How Temperature Fluctuates Throughout the Day

In your weather forecast, high and low temperatures tell you how warm and cool the air will be during a 24-hour period. The daily maximum temperature, or high, describes how warm you can expect the air to be, usually from 7 a.m. to 7 p.m. The daily minimum temperature, or low, tells how much the air is expected to cool, usually overnight from 7 p.m. to 7 a.m.  

High Temperatures Don't Happen at High Noon

There's a common misconception that high temperatures occur at high noon when the sun is at its highest elevation. However, this is not the case. 

Just as the hottest days of summer don't happen until after the summer solstice, high temperatures don't usually happen until the late afternoon — typically 3 to 4 p.m. local time. By this time, the sun's heat has built up since noon and more heat is present at the surface than is leaving it. After 3 to 4 p.m., the sun sits low enough in the sky for the amount of outgoing heat to be greater than that incoming, and so temperatures begin to cool. 

How Late at Night Do Lows Happen?

Just how long after 3 to 4 p.m. will temperatures be at their coolest? 

While you can typically expect the air temperature to drop as the evening and nighttime hours wear on, the lowest temperatures don't tend to happen until just before sunrise. 

This can be quite confusing, especially since the low is often listed along with the word "tonight." To help make it a little clearer, consider this. Let's say you check the weather for Sunday and see a high of 50°F (10°C) and a low of 33°F (1°C). The 33 degrees that's displayed is the lowest temperature that'll occur between 7 p.m. Sunday evening and 7 a.m. Monday morning.

Highs Don't Always Happen During the Day, Nor Lows at Night

We've talked about the times of day when high and low temperatures occur 90% of the time, but it's important to also know there are exceptions to this. 

As backward as it sounds, sometimes the high temperature for the day won't actually happen until late in the evening or overnight. And likewise, the low can happen during midday. In winter, for example, a weather system may move into an area and its warm front sweep across a region late in the day. But by the start of the next day, the system's cold front then enters and sends the mercury dropping throughout the daytime hours. (If you've ever noticed a downward-facing arrow next to the high temperature in your weather forecast, this is what it means.)