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Page 2
Financial Statement:
Surface Radiation Balance: Savings in the financial statement increases when Deposits exceed Withdrawals and decrease when Withdrawals are greater. Similarly, Storage in the above surface radiation balance equation determines whether the surface warms or cools and thus provides a measure of the temperature of the surface and the air near it. When more heat is gained than lost, there is warming. When more heat is lost, there is cooling. Here is a graph of an idealized daily surface radiation balance with the daytime solar radiation curve also plotted. Solar radiation begins flowing after dawn and ends shortly before sunset, reaching a maximum at solar noon (still assuming clear skies). The strength of that incoming solar radiation depends greatly on the solar season, which defines the sun's angle above the horizon, here the data are plotted for the Equinox at 45o N latitude in the Northern Hemisphere. Looking more deeply at the surface radiation balance curve, we can see that it is negative during the night hours indicating the surface is losing radiation and therefore cooling. In the late morning and afternoon, the curve is strongly positive indicating more incoming radiation than heat loss and thus the warming of the surface and its air. (Note this may not be the case in deep solar winter and at high latitudes when the sun's low midafternoon angle cannot overcome the surface heat loss at all during the day.) Somewhere during the early morning period, the radiation balance curve crosses the zero line (indicating a balance of gain and loss and therefore no temperature change) from negative to positive. Here is where the switch is made from nocturnal cooling to daytime heating. And what do we notice about this time (I have blown up this region below)? The time of switching does not correspond with dawn but begins an hour or so later. The reason the switch is not coincident with dawn is that during the first hour or so, the incoming solar energy does not yet have enough strength to reverse the heat loss from terrestrial radiation. (A similar condition arises later when solar energy becomes too weak to balance the heat loss in the hours before sunset.) As a result, the surface and lower atmosphere air are still losing heat for some time following sunrise and thus the coldest temperature of the day has yet to be reached. A general rule of thumb puts the time of coldest temperature about an hour after sunrise.
The copyright of the article Why Is It Coldest After Dawn? - Page 2 in Meteorology is owned by Keith C. Heidorn. Permission to republish Why Is It Coldest After Dawn? - Page 2 in print or online must be granted by the author in writing.
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