Clouds, as beautiful as they can be, are nothing more than water vapor suspended in the air. Their formation requires the presence of adequate humidity in air that is cooled to its dew point; further cooling leads to precipitation, in the form of rain or snow. When this process occurs near the ground, fog develops; when it occurs higher in the atmosphere, we see clouds.
Humid air is less dense than dry air and cannot get as hot or as cold. As a result, we find the highest and lowest temperatures in areas where the humidity is very low, i.e. deserts and the polar regions. Humidity thus moderates air temperature but clouds generally have different effects on our surface temperature in the summer and winter. During the warmer months, clouds reflect the sunlight, shade the ground and provide a cooling effect at ground level; if the humidity is very high, producing a high heat index, this cooling effect will be less noticeable. In contrast, clouds tend to warm the surface air during the winter, when the sun angle is low and the solar heating effect is reduced; in this case, clouds provide a blanket effect, reflecting radiant heat from the ground back toward the surface. Such cloud blankets are especially noticeable on cold, winter nights; in dry areas of the West, where clouds tend to dissipate through the evening, overnight lows often plummet into the teens and single digits.
This morning dawned clear as a bell, not a cloud in the sky. The temperature was 21 degrees!
