Massive movements of the atmosphere
Weather instruments and how they are used
The origins, development, and effects of storms
How to interpret weather maps
Full-color illustrations and up-to-date facts help you understand the fascinating phenomena of weather, and how changes are predicted.
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Air Masses â" Clouds â" Rainfall Storms â" Weather Maps â" Climate
By Paul E. Lehr, R. Will Burnett, Herbert S. Zim, Harry Mcnaught
St. Martin's PressCopyright © 1993 St. Martin's Press
All rights reserved.
WHAT MAKES THE WEATHER?
Weather is the condition of the atmosphere in terms of heat, pressure, wind, and moisture. These are the elements of which the weather is made. Where the atmosphere thins to near vacuum, high above the earth, there is no weather. But near the surface of the earth, where the atmosphere is dense and heavy, you see the everchanging, dramatic, and often violent weather show.
But it takes more than air to make weather. If the earth's atmosphere were never heated, mixed, or moved about, there would be no weather — or, more properly, there would be no changes in the weather. There would be no winds, no changes in air pressure, no storms, rain, or snow.
Heat is the spoon that mixes the atmosphere to make weather. All weather changes are brought about by temperature changes in different parts of the atmosphere.
WHAT HAPPENS TO THE SUN'S HEAT is shown in the diagram above. This is for average weather — that is, 52 per cent cloudiness in the sky. A typical cloud reflects back into space 75 per cent of the sunlight striking it. On overcast days, only about 25 per cent of the sun's energy hits the ground. Energy that does reach the ground is absorbed and reflected in varying degrees. Snow reflects about 75 per cent, absorbs only 25 per cent; this partly accounts for the cold of polar regions. Dark forests absorb about 95 per cent of solar energy and change it to heat. Such differences in absorption and reflection account, in part, for regional differences in temperature and climate.
EARTH AS A GREENHOUSE The glass of a greenhouse lets the short solar rays pass through. These are absorbed by objects inside and are re-radiated as long heat rays. But these long heat rays cannot get through the glass. The heat rays are continually reabsorbed and re-radiated inside. This helps keep the greenhouse warm on cold days. Some heat is lost by conduction through the glass.
Like a greenhouse, the earth's atmosphere admits most of the solar radiation. When this is absorbed by the earth's surface, it is re-radiated as heat waves, most of which are trapped by water vapor and carbon dioxide in the atmosphere. Thus the earth is kept warm.
THE ATMOSPHERE AS A THERMOSTAT controls the earth's heat as automatically as in any heating system. It protects the earth from too much solar radiation during the day, and screens out dangerous rays. It acts as an insulating blanket which keeps most of the heat from escaping at night. Without its thick atmosphere the earth would experience temperatures like the moon's. The moon's surface temperature reaches the boiling point of water (212° Fahrenheit) during the two-week lunar day. It drops to 238°F below zero during the long lunar night.
The earth cools faster on bright clear nights than on cloudy nights, because an overcast sky reflects a large amount of heat back to earth, where it is once again re-absorbed.
HEAT AND AIR MOVEMENTS The air is heated mainly by contact with the warm earth. When air is warmed, it expands and becomes lighter. A layer of air, warmed by contact with the earth, rises and is replaced by colder air which flows in and under it. This cold air, in turn, is warmed and rises, and it, too, is replaced by colder air. Such a circulating movement of warm and cold fluids is called "convection." You can see convection currents if you drop small bits of paper into a glass container in which water is being heated.
The air at the equator receives much more heat than the air at the poles (here). So warm air at the equator rises and is replaced by colder air flowing in from north and south. The warm, light air rises and moves poleward high above the earth. As it cools, it sinks, replacing the cool surface air which has moved toward the equator. If the earth did not rotate, the air would circulate as shown. Because the earth does rotate, the circulation is different (here).
CONVECTION causes local winds and breezes. Different land and water surfaces absorb different amounts of heat. Dark, plowed soil absorbs much more than grassy fields. Mountains absorb heat faster during daylight than nearby valleys, and lose it faster at night. Land warms faster than does water during the day and cools faster at night. The air above such surfaces is warmed or cooled accordingly — and local winds result.
WATER IN THE ATMOSPHERE Water is always present in the air. It evaporates from the earth, of which 70.8 per cent is covered with water. In the air, water exists in three states: solid, liquid, and invisible vapor.
The amount of water vapor in the air is called the "humidity." The "relative humidity" is the amount of vapor the air is holding expressed as a percentage of the amount the air could hold at that particular temperature. Warm air can hold more water than cold. When air with a given amount of water vapor cools, its relative humidity goes up; when the air is warmed, its relative humidity drops.
As the table below shows, air at 86°F is "saturated" when it holds 30.4 grams of water vapor per cubic meter. (In other words, it has a relative humidity of 100 per cent; it has reached its dew point.) But air at 68° is saturated when it holds only 17.3 grams per cubic meter. That's a difference of 13.1 grams per cubic meter. So every cubic meter of 86° saturated air that is cooled to 68° will lose 13.1 grams of water vapor as cloud droplets which, if conditions are right, will fall as rain or snow.
HOW CLOUDS ARE FORMED When air is cooled below its saturation point the water vapor in it condenses to form clouds. When water vapor at a teakettle spout is cooled by the air around it, a small cloud forms. Your warm moist breath forms a miniature cloud when it hits the cold winter air. The clouds you see nearly every day form in several ways but all form by the same general process — cooling of air below its saturation point.
CLOUD CLASSIFICATION Clouds are classified according to how they are formed. There are two basic types: (1) Clouds formed by rising air currents. These are piled up and puffy. They are called "cumulus," which means piled up or accumulated. (2) Clouds formed when a layer of air is cooled below the saturation point without vertical movement. These are in sheets or foglike layers. They are called "stratus," meaning sheetlike or layered.
Clouds are further classified by altitude into four families: high clouds, middle clouds, low clouds, and towering clouds. The bases of the latter may be as low as the typical low clouds, but the tops may be at or above 75,000 ft.
CLOUD NAMES The names of clouds are descriptive of their type and form. The word "nimbus," meaning rain cloud, is added to the names of clouds which typically produce rain or snow. The prefix "fracto-," meaning fragment, is added to names of wind-blown clouds that are broken into pieces. "Alto-," meaning high, is used to indicate middle-layer high clouds of either stratus or cumulus types. The pictures and captions on the next four pages will help you to identify major cloud types and to understand better their relationship to the weather.
HIGH CLOUDS are composed almost entirely of tiny ice crystals. Their bases average about 20,000 ft. above the earth. Three types exist:
Cirrus clouds, thin, wispy, and feathery, are composed entirely of ice crystals. Cirrus clouds usually form at 25,000 ft. and above, where the temperature is always far below freezing. These clouds are frequently blown about into feathery strands called "mares' tails."
Cirrocumulus clouds, generally forming at 20,000 to 25,000 ft., are rarely seen. These thin, patchy clouds often form wavelike patterns. These are the true mackerel sky, not to be confused with altocumulus rolls. They are often rippled and always too thin to show shadows.
Cirrostratus clouds form at the same altitudes as cirrocumulus. These are thin sheets that look like fine veils or torn, windblown patches of gauze. Because they are made of ice crystals, cirrostratus clouds form large halos, or luminous circles, around sun and moon.
MIDDLE CLOUDS are basically stratus or cumulus. Their bases average about 10,000 ft. above the earth.
Altostratus are dense veils or sheets of gray or blue. They often appear fibrous or lightly striped. The sun or moon does not form a halo, as with higher, ice-crystal cirrostratus, but appears as if seen through frosted glass.
Altocumulus are patches or layers of puffy or roll-like clouds, gray or whitish. They resemble cirrocumulus, but the puffs or rolls are larger and made of water droplets, not ice crystals. Through altocumulus the sun often produces a corona, or disk, generally pale blue or yellow inside, reddish outside. The corona's color and spread distinguish it from the cirrostratus halo — a larger ring, covering much more of the sky.
LOW CLOUDS have bases that range in height from near the earth's surface to 6,500 ft. There are three main kinds:
Stratus is a low, quite uniform sheet, like fog, with the base above the ground. Dull-gray stratus clouds often make a heavy, leaden sky. Only fine drizzle can fall from true stratus clouds, because there is little or no vertical movement in them.
Nimbostratus are the true rain clouds. Darker than ordinary stratus, they have a wet look, and streaks of rain often extend to the ground. They often are accompanied by low scud clouds (fractostratus) when the wind is strong.
Stratocumulus are irregular masses of clouds spread out in a rolling or puffy layer. Gray with darker shading, stratocumulus do not produce rain but sometimes change into nimbostratus, which do. The rolls or masses then fuse together and the lower surface becomes indistinct with rain.
Cumulonimbus are the familiar thunderheads. Bases may almost touch the ground; violent updrafts may carry the tops to 75,000 ft. Winds aloft often mold the tops into a flat anvil-like form. In their most violent form these clouds produce tornadoes (here).
Cumulus are puffy, cauliflowerlike. Shapes constantly change. Over land, cumulus usually form by day in rising warm air, and disappear at night. They mean fair weather unless they pile up into cumulonimbus.
Cumulus and Cumulonimbus are both clouds of vertical development, unlike the layered clouds described on previous pages. Clouds of the cumulus type result from strong vertical currents. They form at almost any altitude, with bases sometimes as high as 14,000 ft.CHAPTER 2
RAIN, SNOW, DEW, AND FROST
PRECIPITATION such as rain, snow, sleet, and hail can occur only if there are clouds in the sky. But not all kinds of clouds can produce precipitation. Temperature, the presence of tiny foreign particles, or of ice crystals, all help determine whether precipitation will occur and what form it will take. For example, snow will not form unless air is supersaturated (cooled below its saturation point or dew point without its water vapor condensing) and is considerably below the freezing point of water.
WHAT MAKES IT RAIN? Rain falls from clouds for the same reason anything falls to earth. The earth's gravity pulls it. But every cloud is made of water droplets or ice crystals. Why doesn't rain or snow fall constantly from all clouds? The droplets or ice crystals in clouds are exceedingly small. The effect of gravity on them is minute. Air currents move and lift droplets so that the net downward movement is zero, even though the droplets are in constant motion.
Droplets and ice crystals behave somewhat like dust in the air made visible in a shaft of sunlight. But dust particles are much larger than water droplets, and they finally fall. The cloud droplet of average size is only 1/2500 inch in diameter. It is so small that it would take 16 hours to fall half a mile in perfectly still air, and it does not fall out of moving air at all. Only when the droplet grows to a diameter of 1/125 inch or larger can it fall from the cloud. The average raindrop contains a million times as much water as a tiny cloud droplet. The growth of a cloud droplet to a size large enough to fall out is the cause of rain and other forms of precipitation. This important growth process is called "coalescence."
Coalescence occurs chiefly in two ways: (1) Droplets in clouds are of different sizes. Big drops move more slowly in turbulent air and in paths different from the paths of small droplets. Bigger, heavier drops are not whipped around so rapidly. So drops collide, become bigger, and finally drop as rain. This is probably the main cause of rainfall from nimbostratus and other low clouds.
(2) The most important type of coalescence occurs when tiny ice crystals and water droplets occur together (as near the middle of cumulonimbus clouds). Some water droplets evaporate and then condense on the crystals. The crystals grow until they drop as snow or ice pellets. As these drop through warm air, they change into raindrops.
(3) Lightning discharges in a thunderstorm form oxides of nitrogen that are extremely hygroscopic (water-absorbing). These oxides are added to the atmosphere and become one of the kinds of nuclei for future condensation and eventual coalescence and rainfall. But the two processes mentioned above are the main and perhaps the only causes of coalescence and hence precipitation. Research may show other possibilities.
SNOW Very small particles in the air may act as nuclei upon which water vapor will crystallize to form snow. Air must be supersaturated with water vapor and below the freezing point. Microscopic bits of soil, clay, sand, and ash are common nuclei. Cloud temperatures must generally be from +10° to –4°F before snow begins to form. Vapor changes to snow even without nuclei at high altitudes in supersaturated air at –38°F.
SNOW PELLETS (or granular snow) are white, and of various shapes. Although much like soft hail, pellets are too small and soft to bounce. A single pellet generally forms from many supercooled cloud droplets which freeze together into crystalline form.
ICE PRISMS form hexagonal plates, columns, and needles that sometimes glitter like diamonds as they are blown about. Because of their small size they fall very slowly. Ice needles often make halos around sun or moon. In very cold climates ice-needle fogs form on the ground.
ICE PELLETS (sleet) consist of transparent or translucent beads of ice. Sleet occurs when rain, dropping from upper warm air, falls through a layer of freezing air. Raindrops first become freezing rain (supercooled) and when striking the ground in this condition form glaze (here). But further cooling produces ice pellets, or true sleet, which bounces on hitting the ground.
HAIL forms as frozen raindrops, formed high in the clouds, move through areas of supercooled water droplets in thunderclouds. Hailstones were long thought to develop their onionlike structure by being alternately forced upward by vertical winds in the thunderhead to a freezing level, then dropped down to where more water was picked up. Such up-and-down trips do occur, but the growth of hailstones results mostly from ice pellets' picking up water in the supercooled middle and upper regions of the cloud. The layers result from differences between the freezing rate and the rate at which water accumulates on the pellets.
ICE STORMS are characterized by glaze. Glaze is as destructive as it is beautiful. It occurs when rain or drizzle that has been supercooled (cooled below 32°F but not yet frozen) falls on cold surfaces and immediately freezes. Glaze ice formed from this freezing rain can snap branches, wires, and poles and cause hazardous driving conditions.
DEW does not fall. It is water vapor that condenses on solid surfaces that have cooled below the condensation point of the air in contact with them. This cooling by radiation occurs usually on clear nights. The "sweat" that forms on the outside of a glass of cold lemonade on a hot day is also dew.
Excerpted from Weather by Paul E. Lehr, R. Will Burnett, Herbert S. Zim, Harry Mcnaught. Copyright © 1993 St. Martin's Press. Excerpted by permission of St. Martin's Press.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Table of Contents
ContentsWHAT MAKES THE WEATHER? The effects of heat, pressure, wind, and moisture ... Clouds,
RAIN, SNOW, DEW, AND FROST Their nature, types, and origins ... Rainmaking,
THE ATMOSPHERE-RESTLESS OCEAN OF AIR ... Its structure and weather function,
THE EARTH'S MOTIONS AND WEATHER Seasonal changes and the earth's rotation as they affect winds,
HIGHS AND LOWS Pressure cells, their winds, and associated weather,
AIR MASSES Major air masses of the world, their identification, and their role as a source of our weather,
FRONTS AND FRONTAL WEATHER How fronts form, move, and change ... The kinds of weather associated with each type,
STORMS The origin, development, and effects of thunderstorms, tornadoes, and hurricanes,
WEATHER FORECASTING Weather instruments and how they are used in forecasting,
WEATHER MAPS How data are plotted and charted ... How to read maps and make your own forecast,
WEATHER AND CLIMATE Average weather conditions worth knowing and using day by day,
BOOKS FOR MORE INFORMATION Books and magazines to read,