How do cloud cover and humidity affect the diurnal temperature range in a given area?

• A. More cloud cover and higher humidity reduce the diurnal temperature range by trapping heat at night and reflecting some solar radiation during the day.
• B. Cloud cover increases the diurnal range by trapping heat at night.
• C. Humidity has no effect on diurnal range.
• D. Cloud cover has no effect; diurnal range is fixed by latitude.

Answer: (A)

The diurnal temperature range is the difference between the day's highest temperature and the night's lowest. Clouds and humidity alter this by changing how energy from the sun and from the Earth escapes.

More cloud cover lowers daytime temperatures because clouds reflect or scatter some of the incoming solar radiation, so the surface receives less energy. At night, clouds trap outgoing longwave radiation, reducing cooling and keeping surface temperatures higher. Put together, this makes the day warmer than it would be with no clouds and the night warmer as well, narrowing the gap between the highest and lowest temperatures.

Humidity works in the same direction to shrink the diurnal range because water vapor is a greenhouse gas that traps heat, especially after sunset. While its daytime effect isn’t as strong as clouds in reducing the maximum temperature, the overall effect across a 24-hour cycle is to lessen how far temperatures swing from day to night.

So, higher cloud cover and higher humidity both tend to decrease the diurnal temperature range by cooling the day and warming the night relative to clear, dry conditions.