Just as a pilot needs charts showing a flight's complete route to successfully navigate from airport to airport, the weather student should have a "chart" giving an overview of the learning route to be followed.
The eight general principles below can help students to keep their bearings while navigating a weather course. Student pilots can use this guide to help them grasp the bigger picture - much the way they would climb to a higher altitude if they became lost on a cross-country flight. Terrain that's confusing when you're flying 2,000 feet above the ground usually makes more sense if you climb to 5,000 feet. There you might see lakes, rivers, cities, the coastline, or other features that give you the big picture and help you to figure out where you are. Instructors can use these principles to introduce weather topics to their students. If the details of the weather become confusing, returning to these eight basics can help you to regain your bearings.
1. The Sun Heats The Earth Unequally.
The biggest differences are caused by the seasons. During the Northern Hemisphere's winter, the North Pole is tilted away from the sun, bringing shorter days and causing the sun to be lower in the sky. During summer, the North Pole is tilted toward the sun, which means that the sun is higher in the sky and the days are longer. The sun is high in the sky in the tropics all during the year. This is why the Earth's middle latitudes have distinct cold and warm seasons while the tropics are warm all year.
Unequal absorption of solar heat by different kinds of surfaces also causes some areas to be warmer than others, even though the same amount of sunlight reaches them. Anyone who's ever been to the beach on a hot afternoon knows about this. As you walk barefoot onto dry sand, it burns your feet. What do you do? Most people run across the hot sand to the water, which they know will be cooler. Yet, the sun has been shining with equal intensity on both the hot sand and cool water all day. In brief, the very top layers of the sand absorb all of the sun's heat reaching it - you don't have to dig far down to find cooler sand. Solar radiation penetrates farther into water than sand or dirt, warming a deeper layer. Since a given amount of solar energy warms more water than sand, the water's temperature doesn't climb as high as the sand's.
2. Air Temperature Differences Start The Winds Blowing.
The variations in ground or water temperatures create different air temperatures around the globe and, to a smaller degree, over land and water.
Warm air is light and tends to rise, while cold air is heavy and tends to sink. On a global scale, this means that air tends to rise in the tropics and sink over cooler parts of the globe. If the Earth were not rotating, warm air would be rising in the tropics and flowing at high altitudes to the north and south where it would sink in the polar regions. Cool air from the northern and southern parts of the globe would flow across the Earth's surface to replace the rising air in the tropics.
On a local scale, unequal heating causes "sea breezes" near the oceans or large lakes. As the land heats up during the day, air heated by the warm land begins rising, and cooler air flows in to replace it, creating a cooler breeze from the water to the land.
At least a couple of questions on the FAA's private pilot written exam require applicants to understand that all weather begins with the unequal heating of the Earth. One of these questions, for instance, says: "Every physical process of weather is accompanied by, or is the result of, a: A - movement of air; B - pressure differential; C - heat exchange." In this case, C is the correct answer because air movements and pressure differences, like all aspects of weather, depend on unequal heating, or a heat exchange.
3. The Earth's Rotation Destroys The Simple Pattern Of Winds Flowing To The Poles Aloft And Toward The Equator At The Surface.
The effect of the Earth's rotation, known as the Coriolis force, combines with the other forces that drive the winds to create huge wind spirals known as high- and low-pressure areas. On the real, spinning Earth, air that rises in the tropics descends over the subtropical regions between about 20 and 30 degrees latitude north and south. In the middle latitudes, the Coriolis force distorts the winds so much that it creates high- and low-pressure areas that follow each other from west to east with ever-changing weather.
In the Northern Hemisphere, the air flows counterclockwise around low-pressure areas and clockwise around high-pressure areas. The flows in the Southern Hemisphere are in the opposite directions.
4. Cool Air Can "Hold" Less Water Vapor Than Warm Air, Which Is Why Cooling The Air Creates Clouds, Dew, Fog, Drizzle, Rain, Frost, Snow, Freezing Rain, Or Sleet.
When the air is relatively warm, water evaporates into it, which means the water becomes an invisible gas known as water vapor. If the air is cooled, some of the vapor condenses to form dew or the tiny water droplets that make up fog and clouds. (Fog is merely a cloud that's on the ground.) When conditions are right, cloud drop-lets come together to create small drops called drizzle or larger drops called rain.
When the temperature is cold enough, water vapor can sublimate directly into ice to create frost on objects or snow in clouds. Freezing rain and sleet are created when rain falls from air that's warmer than 32 degrees into freezing air.
5. Air Pressure Decreases With Altitude.
The decrease of air pressure with height is important to pilots both because of the role that it plays in weather and because aircraft altimeters use decreasing pressure to indicate height above sea level. Also, the lower pressures at altitude affect aircraft performance.
Air pressure decreases with altitude because pressure depends on the weight of the air above the point where the pressure is being measured. The higher you go, the less air there is above you.
6. Lowering The Air's Pressure Causes Air To Cool. Increasing The Air's Pressure Causes The Air To Warm.
This basic fact of nature can create confusion in a couple of ways. First, this statement applies to air only when its pressure is changing. For example, if you use a bicycle pump to inflate a tire, the pump becomes warm because you are increasing the air's pressure each time you push the plunger. If you push down on a tire's valve to let the air out, the air coming from the tire feels cool because its pressure is decreasing as it rushes from the high pressure in the tire to the lower pressure outside.
Second, areas of low pressure are not necessarily cool and areas of high pressure are not necessarily warm. Hurricanes are areas of extreme low pressure, yet their centers are warmer than the surrounding air. In winter storms, on the other hand, the low-pressure center is cooler than the surrounding air. The highest air pressures measured at the Earth's surface are in very cold air, such as over Siberia.
To understand the weather, you need to know that as the pressure of a blob or parcel of air decreases, the air cools. As the pressure the parcel of air increases, the air warms.
By the way, the lower pressure of air at higher altitudes is not the reason that the air usually cools with altitude. Sunlight hardly warms the air at all. Instead, solar energy warms the Earth's surface and the air is warmed where it is in contact with the surface. The farther from the surface, the cooler the air most of the time.
7. Rising Air Causes Clouds And Precipitation. Sinking Air Tends To Clear The Sky.
As a parcel of air rises from the surface, the pressure of the surrounding air decreases with altitude. The pressure of the rising air decreases to match the pressure of the surrounding air. As a result, the rising air cools. If there is enough water vapor in the air, it begins condensing to form clouds and maybe precipitation.
When air is sinking, its pressure increases as it descends into the higher-pressure air at lower altitudes. This causes the air to warm. If there are any clouds, they begin to evaporate as the air warms. The warming air, of course, will also keep clouds from forming.
8. Air Is Rising From Areas Of Low Air Pressure At The Earth's Surface And Sinking Into Areas Of High Pressure At The Surface.
As a result, low-pressure areas tend to bring clouds and precipitation while the sky is normally clear in areas of high pressure.
At the Earth's surface, air spirals into areas of low air pressure where it rises. As the air rises, it cools, and the water vapor in it begins condensing to form a widespread area of clouds and precipitation. Air that rises in low-pressure areas flows into the upper atmosphere until it eventually begins sinking to form areas of high pressure at the surface. While the sinking air keeps clouds from forming, it doesn't always bring completely clear skies. Sometimes, the high pressure can trap pollution, which can create low visibility in haze. Warming the air at the Earth's surface is one way to make air rise, but it's not the only one. Hurricanes depend on warm, humid air rising from the surface; this is why they form only over warm oceans.
Other storms are much more complicated, with various factors causing air to rise in areas of low pressure. These are related to the movements of upper atmospheric winds. The source of these winds, in turn, go back to the unequal heating of the Earth's surface and the global-scale winds that this set into motion.
Weather is complicated, and to fly safely - or to know when not to fly - pilots do have to learn a fair amount about the science of the atmosphere. Of course, the required knowledge goes far beyond the brief outline of the basic principles here, but these principles will help you to make sense of the details.
