IF YOU'RE under the impression that this has been a rainier spring than usual, you're right. It's rained almost every weekend this spring, and temperatures have been below average. And it's not only here that the weather has caused problems. Storms destroyed vegetable crops in California in March and mudslides destroyed houses in Utah this week. The Pearl River flooded in Mississippi in April and there were blizzards in Wyoming and Montana in May. Mountains and hills from the Sierra Nevada to the Berkshires are covered with unusually large amounts of snow and water, and weather experts predict fierce floods all over the place for the next month or so.

You can blame all these things, or so weather experts tell us, on El Nino. That's the name a Peruvian fisherman gave the unusual winds and ocean currents he observed during the Christmas season. This year El Nino continued long past the Christmas season, and as a result the jet stream, which usually encounters North America somewhere in Canada, instead came flying directly across California and the central part of the continental United States. That meant that we didn't get much cold Arctic air in this country and had an unusually mild winter. Then in March the jet stream began to meander up and down the United States slowly, spinning off lots of storms and creating lots of cloud cover, and so we have had that unusually wet spring.

When are we going to get normal weather again? No weather forecaster can tell for sure, and perhaps one reason is that we can't really be sure what normal weather is. Statistically, it means weather that, at a particular time of year, matches the average temperature, precipitation, and humidity recorded in the same locality since people started making records. But for most of the United States, that's only about 100 years--an instant in geological time.

Who's to say that such averages are the same as the averages you'd get if you had records going back 10,000 years? Historical research points to the conclusion that we have long cycles of different weather. The late 1600s were unusually cold in Europe-- which may be one reason Europeans were willing to extend hegemony over the vast and, usually in comparison to Europe, uncomfortably cold expanses of North America and Siberia. The early 1900s were unusually warm, with peak warmth and sunniness in the early 1940s--weather that made it easier for Hitler's Panzer divisions to overrun continental Europe.

But for most of us, "normal" weather is not determined by statistics, but by what we remember from growing up. We've blotted out the memory of those dreary afternoons, spent noisily indoors with parents driven to distraction; we remember those golden sunny afternoons when the world seemed perfect. Now we feel robbed when every weekend doesn't turn out that way or when, in fact, no weekend seems to, when "normal" weather in the old- fashioned meaning of the word has become what you get from Monday through Friday.