Click
here to get to the group web page
Cloud cover and precipitation
First, cloud cover can affect climate by reflecting solar radiation back up and out towards space. If there is less solar radiation the temperatures will be cooler. Clouds can also raise temperatures at night though. Clouds have an insulating effect to the heat radiating out of the earth at night. This is partly why areas with more cloud cover usually have less of a fluctuation between day and night time temperatures. As everybody knows, rain comes from clouds, so its not surprising that there seems to be a general correlation between the number of cloudy days at a given station and the number of days with precipitation.
If you look at the table you
will notice that the farther south the stations are located the less the
number of cloudy days. This is because in the northwest moisture laden
air is brought in off the pacific by prevailing westerly winds. When the
winds reach the mountains the air is forced to rise in elevation. With
the higher elevation and cooler temperatures the air can hold less moisture
so it starts to condense as clouds and rain. The southern part of California
however, is located near the thirty-degree latitude line. At this area
there is a wind cycle that circulates the air straight towards the ground.
If the air can not rise clouds can not be formed.
Mean # of Days Yearly:
| City | Clear | Partly Cloudy | Cloudy | Precipitation.01 inches or more |
| Quillayute, WA | 51.2 | 75 | 239 | 208.1 |
| Olympia, WA | 49.5 | 76.5 | 239.2 | 161.6 |
| Seattle, WA | 57.8 | 81.6 | 225.5 | 154.2 |
| Yakima, WA | 109.2 | 92.4 | 163.6 | 69.2 |
| Spokane, WA | 86.4 | 87.5 | 191.4 | 112.1 |
| Portland, OR | 68.4 | 74.5 | 222.4 | 150.9 |
| Astoria, OR | 52.3 | 84.4 | 228.5 | 190.3 |
| Salem, OR | 76.7 | 80.3 | 208.2 | 145.7 |
| Eugene, OR | 75.2 | 81.5 | 208.5 | 136.9 |
| Medford, OR | 117.6 | 78.5 | 169.2 | 100.7 |
| Pendelton, OR | 101.3 | 90.9 | 173.1 | 97.7 |
| Eureka, CA | 78.4 | 98.7 | 188.1 | 117.7 |
| Sacremento, CA | 188.7 | 75.7 | 100.8 | 57.7 |
| Stockton, CA | 184.4 | 76.8 | 104.3 | 51.6 |
| San Fran, CA | 180.1 | 100.4 | 104.8 | 62.5 |
| Fresno, CA | 194.3 | 73.1 | 97.9 | 44.8 |
| Bakersfield, CA | 191.5 | 81 | 92.8 | 37.1 |
| Santa Maria, CA | 175.6 | 109.7 | 79.9 | 45.8 |
| Long Beach, CA | 158.6 | 119.2 | 87.3 | 32.2 |
| Los Angelos, CA | 146.9 | 115.7 | 102.5 | 35.4 |
| San Diego, CA | 146.3 | 116.7 | 102.2 | 42.2 |