How old is too old for weather reports
After taking off with no one else in the airplane and flying the required 150 nautical miles, including landings at two airports before returning home, you feel more like a "real" pilot. You've flown far over the horizon and back, relying only on your own skills and knowledge, including your knowledge of weather.
In fact, nothing sparks your interest in weather forecasts like scheduling your long solo cross-country. While you're likely to begin studying the forecasts a week or more in advance, you and your flight instructor won't finally decide that it's a good day to go until an hour or so before you take off. This is because a basic rule of weather forecasting is the shorter the time between when the forecast is made and the time it's for, the better it's likely to be.
By the time you're ready to go on your long solo you should be able to make a wise go/no-go weather decision. If you don't make wise decisions, federal aviation regulations give your flight instructor a veto over the flight.
Of course the instructor could decide you shouldn't go today if you're obviously not in good health, something is wrong with the airplane, or you've made a hash of preflight planning. A forecast for marginal if not downright dangerous weather, however, is more likely to delay your flight. Once you've earned your private pilot certificate, weather forecasts probably will be the main causes of postponed flights.
Since weather forecasts are such an important part of a pilot's decisions, you'll be able to make better use of them if you know a little about how they're made. The key idea of weather forecasting is seeing what the weather is doing now and then using some kind of scientific method to estimate what it will do in the future. For relatively short time periods and small distances, the forecasting method doesn't need to be sophisticated.
The simplest forecasting method is persistence. That is, in many cases forecasters can assume that the weather will continue doing what it's doing now for some time in the future. During late summer and early fall in much of the United States, when large weather systems tend to move slowly, persistence can work pretty well for maybe the rest of a day that started with light winds and only a few puffy clouds in the sky.
Another method that can work for periods of a few hours is to assume that a weather system that is going to affect you will continue to move at the same speed as it has been moving.
During the winter, the weather system you're watching is likely to be a large extra-tropical storm, which might be spreading snow across the Plains toward the Ohio Valley, possibly aiming at the Northeast. Depending on the circumstances, expecting the system to stay on course with no big changes in intensity might work for a day or so, but you wouldn't want to plan a flight based on nothing but this expectation.
In spring and summer, thunderstorms become the major aviation weather hazard. Once thunderstorms form, either as individual storms or in a line or cluster, forecasters track them with Doppler weather radar, satellites, the U.S. National Lightning Detection Network, and other observations. Tracking the storms enables forecasters to use the direction of movement and data on upper-altitude winds to forecast where storms are likely to hit in the next hour or so. But observations alone won't tell forecasters when thunderstorms will begin to strengthen or weaken.
If you watch a local television station when severe thunderstorms threaten the station's area, you might see the station's weathercaster point to a red blob on the radar image and say something like: "This very dangerous storm will be moving into the Smithville area in the next 10 to 15 minutes. If you live there, take shelter in your basement or somewhere inside and away from windows."
Imagine that you're on your long cross-country flight and see what looks like a line of thunderstorms in the distance off to your right. You radio Flight Watch on the frequency 122.0 MHz, identify yourself, say where you are and where you're heading, and ask about the cloud tops you see.
You hear the clouds are a line of thunderstorms with some of them growing to severe levels. Continuing on your current heading will bring you within five or 10 miles of the line. The forecast estimate of where the storms are heading helps to ensure that when the thunderstorms cross your original flight path, you're safely on the ground.
Using persistence or assuming that the current system will continue doing the same thing for at least a while can carry you only so far, however.
Numerical forecasting, developed in the 1950s, now dominates weather prediction. This method uses computers to compile as much data as possible about the current state of the atmosphere, solve the equations that describe the atmosphere's changes and motions, and predict what will happen at various times in the future. The physical properties the equations use include, but are far from limited to, the forces of air pressure differences, the Earth's rotation, and other forces pushing the air in various way to create wind, temperature differences, and the air's buoyancy that cause the air to move up and down; heat added to the air or taken from it as water changes phases among ice, liquid, and vapor; the sun's heating of the Earth; heat radiating away from the ground, and so on.
All of this is done in the framework of a grid. To picture it, imagine the frame of a skyscraper. Each point where vertical and horizontal beams meet is a grid point.
Information from a wide variety of weather observations is supplied to the model for each grid point and the computer solves the equations to see how the atmosphere will change over a short period of time, say five minutes. Then the information about what happens at each grid is fed into the adjoining grids. For instance, if calculations show warm air will rise at one point, data on how much air is rising and its temperature is supplied to the point above.
The equations are solved for each grid point for the next time step and on and on and on until you reach a time period you're interested in, say an hour from when the model started. The computer system produces maps and text describing various aspects of the weather at that time and keeps on calculating.
In the United States the National Weather Service's National Centers for Environmental Prediction (NCEP) headquartered in Camp Springs, Maryland, runs the computer models that all forecasters use.
The NCEP models are used by local forecasters, including television meteorologists, local National Weather Service offices that prepare terminal forecasts, and the NWS Aviation Weather Center, which produces more general forecasts such as the area forecasts for aviation.
When you obtain a weather briefing for a flight, one of the major sources of information is likely to be the Rapid Update Cycle (RUC), which is updated each hour with forecasts for one, two, three, six, nine and 12 hours ahead. It's designed for aviation and forecasters who are concentrating on thunderstorms.
Figure 1 (see p. 41) is a three-hour RUC prediction of surface air pressure and winds, and Figure 2 is the RUC three-hour prediction of the water in clouds below 6,000 feet.
In Figure 1, the black lines are isobars, lines of equal air pressure. You can see Ls over the Dakotas and Missouri marking predicted low-pressure centers and an H over Idaho for a high-pressure center. The blue shading shows where surface winds are forecast to be between 20 and 30 knots.
In Figure 2, the yellow and green colors represent the amounts of water in clouds with the darker green representing the most. The stripes show where the water is forecast to be supercooled; that is, below 32 degrees Fahrenheit, but still liquid. Such water freezes when it hits an airplane, which means dangerous aircraft icing is possible in those clouds.
You can access the outputs from the various forecasting models through the National Center for Atmospheric Research Web site (www.rap.ucar.edu/weather/model/).
The latest version of the RUC model uses grid points that are 13 kilometers (about eight miles) apart with 50 levels of the atmosphere. You can see why running weather forecasting models requires some of the world's most powerful computers to solve billions of equations by doing trillions of calculations each second.
Still, anyone trying to forecast the weather faces some formidable obstacles, the major one being the atmosphere's chaos. The atmosphere is a complicated system with several variables affecting each other in different ways. This means that any computer program designed to forecast the weather will have what atmospheric scientists call "a sensitive dependence on initial conditions." That is, little changes now can lead to big changes in the future.
While the RUC model has one of the tightest grids of any model used to forecast the weather for areas as large as individual states, the grid points are still miles apart-- while some of the important processes, such as water vapor condensing, happens on the molecular level.
Even if complete weather observations could be taken at every point of the model's grid--impossible with any current technology--important things could be going on between the grid points. For example, a small thunderstorm could be occurring in the middle of one of the RUC's eight- by eight-mile grids.
This is why the weather--especially small but powerful phenomena like thunderstorms--can surprise forecasters. Most of the time you can expect the forecast you obtain an hour or less before taking off to be very accurate about the general picture for a two- or three-hour flight, and pretty accurate about the small details.
Still, as you cruise along on your long solo cross-country, enjoy the view. Keeping an eye on the ground will help to ensure you don't get lost. Keeping an eye on the sky will help ensure the weather doesn't spring any surprises on you. And if you do see anything, such as growing clouds on the horizon, turn your radio to 122.0 MHz, key the microphone, and ask Flight Watch about the weather. You'll be exercising the good judgment that pilots are expected to exhibit.
Jack Williams is coordinator of public outreach for the American Meteorological Society. An instrument-rated private pilot, he is the author of The USA Today Weather Book and The Complete Idiot's Guide to the Arctic and Antarctic, and co-author with Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.
Related Articles
