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Rain would be welcome

yields already hurt

July 24, 2007|By JEFF SEMLER

All the dry weather we've been having has brought up the question of cloud seeding.

So at the suggestion of a reader, I did a little research.

I must admit that with cloud seeding, you either believe it occurs or it is in the category of Big Foot and UFOs.

My research surprised me.

I was always led to believe that clouds were seeded in order to disperse the rain. I know most people hold to that belief, but my research found that cloud seeding was developed to increase precipitation and reduce hail, not disperse the clouds.

Research work at Colorado State University has this to say about cloud seeding. "The main objective of the 'static mode' of cloud seeding is to increase the efficiency of precipitation formation by introducing an "optimum" concentration of ice crystals in supercooled clouds by cloud seeding.

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"It was originally thought that clouds were deficient in ice nuclei and therefore additions of modest concentrations of ice nuclei should result in a more efficient precipitation-producing cloud system.

"All that was needed was to introduce seeding material from the ground or at the base of clouds which would then enhance ice crystal concentrations and thereby increase rainfall."

The National Oceanic and Atmospheric Administration's (NOAA) Hurricane Research Division of the Atlantic Oceanographic and Meteorological Laboratory puts it this way, "In mid-latitude clouds, the usual seeding strategy has been based upon the vapor pressure being lower over water than over ice.

"When ice particles form in supercooled clouds, they grow at the expense of liquid droplets and become heavy enough to fall as rain from clouds that otherwise would produce none."

Quite to the contrary of what I have been led to believe. But I am not writing this to stir up debate, just to share facts that have been shrouded in myth.

No matter what your beliefs, it is dry.

Many folks ask, how far behind are we on annual rainfall?

It really doesn't matter. Sure, annual rainfall is a nice figure but it is most times irrelevant.

Let's look at two periods. From 1960 to 1979, the annual average rainfall was 38.27 inches. From 1980 to 2006, the average annual rainfall was 41.38 inches.

Are we getting wetter? From the averages you would think so.

But if you take out 1996 (76.66 inches), then the average drops to 40.03 inches. So averages tell us very little.

What is more important is when the rain falls and how it falls.

Hard thunderstorms provide only a small amount of relief because the rain comes too fast. A half-inch of steady rain does more good than a one-inch downpour because most of that precipitation runs off.

Don't believe me? Look at your lawn and you will notice the grass at the end of the spouting and where water runs off patios and driveways is much greener - the result of the concentration of those thunderstorms into slower moving deeper water.

The other important scenario is, rain needs to fall in the growing season, April to early September.

Does rain in March and October help? Sure it does, but not to the extent a June or July rain does.

Think about our current corn crop.

Will 11.45 inches of rain in September do it any good? That is what we received in September of 2003.

The 4.86 inches in July of 2005 would be of much greater benefit.

At the current totals reported for July (up to the 15th), we have received 0.00 inches of rain.

By September, most crops are finished. Those fall rains do benefit crops like wheat, barley and rye.

The next question on everyone's mind is, will rain at this point help?

It depends on the condition of the crop. I have seen some fields that no amount of rain will help, while others will benefit greatly by showers as the ears of corn pollinate and fill and as the soybeans set pods.

Perennial crops such as alfalfa, grasses and orchards will bounce back from rains regardless of when they fall.

Whether it will be in time for this year's harvest is anybody's guess.

So what's my point?

It is that we are short on soil moisture and we need a few days of soaking rain. Our crop yields will be lower this year. How much less remains to be seen.

Jeff Semler is an Extension educator, specializing in agriculture and natural resources, for the University of Maryland Cooperative Extension. He is based in Washington County. He can be reached weekdays by telephone at 301-791-1404, ext. 25, or by e-mail at jsemler@umd.edu

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