Hottest rain on record? Rain falls at 109°F in Saudi Arabia
Pilgrims to the holy city of Mekkah (Mecca), Saudi Arabia must have been astonished on Tuesday afternoon, June 5, when the weather transformed from widespread dust with a temperature of 113°F (45°C) to a thunderstorm with rain. Remarkably, the air temperature during the thunderstorm was a sizzling 109°F (43°C), and the relative humidity a scant 18%. It is exceedingly rare to get rain when the temperature rises above 100°F, since those kind of temperatures usually require a high pressure system with sinking air that discourages rainfall. However, on June 4, a sea breeze formed along the shores of the Red Sea, and pushed inland 45 miles (71 km) to Mekkah by mid-afternoon. Moist air flowing eastwards from the Red Sea hit the boundary of the sea breeze and was forced upwards, creating rain-bearing thunderstorms. According to weather records researcher Maximiliano Herrera, this is the highest known temperature that rain has fallen at, anywhere in the world. He knows of one other case where rain occurred at 109°F (43°C): in Marrakech, Morocco on July 10, 2010. A thunderstorm that began at 5 pm local time brought rain at a remarkably low humidity of 14%, cooling the temperature down to 91°F within an hour.
Figure 1. Thunderstorms at 109°F? This true-color satellite image of Saudi Arabia taken at 2:10 pm local time (11:10 UTC) on June 5, 2012, shows a line of thunderstorms that developed along the edge of the sea breeze from the Red Sea. Three hours after this image was taken, Mekkah (Mecca) recorded a thunderstorm with rain and a temperature of 109°F (43°C.) Image credit: NASA.
More like a hot shower than a cooling rain?
Thunderstorms often produce big drops of cold rain, since these raindrops form several thousand meters high in the atmosphere, where temperatures are much cooler than near the surface. Some drops even get their start as snow or ice particles, which melt on the way to the surface. Additional cooling of the drops occurs due to evaporation on the way down. However, in the case of the June 4, 2012 Mekkah storm, I think the rain was probably more like a hot shower. Large raindrops, like the kind thunderstorms produce, fall at a speed of about 10 meters per second. A balloon sounding of the upper atmosphere taken at 3 pm local time at a nearby station (Al-Midinah) found that the bottom 1000 meters of the atmosphere was 97°F (36°C) or warmer. Thus, the thunderstorms' raindrops would have been subjected to 100 seconds of some very hot air on the way to the surface, likely warming them above 100°F by the time they hit the ground. A classic 1948 study of raindrops found that, in many cases, raindrop temperatures start off cold in the first few minutes of a rain shower, then warm up to within 1°C (1.8°F) of the air temperature within a few minutes. With the air temperature a sizzling 109°F (43°C) at the time of the June 4 thunderstorm in Mekkah, the raindrops could easily have been heated to a temperature of over 105°F (41°C) by the time they reached the surface!
How hot can it be and still rain?
If substantial amounts of liquid water are present on the Earth, the planet will experience rain, as long as some mechanism to lift the warm, moist air and cause condensation can be found. If the climate continues to warm as expected, we should see an increasing number of cases where it rains at temperatures well above 100°F. On Saturday, June 2, the temperature in Mekkah hit 51.4°C (124.5°F), a new record for the city, and just 1.1°F (0.6°C) below the all-time hottest temperature record for Saudi Arabia (125.6°F, or 52°C, recorded at Jeddah on June 22, 2010.) I expect that 20 - 40 years from now, we'll begin seeing occasional cases where rain falls at a temperature above 117°F (47°C) in the desert regions of North Africa and the Middle East.
I'll have a new post by Friday afternoon.