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Wednesday, August 15, 2012

Jeff Masters: Hottest rain in recorded history -- 115 °F in Needles, California; the lowest humidity rain has ever occurred at anywhere on Earth in recorded history

by Dr. Jeff Masters, WunderBlog, August 15, 2012

A searing heat wave rare even for the Desert Southwest sent temperatures soaring to record levels on Monday, with Needles, California, tying its record high for the date of 118 °F (47.8 °C). The temperature might have gone higher in Needles, but a thunderstorm rolled in at 3:20 p.m., and by 3:56 p.m. PDT, rain began falling at a temperature of 115 °F (46.1 °C). Most of the rain evaporated, since the humidity was only 11%, and only a trace of precipitation was recorded in the rain gauge. Nevertheless, Monday's rain at 115 °F in Needles sets a new world record for the hottest rain in world history. I don't think many people were outside to experience to experience the feeling of rain falling at 115 °F, but if they were, it must have been an uncomfortable, sauna-like experience! Thanks go to Dr. Warren Blier of the NWS Monterey office for pointing out this remarkable event to me.

It is exceedingly rare to get rain when the temperature rises above 100 °F, since those kinds of temperatures usually require a high-pressure system with sinking air that discourages rainfall. Monday's rain in Needles was due to a flow of moisture coming from the south caused by the Southwest U.S. monsoon, a seasonal influx of moisture caused by the difference in temperature between the hot desert and the cooler ocean areas surrounding Mexico to the south. According to weather records researcher Maximiliano Herrera, the previous record for hottest rain, which I blogged about in June, was a rain shower at 109 °F (43 °C) observed in Mecca, Saudi Arabia, on June 5, 2012, and in Marrakech, Morocco, on July 10, 2010. The 11% humidity that accompanied Monday's rain shower at 115 °F in Needles was the lowest humidity rain has ever occurred at anywhere on Earth in recorded history, according to Mr. Herrera.

Figure 1. True-color MODIS satellite image of California and Arizona taken at 1:25 p.m. PDT, August 13, 2012. Developing thunderstorms surround Needles, CA, and the line of clouds to the southwest of the city would develop into a thunderstorm that brought rain to the city at 4 p.m. PDT, at a temperature of 115 °F. Image credit: NASA.

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