Dec 042012
 

Several morning last week, I noticed the road were wet even though no rain or snow had fallen over night. I think a couple of things happened to cause this. First, the surfaces cooled a bit more than the air overnight. Surfaces can radiate their heat via black body radiation. Most nights last week were very clear, allowing radiation to escape to by this method.

Second, the dew point was fairly high in the upper 20s. As you see in the graph below, right around dawn each day, the dewpoint and the temperature are very close. Friday is the best example of this. To form dew requires only a slight temperature drop. Notice that each day had an increase in the dew point temperature after sunrise. As the dew and frost evaporate with heating, more water is put into the atmosphere and the dew point rises.

A couple more thoughts, many cars formed frost last week. Since they don’t have as much mass as the ground, they don’t store as much heat. So, cars formed frost, the roads generally had dew. However, black ice could have formed with a preceding period of colder temperatures.

My cars typically don’t get frost or dew on them. Why am I so special? Large ponderosa pines surround my driveway. They reflect the black body radiation back down and keep the cars slightly warmer.

Weather conditions for the week of November 25 to December 1, 2012 from my weather station.

Weather conditions for the week of November 25 to December 1, 2012 from my weather station.

 

Stu

 Posted by at 4:08 am

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