The difference in air pressure creates what we call high- and low-pressure areas. These areas are what we call weather systems, and they are constantly in motion about the surface of the earth. Their broad direction of movement is from west to east, the same direction as the earth’s rotation. But their paths are affected by many factors. Huge streams of air are constantly in motion high in the troposphere and in the stratosphere, and they affect the movements of weather systems. So does the land itself, mountain ranges, river valleys, deserts. And warm air is constantly rising in the tropics; heavier cold air is constantly flowing down from the polar regions. Warm air anywhere, but especially in the tropics and over oceans and lakes, absorbs water vapor, and cold air chills and precipitates this moisture. All these factors, and still more, affect the size, direction, character, and speed of weather systems.
Weather systems move across the earth on more or less regular schedules. In the Summer the tendency is for one such system to follow another at seven-day intervals. In Winter this speeds up to five-day intervals. The difference is caused by the fact that warm Summer air is lighter than cold Winter air and moves somewhat more slowly. The Winter wind, cold air, is literally heavier than the warm wind of Summer. Tests have shown that a twenty-mile Winter wind, for instance, exerts more pressure on a windmill than a twenty-mile Summer wind. When we have a Summer that seems to bring rain every week end, it is not mere coincidence. Summer weather tends to repeat itself in seven-day cycles. But don’t count on it. One storm may be slowed up on the way and a new sequence will follow.
Storms are the most violent and dramatic of all weather phenomena. They vary from the local thunderstorm to the widespread devastation of a hurricane.
Thunderstorms are most frequent along the eastern part of the Gulf Coast and in the southern part of the Rockies and the nearby Plains area. Parts of Mississippi, Alabama, and Florida sometimes have ninety such storms a year. The upper Midwest averages about forty a year, and most of the Northeast has around thirty.
The thunderstorm is essentially local. It is caused by violent vertical movements of air, hot spots at the earth’s surface often contrasted with nearby cool spots, as where plain and mountain meet or warm shoreline and cool ocean. Cumulus clouds build into thunderheads sometimes 75,000 feet high, whose interiors seethe with warring winds. Brief, heavy rain is typical of such storms, and so is violent lightning. You can estimate the distance of such a storm by timing the lag between the sight of a lightning flash and the sound of its thunder. The sight is almost instantaneous, but the sound travels only about 1,100 feet a second. There is a five-second lag if the storm is a mile away, ten seconds for two miles. Another warning is the wave of cool air ahead of the storm, usually about three miles. When you first feel the chill, the storm is probably still three miles away.
Lightning tends to strike the highest object. Never take shelter under a lone tree in an open field in a lightning storm. One is much safer lying flat in the open, even at the expense of a drenching. The mast on a sailboat is a prime target. If you are in a boat on the water when such a storm approaches, get to land as soon as possible. You are safe inside an automobile in the open, but not under a tree. You are safe in any steel-frame building or any building with lightning rods properly installed. A TV antenna is a prime target unless it is properly grounded. In the woods, stay away from the taller trees.
Tornadoes are the most violent of all storms, though their paths seldom are more than an eighth of a mile across. They are marked by twisting, funnel-shaped clouds inside which the wind force is incredible. They usually accompany heavy thunderstorms and heavy rain, sometimes severe hailstorms. They are most frequent in the lower Mississippi valley and the Midwest. The area west of Texas and Nebraska has almost none, West Virginia and Maine have very few, New England and the Northeast have only ten or fifteen a year. The extreme low pressure inside a tornado’s vortex can cause even a large building literally to explode, and the winds can uproot huge trees, even knock a train and locomotive off the track. In recent years the Weather Bureau has been tracking tornadoes by radar and issuing warnings to all in their path.
In a sense, a hurricane is a tremendous tornado. Hurricanes originate near the equator and are created, at least in part, by the twisting forces of the earth’s rotation. A mass of warm air, perhaps twenty or thirty miles in diameter, rises and starts swirling. It feeds on its own motion, condensing its moisture, generating more hot, turbulent air, drawing more and more surrounding air into its giant whirlpool. At first it is something like a big thunderstorm, but it becomes a tremendous wheel of growing winds and heavy rain. The winds may reach 200 miles an hour and the path of destruction may be a hundred miles or more wide. The storm may move only ten miles an hour at the start, but as it gains strength it may travel thirty miles an hour. Some hurricanes travel 4,000 miles before they die out.
Late August, September, and early October are the worst time for hurricanes in our area. The worst of their damage in the Northeast usually is within a hundred miles of the coast, and most of it often is caused by flooding. Now and then a hurricane, as the vicious one of 1938, moves inland in the Northeast and leaves a broad path of devastation caused by both wind and flood. The Weather Bureau now maintains a hurricane watch and warning system.
The northwestern part of the Pacific has more hurricanes, known there as typhoons, than any other part of the earth. But Australia and New Zealand have them, and so does Africa. The hurricane winds revolve counter-clockwise in our northern hemisphere, clockwise in the southern hemisphere.
All these violent storms are accompanied by clouds, but there can be clouds without storms. Often the clouds make the sky intensely beautiful and they always add interest to the sky. Clouds are responsible for colorful sunrises and sunsets, which are among the most beautiful of all natural phenomena.
Clouds are formed of water vapor in some state or another. The air always contains water vapor, even on a dazzling, cloudless Autumn day. Clouds form when the air cools below its saturation point and the water vapor condenses. The steam from a boiling kettle in a cool room is a miniature cloud. So is your breath on a cold morning. Fog is a cloud at the earth’s surface.
Clouds are among the earth’s greatest travelers, constantly in motion, constantly being renewed. I look out my window now and see cirrus clouds, mare’s-tail streamers, riding the wind several miles high. Only six or eight hours ago those clouds probably were over Chicago, nearly a thousand miles away. If conditions are right tonight I may look up and see noctilucent clouds traveling in the stratosphere at the speed of a jet plane, yet so far away that they seem to be standing still among the stars.
These are high-flying clouds, which zip along at spectacular speeds up where eternal gales are blowing. But the everyday clouds of the lower atmosphere are also travelers of consequence. Scud, the ragged gray cloud which flies so low just before or after a rainstorm, is whipped along by winds of thirty, forty, and fifty miles an hour. The most spectacular of all, the big cotton-ball cumulus clouds, sometimes travel hundreds of miles, though at a stately pace of fifteen or twenty and rarely more than thirty-five miles an hour.
Cloud names come from the Latin. Cumulus means a heap, and cumulus clouds look like giant heaps of meringue. Nimbus means rain, and nimbo-stratus clouds are those gray masses that close in the earth, darken everything, and finally pour out a drenching rainstorm. Stratus means layer, and stratus clouds look like successive layers or bands. Cirrus, meaning a curl or strand of hair, aptly describes the highflying cloud strands which sometimes resemble straight mare’s-tails and often curl up at the ends in beautiful swirls. Noctilucent clouds are night-luminous, and Nacreous clouds are like mother-of-pearl. Both are visible only at night, since they occur at heights of sixteen to fifty miles, where they catch and reflect the sun’s glow long after sunset. We aren’t sure, but it seems likely that noctilucent clouds are composed of fine dust similar to that of comets and meteors rather than of water vapor. All other clouds are
vapor clouds, though some contain smoke and other solid materials. Vast forest fires and volcanic eruptions create smoky clouds that sometimes completely circle the earth. The clouds created by the explosion of Krakatoa in the Dutch East Indies in 1883, the most violent of modern times, circled the globe within a week and remained in the atmosphere, creating brilliant sunsets, for nearly a year.
There are two broad classes of clouds—the puffy, piled-up cumulus clouds formed when warm, saturated air rises swiftly and the moisture is condensed into visible particles by the cool upper air, and the sheets of foglike layer-clouds formed when a layer of warm, saturated air is cooled without much vertical movement.
Clouds are also classified by their altitude—low clouds, middle clouds, high clouds, and towering clouds.
There are three kinds of low clouds, which are seldom much more than a mile above the earth. Stratus clouds are a low, uniform cloud sheet, often dull gray. They make a heavy, dull sky but bring only drizzling rain. Nimbo-stratus are the rain clouds, darker than ordinary stratus and having a “wet” look. Streaks of rain often can be seen in them, reaching to the ground. Nimbo-stratus clouds often are accompanied by fractostratus, or low, scudding clouds whipped along by a strong wind. Strato-cumulus clouds are puffy masses of cloud that form a moving, sometimes rolling layer, usually gray with deep, dark shadows. Strato-cumulus do not produce rain, but if they roll into great masses and form nimbo-stratus they can and usually do become the source of rainstorms, sometimes mild thunderstorms.
Middle clouds are nearly always either stratus or cumulus and they ride about two miles above the earth. Alto-stratus are veils or sheets of cloud, either blue or gray in color. They often give a striped appearance. The sun or moon seen through them looks as though it were behind frosted glass but does not form a halo. Alto-cumulus are patches or layers of fluffy clouds. They may be dazzling white or pearly gray. They seem to have no special form, are clots of cloud that never seem to come together. The sun may appear as a disc with a corona of blue or yellow with a reddish rim when seen through alto-cumulus clouds. Neither alto-stratus nor alto-cumulus clouds produce storms.
High clouds are composed almost entirely of minute crystals of ice and they fly about four miles above the earth. Cirrus clouds are feathery, wispy. They form the mare’s-tails I mentioned earlier. Cirro-cumulus are rare. They are streaky clouds that form waves and ripples and create the true “mackerel sky.” Cirro-stratus clouds look like ragged patches of veiling, very high and gauzy. They are the clouds that create large halos, big luminous circles, around the sun or moon.
The towering clouds are the thunderheads, the cumulo-nimbus, which may have their bases close to the ground and their tops, often flattened and anvil-shaped, as much as fifteen miles above the earth. They are frothy and turbulent, white with deep, dark shadows. Ordinary cumulus clouds are quite harmless and sail like ships with towering sheets of canvas across the Summer sky. They form at almost any altitude, sometimes as much as three miles up but more often half that high or less. They are the great clouds of the West, the picturesque ones that make the clear sky look twice as blue, and they often accompany a brilliant, sunny Summer day. When in the western sky at sunset they can be dazzlingly beautiful, shot with silver and gold and even with red and orange, but more often with pink and yellow.
The droplets of moisture in a cloud average only 1/2,500 inch in diameter, so small that they float in air that has any motion at all. The average raindrop contains about one million times as much water as one of those cloud droplets. Hail is formed when raindrops are frozen into ice particles in high, turbulent clouds, then carried up and down by the turbulence through warm, moist zones and cold, dry ones, building bigger and bigger all the time, until they finally are hailstones and come pelting down. Hailstones as big as a hen’s egg have been found. Sleet occurs when rain falls from high clouds through a layer of freezing air. Sleet actually consists of ice pellets, usually very small as contrasted to hail. Snow remains something of a mystery. As far as is known, droplets of water vapor in saturated air attach themselves to very small particles of dust or other matter, then are frozen into crystals. Those crystals are six-sided and amazingly delicate and beautiful. But it has also been found that at high altitudes snowflakes will form without those dust particles if the air is supersaturated and the temperature is at least 38° below zero. Snow pellets are like soft sleet and are differentiated from sleet, usually, only by meteorologists. Ice prisms are tiny ice crystals, often needles and columns, so small that they fall very slowly. They are seen as a dazzling shimmer in the air on very cold Winter mornings and often accompany a Winter morning fog. They sometimes create beautiful colored halos around the sun.
Dew and frost form from moisture in the air at the earth’s surface. They form on clear nights. The “sweat” that collects on a glass containing a cold drink on a hot day is a form of dew. Frost out of doors, especially near a river, a pond, or other source of water vapor, sometimes makes a spectacular display. This is sometimes called hoarfrost. It often takes one of two typical forms, tabular or spicular. Tabular frost forms in tiny plates, usually on edge like the leaves of a partially opened book. Spicular frost consists of spikes or needles, smaller than a pin in diameter and bristling on a grass blade or weed stem. Either frost or dew can make spider webs into fantastically beautiful, delicate designs of crystal. Another form of frost forms on window-panes in a room with moist air on a very cold night. These frost patterns often are feathery, intricate beyond belief, and remarkably beautiful. They are found in our centrally heated houses usually in bedrooms, where our own breath has created enough moisture for them to form. Elsewhere in the house the air is too dry, though sometimes they will appear on a kitchen window, where cooking has moistened the air with its steam.
Man’s own works have somewhat altered the weather and climate. Cities, with their concentration of heat-absorbing structures and of heating machinery, are warmer both Summer and Winter than the country. The haze over cities and factory areas also holds the heat. City air has more carbon dioxide than the air of the open country, relatively less oxygen.
Both climate and weather will vary somewhat within a region because of local differences of geography and plant life. Valleys are generally cooler than hilltops, especially at night and in the Winter, because cold air flows downhill. Lakesides and riversides are more humid than inland areas, have more dew, often have later Autumn frosts. Woodlands are cooler in Summer, warmer in Winter, than open land.
The question of whether our climate is growing warmer or not has often been argued, to no conclusion. Some say we are still emerging from the last Ice Age and the trend is toward a warmer climate. Others say we are moving into a new Ice Age and the long-time trend is colder. The evidence seems inconclusive. Despite the grandfather tales of bitter winters in the past, we have only about one century of weather records made with comparable instruments, by no means enough to make a judgment either way. It does appear that the West Coast of the United States is getting somewhat cooler and the East Coast somewhat warmer, but the evidence is slight and short-term. We who are living today probably never will know the answer, but perhaps our great-great-grandchildren will.
When we come to matters outside the earth’s atmosphere, there is the whole universe to consider, and that is a subject for more awe and speculation than precise information. Perhaps we can touch a few of the highlights. Let’s start with the moon, the earth’s own satellite, our companion in the annual trip around the sun.
The moon is about one quarter the diameter of the earth. It has an elliptical orbit but averages about 238,000 miles from the earth. The moon is a “dead” planet with little or no atmosphere, no light of its own, and probably with no form of life we would recognize as life. The moonlight we see is reflected from the sun. Because the moon rotates only once in each circuit of the earth we always see the same side of it.
The moon’s phases are created by the fact that the moon revolves around the earth just
a little slower than the earth revolves on its own axis. The difference averages about fifty minutes a day. That is another way of saying that the moon rises an average of fifty minutes later each day. When we have a new moon, the moon rises in the east soon after sunrise, is invisible all day in the sun’s overpowering brilliance, and can be seen at dusk as a thin crescent low in the west, its horns pointing to the east. The crescent is the reflection of the sun on the moon’s lower side.
Each day the moon rises a little later, and about seven days after new moon it appears overhead as a half moon soon after sunset. Now the whole lower half reflects the sun there below the horizon. Another seven or eight days and the moon rises in the east at about the time the sun sets in the west. Now it is entirely illuminated by the sun and we see it as a full, round disc. This is the full moon and it remains in the sky all night long. Still rising later each night, in another seven days it doesn’t appear until midnight and is a half moon again, waning. And, after still another week, it rises just before sunrise, a crescent once more but this time with the horns pointing west. After that it is out of sight about seven days because it rises so near sunrise that it is invisible. This is what we call the dark of the moon. Finally its daily lag sets it in the western sky just after sunset and we have the crescent new moon again. Each phase of the moon is repeated about every four weeks.
The daily lag of the moon varies from season to season. It is at its minimum, only about twenty-three minutes a day, in the Northeast at about the time of the Autumnal Equinox. So the full moon nearest the Autumnal Equinox gives us four or five days in a row with the moon practically full and in the sky almost all night. This is the Harvest Moon, and it occurs only then.