Aviation Weather Basics
What weather means to pilots
Even novice pilots understand that weather affects a plane in flight very differently than it affects a car driving down the freeway. Although weather conditions can compromise your safety both in the air and on the road, the stakes are higher in flight because you can't pull over to the shoulder if the severity of weather exceeds your abilities.
To better understand the forces that weather exerts on planes, it's important to realize that the atmosphere is not empty space. Heat from the sun and friction from the earth's surface act upon the gases and water in the air, creating a variety of weather conditions.
A number of factors—wind, cloud layers, temperature, dew point, barometric pressure, and visibility—affect your Flight Simulator experience. You've probably heard your local weather forecaster use these terms, but as a pilot, it's important that you understand their precise meanings.
Wind is one of the basic weather factors affecting flight. Depending on its severity, wind can be a nuisance—pushing you off course by affecting your heading, airspeed, or altitude—or a true hazard, creating dangerous flight conditions. Wind can also make takeoffs and landings more challenging if it's not aligned with the active runway. And, although it seems basic, remember that if you're flying into the wind, you're going to fly slower; flying with the wind will move you along faster.
Friction on the ground acts as a kind of brake on the wind. Weather systems don't move in a straight line and often rotate, so wind in one area may not be blowing in the same direction as wind in another area nearby. Winds can also change quickly across a weather front. Because there can be dramatic differences between the direction and velocity of the wind on the ground and at high altitude, aviation weather reports include information on both surface winds and winds aloft.
Wind shear refers to situations when the wind direction and speed are shifting rapidly over a short distance. Encountering wind shear on final approach to landing can add unwelcome excitement to your flight. If the wind shifts dramatically from a headwind to a tailwind, for example, you may experience severe downdrafts. Because a plane's airspeed is already relatively low on approach to landing, wind shear can create a dangerous situation.
You also need to exercise care when taking off or landing in crosswinds, and you'll need to adjust your navigation if wind coming from one side blows you off track. If you're flying into a headwind, you'll need to keep fuel consumption in mind so that your plane doesn't run out of fuel before reaching your destination.
Knowing the current conditions of winds aloft can help you plan your route and altitude to either take advantage of a tailwind or to avoid the worst of a headwind. Let's say the wind at an altitude of 15,000 feet is blowing 360 degrees at 15 knots and the wind at 25,000 feet is blowing 320 degrees at 30 knots. Assume for this example that your course for the flight is 340 degrees.
Assuming further that the aircraft you choose to fly performs well at either altitude, you might want to plan your flight for 25,000 feet, since you'll get a speed boost from a quartering tailwind (a quartering wind comes at you from 45 degrees off your tail or nose). If your course was 160 degrees, you might plan to fly at 15,000 feet to minimize the effect of a quartering headwind.
When using the Real-world weather or Weather themes options in Flight Simulator's Weather dialog box (an option within the Create a Flight dialog box), surface winds and winds aloft will be set for you. You can set surface winds and winds aloft yourself using the User-defined weather option.
Quiz: If you're taking off from runway 36 at heading 360 degrees and you want to practice a crosswind takeoff, you would set the wind in Flight Simulator to:
Answer: B would give you a wind blowing directly off your right wing. D would give you a wind blowing from behind your left wing.
To learn how to set wind conditions, see User-defined Weather.
Even to the most casual observer, scattered, puffy clouds are obviously different than solid overcast conditions. But there's more to it than simple scenery. Clouds vary in appearance due to the different atmospheric conditions that cause them to form. Understanding the conditions that create certain cloud types helps pilots determine what conditions they may encounter up ahead. Conversely, researching weather conditions ahead of time can give pilots an idea of what kinds of cloud layers to expect during flight.
Because the air in which stratus clouds form is relatively stable, stratus layers have a flat, even appearance and cover much of the sky.
Although you will likely enjoy a smooth ride in these conditions, visibility is often less clear when the air is stable. Smoke, smog, and moisture can stay suspended in stable air, reducing visibility. (You can use the Visibility setting to determine the visibility distance; the type of cloud formations alone will not affect visibility.).
Stratus clouds can be high and transparent, or low and dark gray. The lowest layers of stratus, nimbostratus clouds, are rain clouds. Higher, less-dense stratus layers can produce rain, but it tends to be more showery than the drizzle produced by low stratus layers.
Cumulus clouds are the puffy, pretty ones.
They aren't, however, always benign. Cumulus clouds form in unstable air, which allows them to have greater vertical development than stratus clouds. Most frequently they have flat bases and lumpy tops, and form below 6,000 feet (1,828 meters). The more unstable the atmosphere, the higher cumulus can build.
Cumulonimbus clouds can tower to greater than 50,000 feet (15,240 meters). Cumulonimbus are the clouds that make thunderstorms.
Unstable atmosphere with rapidly rising masses of air creates the conditions necessary for thunder and lightning. Pilots should avoid these hazards whenever possible.
Lightning strikes are actually not the most dangerous aspect of thunderstorms. Severe updrafts in a thunderstorm can overstress an aircraft's airframe, and violent hailstorms often associated with thunderclouds can cause damage to an aircraft's skin and windshield.
Thunderstorms can line up along a weather front, creating a squall line. Aircraft must navigate around squall lines, sometimes taking them hundreds of miles off their intended course. Flight Simulator will create squall lines if they currently exist when you click on the Real-world weather option in the Weather dialog box. If you click on the Weather themes option, you can also select the Squall Line theme. If you want to create your own squall lines, you'll have to create thunderstorms along a line of weather stations by clicking on the Customize button under the User-defined weather option.
Cirrus clouds are characterized by a wispy appearance and very high altitude. Cirrus are composed of ice crystals. Aircraft don't often fly at the extreme altitudes at which the cirrus reside.
Quiz: On a flight from Dublin to Paris, how would you create conditions so that you'd have to take off into a low cloud layer, navigate around thunderstorms near London, and break out into the clear over Paris?
Answer: You could set a low stratus layer at Dublin, create thunderstorms over London, and then set cumulus clouds at 3/8 coverage at Paris. Once you'd set these conditions for those stations, you could select another type of weather and apply it to all other weather stations in the world. That way, in between the custom conditions you created, you'd still have interesting weather to fly in.
Cold and Warm Fronts
Cold fronts aren't necessarily cold. It's actually just the boundary between a mass of cold air and a mass of warm air in conditions where the cold air is moving in to replace the warm air. In the summer, the "cold" air is just cooler than the warm air and may not seem cold to you at all. Warm fronts are just the opposite; a warm air mass moving in to replace cooler air.
Cold fronts are represented on weather maps such as those that you'll find in Flight Simulator's Weather dialog box as a blue line with triangles pointing in the direction the front is moving. Warm fronts are represented by red lines. You will only see fronts depicted on the map when you use Real-world Weather.
As a pilot, you need to be aware of how cold and warm fronts affect the conditions you'll be flying in. The cloud conditions and weather you'll encounter once the front passes depend on the stability and moisture content of the air mass the front is replacing.
You won't create fronts using the User-defined weather option, but you'll see fronts (as well as high and low pressure areas) depicted on the weather map when using the Real-world weather option.
Barometric pressure is important to pilots for two reasons: it affects how the plane's altimeter works and it affects the weather. In order to measure how high above sea level your aircraft is, its altimeter is set to local barometric pressure. When an air traffic controller says, "Altimeter is 2992" she's saying the local barometric pressure is 29.92 inches of mercury. She's also telling the pilot to set the Kollsman window of the altimeter to read 2992. The reading in feet or meters on the altimeter is known as pressure altitude.
You really don't need to care about dew, unless you're sleeping under your airplane's wing. But when you're planning a flight you should care about the dew point—the temperature to which the air must be cooled to reach saturation.
The air around us contains invisible moisture known as water vapor. The difference between a location's temperature and its dew point indicates how well the air can currently hold moisture. Because cool air has a harder time holding moisture than warm air, pilots use this information to determine how likely it is that fog, clouds, or precipitation will form.
The temperature/dew point spread is usually given in terms of a percentage of relative humidity. For example, if the temperature is 72 F (22 C) and the dew point is 52 F (11 C), the relative humidity is 49 percent. Using this example, the air is 49 percent saturated with moisture.
When the air is cooled to a point where it reaches 100 percent saturation, the water vapor condenses, forming clouds. If the temperature falls further, water drops form and fall to the ground as rain or, if it's cold enough, ice or snow. The presence of clouds does not guarantee precipitation, but the presence of precipitation guarantees there are clouds in the area.
In Flight Simulator, the Automated Terminal Information Service (ATIS) reports the current temperature and the dew point. The closer the dew point is to the current temperature, the more likely you're going to encounter clouds. If you're flying into an area where the temperature is 80 F (29 C) and the dew point is 65 F (18 C), you don't need to worry about a thick layer of clouds blanketing the area. If the temperature is 65 F and the dew point is 60 F (15 C), watch for clouds.
You can set the temperature and dew point for any temperature layer in Flight Simulator. In practical terms, this means you can create conditions where it is more or less likely that clouds will form (although Precipitation is a separate setting within User-defined weather). Be aware that setting a low cloud layer in Flight Simulator weather will not constitute a layer of fog. To create fog-like conditions, modify the Visibility settings using the User-defined weather option.
Quiz: If the temperature at your destination is 85 F (29.4 C) and the dew point is 80 F (29 C), how likely is it that you'll see clouds in the area when landing?
Answer: Very likely. Any time the temperature/dew point spread is five degrees Fahrenheit or less, the air has become saturated.
Density altitude is pressure altitude corrected for temperature. When the temperature is high, air is less dense. Density altitude affects aircraft performance: An increase in density altitude corresponds to a decrease in engine power output, propeller efficiency, and aerodynamic lift. Density altitude is normally computed using graphs in the pilot operating handbook for each aircraft, but for simplicity's sake, just be aware that if the weather is hot, your aircraft's performance will likely suffer. Turbine engine aircraft and turbocharged reciprocating engines suffer less, but density altitude is still a factor.