Jet trails are changing the weather of the Midwest -- and possibly much of the rest of the country -- creating more clouds, cooler days and warmer nights, according to weather researchers at the University of Illinois.

Cloudiness over the Midwest has been slowly increasing since the turn of the century, but beginning in the 1950s with the introduction of commercial jetliners, cloudiness and its temperature effects have increased sharply, the illinois researchers said.

Today the Midwest has 30 fewer clear days a year that it did in 1900: 90 now compared with 120 then. This increase in cloudiness has also caused a steady narrowing of daily temperatures, so that the high and low temperature readings for a day are steadily coming closer together. The Illinois team now is working to calculate how much more rain and snow, if any, is caused by the increased cloudiness.

"The bottom line on this research is economics," said Richard Semonin, one of the leaders of the research team. "At the end of the study, we hope to calculate the economic benefit or disbenefit from reducing temperature stresses [warming the coldest nights and cooling the hottest days], and from additional rainfall. This may be having a significant impact on energy use and on agricultural production."

The effects of the cloudiness seem so far to be positive, Semonin said, but no final statement on the subject can be made until the study, financed by the National Science Foundation, is finished, which will require several years.

Cloudiness generally has two important effects. In the daytime, clouds block the sun and hold temperatures down, an important fact when air conditioners are running in the summertime. In the winter, the heat generated during the day remains longer at night because clouds prevent warm air from escaping up into the atmosphere. So less heating may be necessary at night.

In addition, many plants are damaged by temperature extremes in the summer and winter. Reduction of these extremes should benefit farmers considerably, Semonin said.

He said that his study only covered midwestern skies, but he believes similar weather effects are probably occurring in other parts of the country and the world wherever there is heavy jet traffic. In the United States, such areas would include the East Coast, part of the South and parts of the West especially Denver.

A jet produces a cloud -- or contrail -- behind it because its exhaust consists primarily of water vapor, carbon dioxide and stray unburned paricles of carbon compounds. The tiny stray particles form the "seeds" on which water collect, forming water beads or ice crystals that can be seen.

About one or two days a week, the air at high alitudes reaches 100 percent humidity so that is is saturated with water in the form of tiny ice crystals. Then the additional vapor poured out of the jet engine does not evaporate quickly, and the water beaded on stray particles changes from a temporary plume behind a jet into a long-lasting contrail cloud.

In the midwestern sky, which is traversed by more jets than any other area of the country, the clouds formed from contrails slowly spread out over hours. Layers of them, some visible and some spread too thinly to be visible, reflect the sunlight upward and deflect rising heat downward.

In measuring the cloudiness from 1950 to the late 1970s, researchers could make detailed comparisons since modern weather bureaus have kept track of cloudiness by looking at the sky every three hours and estimating the percentage of sky covered with clouds. The eight sightings a day are added to get the daily cloud figure. Over a year, the maximum possible cloud cover points would be 2920.

During a year in Moline, Ill., in the early 1950s, the total cloud coverage number was about 500. In the 1970s, the number had risen to 800, Semonin said. Another city in the heart of the jet flyways, Fort Wayne, Ind., also jumped from 500 to 800 in cloud cover over two decades.

Cities on the fringe of the heavily traveled flyways showed much smaller increases. Evansville, at the southern tip of Indiana, had an increase in the cloud cover from 625 to 650. Duluth, to the north of the heaviest flying, increased from 600 to 700.

The temperature change measurements made by the Illinois group were done by taking the high and low temperatures for a day and subtracting them to get a difference. This difference was then averaged for each month.

To determine whether there was a trend in the differences, researchers counted only months that showed a narrowing difference in the daily highs and lows from the month before.

At the turn of the century, only two or three months a year had narrowing temperature differences -- there was no trend.

But starting in about 1952, the numbers began to change dramatically, until by 1968, all 12 months of the year showed narrower temperature differences than the month before. From 1968 to the present, every year has continued the trend.

The benefits of increased cloudiness that may be expected are enough to have gotten another researcher, Dr. Andrew Detweiler of the State University of New York at Albany, to begin looking into the possibility of deliberately causing clouds to form at certain times -- to save crops in some areas and cut severe fuel bills in others.

Detweiler suggests flying an airplane back and forth at high altitudes slightly upwind of the area to be covered. Instead of relying on the plane's contrails to make the clouds, Detweiler would seed the air with small amounts of silver iodide, the conventional cloud-seeding chemical.