I'm sure you've all experienced the discomfort of a July or August day when the relative humidity is near 90% and the temperature is over 90. What makes the day especially awful is knowing that the night will be even worse. Humidity will go up as it begins to cool off and it won't cool off much at all. Soon you will be able to explain why relative humidity increases as it cools a bit with sunset, but why doesn't it cool off on the humid nights the way it does when it the air is dryer?

I. Looking again at the energy balance

A. Describing the relationship between radiation and temperature

B. Water as a greenhouse gas

1. The effect

2. Humid nights in Phila

3. Two other environments

a. Deserts

b. High elevations

Questions: 1. Consider two summer nights, one when the humidity at 6:00 P.M. is 90 % and a second when it is 60 %. Describe the likely changes in temperature and humidity for the two nights after sunset.

2. Explain why the Earth warms up on a cloudy day, but does not cool off much on a cloudy night.

3. Explain why summer nights are generally cooler in deserts and in the mountains than they are in Philadelphia even though the days may be just as hot or hotter.

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