Christine L. Woods, age 12, of Eugene, Oregon, for her question:

How can an almanac predict the year's weather?

This month, Eugene, Oregon, can expect about 19 days of rain or snow and average temperatures between 38 degrees and 47 degrees Fahrenheit. January will bring temperatures about three degrees cooler and perhaps another 19 days of precipitation. Next July, Eugene can expect about three rainy days and average temperatures between 60 degrees and 80 degrees. These predictions are based on the records of many past years    the same sort of statistics that almanacs use to forecast the weather for the coming year.

In a general way, you can predict the broad weather picture for 1972. From December to May, you can predict snow on the ski slopes of Oregon's Mount Hood, because this was so almost every year in the past. You can predict enough mild spring weather to produce strawberries in June. This also happened so often in the past that you can expect it next year. For half a century or so, the Weather Bureau has been keeping records of daily weather events in every locality. These records give a pretty good idea of what can be expected in the future.

The yearly weather is a seasonal parade through spring and summer, fall and winter. As a rule, the coldest weather sets in after the shortest days around Christmas time. The summer's heat builds up after the longest days in late June. Seasonal changes in the global atmosphere often bring breezy days in March, a spell of calm Indian summer after the first frost and, in mid winter, a warm Chinook wind blows down the eastern slopes of the Rockies. Statistics of past years reveal the exact dates of these and other weather events in various localities.

To a casual observer, the fickle weather seems to do as it chooses. But records through many years reveal that its seasonal patterns do not vary by very much. For example, the Weather Bureau issues the dates when the first and last frosts can be expected in your locality. This prediction is based on weather events through many years. True, next year may be an exception. The last frost may come a week after you were told it was safe to sow your vegetable seeds. In this case the local prediction will be readjusted.

An almanac uses these and hundreds of other weather patterns of the past. All the detailed evidence of past records is charted for each locality. The rest is statistics, based on the theory that whatever usually happened on a certain date is quite likely to be repeated. This statistical system is not perfect. But it does tend to be accurate in predicting more than half the weather events through the coming year.

Historians suspect that almanacs were invented thousands of years ago by Eastern astronomers. They predicted the phases of the moon, eclipses and the location of the planets. Later almanacs included useful information for farmers. Special almanacs for sailors included tides and wind charts. Weather predictions were added to almanacs of the last century. And our modern almanacs base their surprisingly accurate predictions on careful statistics drawn from half a century.