Seasonal and Diurnal Variations in Air Temperature

Seasonal temperature patterns are affected principally by latitude, large water bodies, and the general circulation patterns. The latitude effect is due to the angle at which the suns rays strike the earth (see Radiation). In general, the seasonal variation of temperatures near the surface is least in equatorial regions, where there is little difference in solar heating through the year. This seasonal variation increases with latitude to both polar regions, where summer days have a maximum of 24 hours of sunshine and winter days a maximum of 24 hours of darkness. Large water bodies moderate the seasonal temperature cycle because of their great heat capacity. In one area, the general circulation pattern may produce cloudy weather with successive influxes of cold air, and thus a reduction in the monthly or seasonal temperature. In another area, the same pattern may produce opposite effects.

The diurnal temperature variation depends upon all of the factors we have discussed so far. The normal daily pattern at an inland location with level terrain consists of a daily temperature range of 20-30° F near the surface, with the highest temperature in midafternoon and the lowest temperature just after sunrise. This diurnal temperature range decreases with altitude above the surface. Various factors alter this pattern.

• A primary factor is the character of the surface. In general, those surfaces that become warmest during the day also become coldest at night, and the air temperature above them also has a high daily range. Snow surfaces are an important exception.

• Clouds, strong winds, high humidity, and atmospheric instability lower the maximum temperature and raise the minimum temperature, thereby reducing the daily temperature range.

• In mountainous terrain, one finds a greater diurnal variation in temperature in the valleys, and less along the slopes (in the thermal belt) and at higher elevations. Aspect affects the solar radiation and therefore the diurnal temperature range and the time of maxima and minima. Maxima will occur earlier, for example, on east slopes than on west slopes. Even minor shape characteristics of topography have their effects. Concave areas will have a larger daily range than convex areas.

• Large water bodies tend to moderate the daily temperature variation just as they moderate the seasonal variation. Coastal areas have a marine, rather than continental, climate.

Diurnal changes in temperature take place within the limitations of air-mass temperature. The passage of a front, evidence that another air mass has moved into the area, is reflected in the temperature pattern. Temperatures drop when a cold air mass moves in, and rise when a warm air mass moves in. In some cases the diurnal pattern is completely obscured. The temperature may continue to fall throughout the day when a very cold air mass moves in rapidly, or may continue to rise throughout the night when a warm air mass moves in. Along the west coast during the summer, a cool, marine air mass is usually found at low levels, and a warm, dry air mass is usually found above. A change in the vertical height of the boundary layer between these two air masses will appear in the temperature patterns along the slopes.