Weather Blog

California hit by..."dew storm"?

California hit by..."dew storm"?
From Flickr user christopherdown

Someday when people look back on Feb. 18, 2010 in California, they might think it rained in a lot of places. Several sites reported 0.01" of precipitation on Wednesday.

But with high pressure squarely in charge of the West Coast, there was nary a rain cloud to be found down there.  Instead, it was dew and frost around the rain gauge that caused the sensors to record rainfall:

I believe unless it gets manually corrected, those readings would be enough to count as an official "rainy day" and end any dry streaks they have going.

As to what causes dew, during the day, the ground will heat up from the sun. But at night, as the ground begins to cool, the temperature will drop to the dew point -- the temperature at which the air becomes saturated. As this happens, the water vapor near the ground will condense onto objects near the surface -- such as grass, plants, cars, etc.

Grass is an excellent dew collector because it gives off its own water, making the air near the grass easier to condense.

If the dew point is below freezing, the water vapor will condense straight to frozen form -- what we see as frost.

Special thanks to UW Research Meteorologist Mark Albright for pointing out the "dew storm" to me.