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Category Archives: Seasons
Why is fog more common in the early morning?
Fog can occur any time of day throughout the year. In southern Wisconsin, fog is most frequent in the morning, between about 6 and 8 a.m.
Fog formation is the result of two primary processes. First, the air is cooled to the dew point, and the relative humidity reaches 100%. When the air temperature is the same as the dew point temperature, condensation of the atmospheric water vapor occurs forming fog droplets. Another method of fog formation requires water to evaporate from the surface into the air, increasing the relative humidity until condensation occurs. Continue reading
When will we stop seeing 80F this year?
At the time of this writing, Madison is in its 6th straight day of temperatures at or above 80F and it is likely we will have one more to reach seven in a row. As recently as 2022, we have … Continue reading
Has it been unusually rainy so far this summer?
As we head into the last week of August, it is of interest to review aspects of this summer’s precipitation in Madison.
First of all, every one of the three traditional summer months has been wetter than normal with June, July, and August (through Aug 20) coming in at 0.26”, 1.55” and 2.02” above normal for a total surplus of nearly 4”. Continue reading
How does this summer dew point temperature compare with previous years?
The dew point temperature is the temperature to which the air must be cooled, at constant pressure, to get dew to form. As the grass and other objects near the ground cool and reach the dew point temperature, some of the water vapor in the atmosphere will condense into liquid water on the objects. While the dew point temperature is a temperature, it is also a measure of the humidity of the air. The higher the dew point temperature, the greater the amount of water vapor in the air.
To know how close the air is to saturation, we need to know the dew point and the air temperature. The closer the dew point is to the air temperature, the closer the air is to saturation. When the dew point equals the air temperature, the air is saturated, so the dew point temperature cannot be greater than the air temperature. Continue reading
Category: Climate, Meteorology, Seasons
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How far has summer beaten back the areal extent of cold air?
On more than one occasion in this column we have commented on the areal extent of air colder than 23 degrees Fahrenheit at 850 hPa (about 1 mile above the ground) as a measure of the extremity of winter.
In the middle of January, about 68 million square kilometers of the Northern Hemisphere are covered by air that cold at that level. Between about July 5 and July 20 that area shrinks to zero, and the complete absence of such air lasts only a very few days. At the beginning of August, we are just past the point in this summer that this area is beginning to increase again. Thus, despite the fact that we have been through a couple of really hot (and humid) weeks to end July, the return of winter has already begun in terms of this measure. Continue reading
Category: Meteorology, Seasons
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