INVESTIGATING EARTH's WEATHER giving rise to warm and cold air masses with characteristic levels of temperature and moisture As the air masses move they influ ence the weather of the places they pass over Maritime tropical air masses for example form over tropical waters and typically pro duce heavy rain when they move over land At weather fronts the boundaries between air masses the air is unstable and storms are common Severe thunderstorms ice storms and tornadoes all typically form where air masses meet Sometimes the storms are violent In the flat plains of the Midwest for instance there are no mountain ranges to keep air masses from clashing and in spring and summer these collisions often spawn severe thunderstorms and sometimes dan gerous tornadoes In fact this region is often called Tornado Alley In this volume you will learn how these powerful storms develop You will also learn how the science of weather prediction has changed over the years The earliest forecasts were based only on careful observation The invention of the thermometer and the barometer in the 17th century provided the first accurate measure ments of temperature and air pressure Today scientists have much more sophisticated
INTRODUCTION tools for detecting and forecasting weather patterns Doppler radar for instance can measure wind speeds by observing micro waves reflected off raindrops And satellites circle Earth gathering images that are fed into powerful computers and turned into maps Such information is shared by the 180 member nations of the World Meteorological Organization And yet as much as weather forecasting has improved in recent years it is still diffi cult to predict daily weather accurately more than about a week in advance The unpre dictability and power of this awesome force of nature shows why weather continues to be as fascinating to modern people as it was to the sailors and farmers of ancient
U AP TER 1 THE NATURE OF WEATHER Nhe weather concerns everyone and has some effect on nearly every human activity It occurs within the atmosphere the mixture of gases that com pletely envelops Earth Weather is defined as the momentary day to day state of the atmosphere over any place on Earth's surface Climate on the other hand refers to weather averaged over a long period The basic atmo spheric conditions that make up the weather include precipitation humidity temperature pressure cloudiness and wind The air is constantly in movement There also is a continuous exchange of heat and mois ture between the atmosphere and Earth's land and sea surfaces These ever changing condi tions can be scientifically analyzed The science of observing and predicting the weather is known as meteorology THE ATMOSPHERE AND ITs MoVEMENTS Air is compressed by its own weight so that about half the bulk of the atmosphere is 10
THE NATURE OF WEATHER squeezed into the bottom 35 miles 5 6 kilo meters The bottom layer of the atmosphere the troposphere is the site of almost all the world's weather Above its turbulence and storminess is the calmer stratosphere which has little moisture and few clouds Underlying the great variety of atmo spheric motions is a pattern of large scale air movement over Earth The basic cause of these planetary winds or general circulation of the atmosphere is that the sun heats the air over the Equator more than it does the air over the poles The heated air over the equatorial regions rises and flows generally polewardſ in both the Northern and Southern hemispheres In the polar regions the air cools and sinks and from time to time it flows back toward the Equator HIGH AND Low PRESSURE BELTS The upward movement of air results in a belt of low pressure in the tropical regions astride the Equator On either side at about 30° N latitude and 30° S latitude is a belt of high pressure formed as the upper level flow of air from the Equator sinks to the surface From each of these subtropical high pressure II
