Certainly an understanding of weather is essential to aeronautics. An aircraft crew must know what conditions to expect in order to prepare and successfully execute a flight plan. Designers need to understand the cause and effect of weather so that the aircraft is built to overcome potential hazards. Weather is the most familiar expression of the atmosphere. Hot, cold, windy, cloudy, wet, - it's something we can feel, see, hear, touch and taste. And though it's considered a condition unique to the troposphere, weather is actually the result of various exchanges of solar energy between the earth, the entire atmosphere, and space.

The sun radiates enormous amounts of energy, most of it lost or absorbed before reaching the edge of earth's atmosphere. The amount received at the edge varies according to location and season.. When the sun's rays are perpendicular (at a 90 degree angle) to a location at the edge of the atmosphere, that area receives the greatest amount of radiation. Shine a flashlight directly at a spot on a wall and see the bright circle it makes. Now move to the left or right and shine the light at the same spot. You will notice that the light from the more oblique angle covers a wider area that is not as bright. This is how the sun's rays strike the earth's atmosphere, and because of the earth's curvature the equator receives a greater amount of solar energy than the poles. The position of the earth relative to the sun also affects the amount of radiation received. When the northern hemisphere experiences summer the North Pole is tilted toward the sun, resulting in a longer period of daylight and more perpendicular rays. Though the earth in its elliptical revolution is actually at its farthest distance from the sun, the amount of energy and the length of daylight overcomes the distance. When the earth is closest to the sun, the North Pole is tilted away and the northern hemisphere experiences winter: short days and oblique rays.

The transfer of energy from the sun to the earth's atmosphere is accomplished through a process called radiation. Radiation transfers energy in wave motions. The waves can travel through empty space because they do not require an interaction with other molecules. These waves are classified according to their wavelength - the distance between peaks in the waves - from shorter to longer. Differing wavelengths causes differing interactions with the atmosphere; in fact, the amount of energy that enters the atmosphere is greatly reduced before it reaches the earth's surface. This is caused by absorption, reflection, and scattering.

Absorption causes energy to be retained by a substance, and by retaining energy the substance heats up and reradiates. X-rays and gamma rays, which have the shortest wavelengths, are absorbed by oxygen and nitrogen molecules in the upper atmosphere and transformed into ions, which form the ionosphere. Ultraviolet rays of slightly longer wavelength are absorbed by ozone in the stratosphere. Infra-red rays, at the other end of the spectrum, are slightly absorbed by carbon dioxide and water vapor in the troposphere. Wavelengths that are visible to the human eye - violet, blue, green, yellow, orange, and red - are affected by reflection and scattering. Reflection occurs when particles and surfaces that are larger than the incoming waves meet and turn back solar energy. Clouds, snow, and light-colored sand are all reflectors. Scattering occurs when particles the same size as the wavelength of the radiation meet. Scattering causes energy to be redirected in all directions, some of which returns to space. The sky appears blue because the short, blue wavelengths are more easily scattered. Without scattering the sky away from the sun would appear black, similar to outer space. Scattering is also the reason the sun appears red at sunrise and sunset. Because the sun's path through the atmosphere is much longer at this time of day, more of the blue wavelengths are scattered out of its beam, leaving more red light.

Due to these three processes, less than half the solar energy that enters the atmosphere reaches the earth's surface, and only about one-fifth warms the atmosphere directly. Most of the energy that warms our atmosphere comes indirectly from the heated earth. A small amount of the energy absorbed by the earth warms the atmosphere through a process called conduction. Conduction is the transfer of heat within a substance. An example of conduction is a metal rod - if one end is heated the collision of rapidly moving molecules will soon heat the other end of the rod. But unlike solids, gases and liquids are not good conductors, so only the air just above the surface of the earth is heated by this process. More heat is transferred from the surface to the atmosphere through a process called convection. Convection is the vertical transfer of energy by the actual movement of the heated substance. Air rises up a chimney, for example, when it is heated by a fire below and becomes less dense (weighs less) than the air surrounding it. Conduction and convection are called sensible heat transfer processes.

The solar radiation that reaches the earth manages to pass through the atmosphere with little or no interference; yet almost all convected energy is absorbed by the atmosphere. Why? The difference in temperature between the sun and earth changes the wavelength of the convected rays - the lower the temperature, the longer the wavelength. The shortwave visible light rays which passed down through the water vapor and carbon dioxide without obstruction return upward as longwave radiation and are for the most part absorbed by those same clouds. The clouds heat up and reemit energy back to earth as counter radiation - in effect recycling radiation from the earth. This process of trapping longwave radiation has been called the greenhouse effect, and is one of the important ways the atmosphere's temperatures remain within a livable range.

If this was all there was to our heating system, however, scientific models indicate that there would be an imbalance between the amount of energy received and expended. The earth's surface should be gradually warming and the atmosphere cooling. Another factor must be added to the equation. Sensible heat transfers energy from one substance to another - from earth to air, for example. Latent heat transfers energy by changing the state of the matter itself. Water is unique in that it can exist as a solid (ice), liquid (rain) or gas (vapor). Its transformation from one state to another involves the addition or subtraction of energy. When a liquid evaporates into gas, energy is required and cooling occurs: the evaporation of water on the earth's surface lowers the earth's temperature. When a gas condenses into liquid, energy is given off and warming occurs. The rising water vapor is absorbed by the atmosphere and through condensation into liquid the atmosphere is warmed. Balance is again achieved in our scientific model.

The main point is that the amount of energy absorbed by the earth-atmosphere system over the entire globe in a year is equal to the amount emitted by the system. Solar energy is radiated into this system, where it is absorbed, reflected, or scattered. The radiation absorbed by the earth is conducted, convected, or evaporated into the atmosphere. The energy in the atmosphere is absorbed from solar radiation and sensible and latent heat from the earth. Energy is returned from earth and atmosphere to space, either immediately or eventually, through one of these processes.

But now another question arises from our model. At different latitudes an imbalance exists between the outgoing and absorbed radiation of the earth-atmosphere heating system. The poles should be getting colder and tropical regions warmer, but this is not happening. Heat is being transported poleward from areas of surplus radiation, almost equally, by ocean and air. The atmospheric balancing act is achieved by wind systems, which we will look at next.

Explore Space ... Not Drugs!
Hear what astronauts have to say about staying drug-free.
Last modified: Fri Apr 24 13:06:40 PDT 1998

Copyright © 1997 by Cislunar Aerospace, Inc. All Rights Reserved.