After a hot finish to the summer season, we had our coolest morning yet in over a month. The low at the Reno airport was 48 degrees Wednesday morning. Normal is 51. So not too far off from where we should be. The Minden Tahoe Airport even got down to 27 degrees Wednesday morning. Carson City got down to 37. With more sunshine in the forecast we have been able to warm up during the day though. Highs will be in the 80’s Thursday. It was the perfect recipe for a chilly start Wednesday. Skies were clear, winds were calm, and a cooler air mass had just moved in. Radiational cooling is a fancy term for this. This happens a lot in our area because of our dry climate. During the day the sun heats the earth and at night the heat escapes back into the atmosphere. Clear skies can make this particular cooling process easier. Reno sees roughly 158 clear days with no cloud cover a year. Portland, Oregon only sees 68.
Having cloudy skies at night is similar to keeping your car window rolled down with the heat on. All the heat leaves the car. The more you roll the window up, the harder it is for the heat to escape and cool off. Clouds can absorb the heat radiating from the ground at night, and then like a mirror re-emits some of it back to the surface. This further slows the cooling process.
Moisture itself can slow the cooling process. Since moist air can absorb more heat it also requires more energy to cool down. This is why it is usually so much warmer at night in places like Miami.
The dewpoint temperature is a good value to look at for how muggy it is outside. The higher the dewpoint the muggier it will feel. The dewpoint tells you what temperature the air would have to cool to in order for saturation to occur. A high dewpoint means a low amount of cooling. The dewpoint is the coldest temperature the air could get to overnight. Our dry climate during the summer can give us a temperature swing well over 30 degrees between the high and low for the day. For Miami it’s typically less than twenty.
Wind can play a role as well. Strong winds can bring warmer air from above to the surface, similar to mixing a hot drink. You could very well need a warm drink Thursday morning. Temperatures will be in the 40’s for many valley locations. Tahoe will be in the 30’s.
