in the air is the determining factor as to whether a given body of air will become stable or unstable. Colder air is heavier than warm air. Cold air will not rise unless it is surrounded by even colder air. It is the relative temperatures of neighboring air bodies that determine whether air will rise or fall.
When the air contains water it is conditionally unstable and will remain in an unsaturated state until it reaches near the dew point. At or near the dew point it will become visible moisture (cloud). Any uplifting of this conditionally unstable air will eventually cause it to condense, become saturated (visible) and unstable. Any lifting of unstable air will cause an acceleration of the lifting action.
The moist air does not cool as much as dry air. This means that any uplifting is more likely to increase the temperature difference between the warmer moist air and the surrounding air. The rising acceleration of a moist air column increases as the temperature difference increases. This increasing acceleration is a characteristic of unstable air.
A dry air column cools at a standard environmental rate of 5.5 degrees Fahrenheit for every thousand feet rise in elevation. Without moisture it will cool and tend to descend or level off at the same level as the surrounding air. Stability or instability of any air column is determined by its relative temperature lapse rate to that of its surrounding air.
Atmosphere can be either stable or unstable. Any uplift of stable air will result in its return to its original level. Any uplift of unstable (warmer) air will cause it to expand and cool but at a lower rate than the surrounding air. This air mass will continue and accelerate if the air is either relatively warm, moist or both. The earth's surface and its immediate air are heated unevenly. Turbulence is caused when warmer air moves up through cooler air.
The standard environmental lapse rate of 3.5 F or 2 C per 1000 feet is not realistic. We can determine approximate lapse rates by taking thermometer reading every thousand feet. The likelihood of any flight having this standard is slim. We can determine the approximate freezing level above a known elevation by deducting 2-degrees Celsius from the initial Celsius altitude/temperature for every thousand feet, until reaching zero.
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