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The Seasonal Almanac: Summer Rainfall and Mars Blue Sunsets

The Seasonal Almanac covers astronomical events and nature, including summer rainfall and sunsets and Mars blue sunsets.

| June/July 1997

The Seasonal Almanac shares astronomical and nature events, this issue includes information on summer rainfall and sunsets and Mars blue sunsets. 

Summer is the wettest season of the year for the vast majority of the eastern half of the U.S. By wettest I mean greatest total amount of precipitation. But within summer, the months of July and August are also the time of the most torrential rainfalls in this part of the land. I am looking over a listing of record rainfalls for the eastern half of the U.S. Of 10 time-periods ranging from 1 minute up to 1 day, only the record for the longest—1 day—did not occur in either July or August. The record for 1 day-38.7 inches in Yankeetown, Florida in 1950—occurred on September 5-6 . . . just past August.

How hard can it rain? As with most weather records, the figures are generally far more extreme than people would typically guess. Meteorologists have terms for rainfall intensity rates of up to more than 5 inches an hour—though admittedly it is rare for these rates to last for an entire hour. Here in New Jersey, there are typically only one or two days in a year when over three inches of rain falls in the entire day. Often I wonder if an inch has fallen in a single hour, but very rarely does that happen. Bearing this in mind, what do you think is the record for the most intense rainfall in the eastern U.S.? It occurred on the 4th of July in 1956. That day in Unionville, Maryland, 1.23 inches fell—in one minute!

What causes these record rainfalls? If you guessed tropical storms and hurricanes, you might be right in some cases. These storms drop immense amounts of precipitation over enormous areas—widespread rainfalls of 5, 10, or even (in the most extreme cases) 15 or 20 inches can occur over the course of a day or two. But even more intense rainfalls may occur in a highly localized region as a result of a quite different weather mechanism: a severe isolated stationary thunderstorm. I recall driving (not by choice!) through such a storm (a Level 6 on the weather radar) and going from dry road to water well above the tops of my tires all in less than half a mile. I barely managed to get back out, but when I did, I found the same cutoff point, and it never did rain far beyond that point all day.



Another time while searching New Jersey weather records, I found that the second heaviest daily rainfall in a 10-year period had been about 11 inches. But a weather station only about 2 miles away from the one which had recorded this amount had picked up a mere fraction of an inch that same July day. Thunderstorms usually go through their full cycle and die in an hour or so, and they usually move—but not always. And where they linger rainfall amounts can be staggering.

Nonetheless, Eastern rainfall is steadily predictable. Weather expert Richard A. Keen points out that in most years the precipitation which falls on most places in the eastern U.S. is within 20 percent of the average—"a reliability unheard of in most other parts of the world." According to Keen, in some northeastern states the variability from year to year is less than 10 percent—and that is the lowest in the entire world.






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