Weather

Before taking to the air, pilots need to know what the weather is likely to be during the flight. Even for an hour in the traffic pattern doing touch and goes, more than just a quick look at the sky around the airport is needed.

For instance, you need to learn whether a thunderstorm, out of sight over the horizon, could send a gust front toward the airport with powerful, shifting winds. You need two kinds of weather information before flying: observations of what’s happening now, and forecasts of what meteorologists expect to happen along your route during the flight.

Radar: what’s happening now

The best way to get a quick view of what’s happening now just about anywhere in the United States is to look at the Nexrad radar image that includes your airport. Nexrad is the system of Doppler weather radars operated by the National Weather Service (NWS) and the U.S. armed forces. They can be seen on any computer with a browser and an Internet connection. To find the nearest radar, see the NWS stations website (www.weather.gov/organization.php); each weather station has links to Nexrad radars in its area. When you call up a radar image you can click on the “Loop” function to see the movement of weather for the last hour or so.

Areas of blue and dark green show relatively light rain. Yellow, orange, and red show heavier precipitation typical of thunderstorms. Red indicates the heaviest precipitation. Weather radar doesn’t answer all of a pilot’s weather questions since it only shows precipitation. Observations from surface stations will tell you whether fog or clouds with poor visibility or low ceilings are occurring where you plan to fly.

Where forecasts come from. Almost all of the forecasts for pilots (and everyone else) in the United States begin in the computers at the National Weather Service’s Hydrometeorological Prediction Center in suburban Washington, D.C. Its supercomputers run various models that apply the laws of physics to weather data collected from the surface to the top of the atmosphere around the globe to create forecasts for times ranging from an hour in the future to more than 10 days out.

Although forecasts have greatly improved in recent years, any forecasting computer program struggles with what scientists call “a sensitive dependence on initial conditions.” That is, little changes now can lead to big changes in the future. This has two important consequences: The older a forecast, the more likely it is to go wrong; and all weather forecasts contain some amount of uncertainty.

These limitations mean that if you received a weather briefing more than an hour or so before a flight, you should update it shortly before departure. An easy way to do this is to call a flight service station at 800-WX-BRIEF and ask for an update briefing.

If during your flight you see that the weather doesn’t appear as forecast, call Flight Watch on 122.0 MHz and find out what's going on. Also, your pilot report of the conditions you're experiencing can help weather forecasters to improve the accuracy of their products.

Probabilities in forecasts. Weather forecasts for particular airports are called terminal aerodrome forecasts, or TAFs. These forecasts indicate uncertainty in a couple of ways. Information that follows “TEMPO” (for temporary), with a beginning and ending time, tells you that sometime during this time period, such as a low ceiling, will occur. But the forecasters don’t say exactly when it will happen.

When you see the term “PROB40” (meaning probability 40 percent), the indicated weather, such as thunderstorms, has a 30- to 50-percent probability of occuring during the indicated time period.

thunderstorm Probabilities. The NWS Aviation Weather Center produces a Collaborative Convective Forecast Product in collaboration with meteorologists at other AMS offices, especially those who work at FAA Air Route Traffic Control Centers. Intended for those responsible for air traffic management, it can offer a second opinion of thunderstorm forecasts and alerts.

This map shows probabilities and the forecasters’ confidence in their predictions. Areas in blue show that the forecasters have high confidence (50 to 100 percent). Areas in gray show low confidence (24 to 49 percent).

Blue or gray hatching shows that thunderstorms are expected to be sparse, covering only 25 to 39 percent of the area. If the hatching is close together, such as in the small blue area over parts of North and South Carolina (above), thunderstorms are expected to have medium coverage (40 to 74 percent). Solid colors indicate solid coverage (75 to 100 percent) of the area.

Add two zeros to the numbers on the map to get the forecast tops of thunderstorms in different locations. For example, the thunderstorms in the Gulf Coast states are expected to be as high as 40,000 feet. The higher the top of a thunderstorm, the stronger its winds are likely to be.

The NWS Aviation Weather Center’s website is a good source for some of these and other weather observations and forecasts (http://aviationweather.gov/).