Daytime Cloudiness and Temperature

The reporting site is evaluated for the times 15Z, 16Z, 17Z and 18Z to determine if two or more of these times report at least broken clouds (75% cloud cover). When this condition is met, clouds are said to have held down day time temperature (since 18Z is near midday in the United States).

Clouds reflect the radiation from the sun, and as a result, less energy is available during cloudy periods to warm the surface of the earth and the lower portions of the atmosphere. Clouds do not appear instantaneously and also tend to persist for periods of an hour or more. Thus, when 3/4 or more of the sky is reported to have clouds at two or more of the above times, reduced midday temperatures can be anticipated.