Lana Tait, age 11, of Des Moines, Iowa, for her question:
How does Illinois compare with average world rainfall?
Meteorologists consider the rain as one form of precipitation falling on the earth. Moisture from the weathery atmosphere also descends in the form of snow and sleet and pelting pellets of hail, and the word "precipitation" enables us to deal with all these types of moisture.
From north to south, Illinois is a tall state, subject to a variety of weather patterns. Much of its weather is carried on warm, moist winds blowing up from the Gulf of Mexico, but its northern region is modified by mild winter winds and cooling summer breezes from Lake Michigan. Meteorologists figure its annual precipitation from r the average amount of winter snows and sleet, summer rains and hail from sultry thunderstorms. And the average varies in weather regions that stretch in wavy bands across the state. Cairo at the southern tip can expect an annual precipitation of from 44 to 48 inches. Chicago in the north expects from 28 to 32 inches.
Snow, of course, is a frozen mercngue of icy crystals and frothy air. As it melts, it shrinks into a smaller volume of rainwater. Meteorologists estimate snow precipitation in terms of its melted water content. This is a problem. A two foot drift of new, soft and fluffy snow may equal only one inch of rainfall. A six inch layer of moist snow and a two inch crust of fine hard snow may both yield one inch of rainwater. When detailed measurements are impossible, meteorologists add one inch of rainfall for every 10 inches of fallen snow. Southern Illinois can expect 10 inches of winter snow and northern Illinois can expect up to 30 inches of snow.
Weathery precipitation varies tremendously over different parts of the globe, ranging from less than an inch to more than 450 inches per year. California's Death Valley expects an annual precipitation of 1.7 inches. Hawaii's Mount Waialeale expects a yearly downpour of 470 inches and claims to be the wettest spot in the world. Altogether, the cloudy atmosphere dumps about 95,000 cubic miles of moisture on the land an seas every year.
Some parts of the globe get more, far more than their share of moisture and some get less. But it is possible to estimate what every place in the world would get if this water were evenly distributed. Meteorologists say that every spot on the globe could expect an annual precipitation of about 40 inches. The northern region of Illinois would get about six extra inches and the southern region would get about the same amount of precipitation that it now expects to get every year.
The weather, of course, changes with the seasons and the seasons rotate north and south of the equator. Rainy and dry seasons vary from year to year in every part of the world. A11 these variations make it hard to estimate annual precipitations. The tricky job is done by keeping weather records for decades. The final estimate is figured from the average of many, many years. It is not perfect, but it does give us a fairly accurate idea of what we can expect.