What was done
Land-based observations of cloud cover for three regions of the United States (coastal southwest, coastal northeast, and southern plains) were examined in relation to observations of global temperature and the strengths of three permanent atmospheric pressure systems (North Pacific high, Icelandic low, and Azores high) over the period 1900-1987.

What was learned
For the three regions studied, cloud cover was found to have a high correlation with global temperature, with mean cloud cover rising from an initial value of 35% to a final value of 47% while mean global temperature rose by 0.5°C from 1900 to 1987.

What it means
The relationship between cloud cover and temperature demonstrated in this study suggests that Earth's hydrologic cycle tends to moderate the thermal effects of any impetus for warming, as increased cloud cover has been shown to decrease the diurnal temperature range.  With smaller daily swings in air temperature, plants and animals are thus exposed to a less variable environment to which they can more readily adapt.