What’s the most likely time of day for the daily maximum temperature?

Actually, it depends on a lot of things, including how clouds build up over the course of the day. But would you be likely to guess midnight?

In Fairbanks, Alaska at this time of year the likeliest time of day for the temperature to reach its warmest (or coldest) is midnight. Why?

There are a lot of temperature cycles. The seasonal cycle is obvious — warm in summer, cold in winter. Likewise the diurnal cycle: warmest when the sun is heating the ground; coldest when it is not. There is also an irregular cycle of several days or more, when large-scale winds blow first from the north and then from the south, bringing cold or warm air.

A figure from my thesis, showing how the hour-by-hour temperature (middle line) changed over the time period from Nov 24 through Dec 1.

In mid-Alaska in winter, the daily cycle is very weak. If you take the average temperature for a particular hour of the day, there is a slight warming a little after solar noon, but it is only a degree or two. A single day between about December 1 and January 12 rarely has a discernible temperature rise in the daytime. I actually checked this back when I was preparing my M.S. thesis on ice fog, and determined that during this time period the temperature changes were controlled almost entirely by thermal radiation, with sudden warming when clouds or warm air aloft came in and abrupt cooling when the sky cleared or cold air arrived. Not too surprising, as the sun this time of year is never more than 3 ½ ° above the horizon.

That leaves the seasonal cycle, which is slow, and the irregular variations due to clouds and warm or cold air advection, which can be very large – 100°F over a period of 3 to 4 days, in the extreme. Daily maximum and minimum temperatures are primarily controlled by these longer variations rather than by the very slight solar heating around midday.

Suppose it is warming up, as was happening around November 30 in the graph. The daily minimum for November 30 was at midnight at the beginning of the day. The daily maximum was also at midnight, but at the end of the day. This maximum was also the minimum for the next day, December 1. The opposite would be true if there were a cooling trend. It all happens because maximum and minimum daily temperatures are defined for a 24-hour period from midnight to midnight.

In most places this affects only the minimum, which tends to be just before dawn. In wintertime Fairbanks the effect is enough to completely decouple the daily maximum and minimum temperatures from the solar cycle.

About these ads