It tends to be earlier in winter and later in summer — Page 62, Lesson 58

I want to walk you through the daily temperature cycle and what drives it. We start with a key fact: the Earth is receiving more solar radiation during the day than it is giving up as terrestrial radiation — that's the longwave heat the Earth emits back. That imbalance doesn't last all day. A balance between incoming solar radiation and outgoing terrestrial radiation is reached, on average, at 1500 local time. That's the moment when we can expect maximum temperatures. Now, a note on that timing: the actual time of maximum temperature varies with latitude and with the time of year. It tends to be earlier in winter and later in summer. But 1500 local time is a good average for temperate latitudes — that's the rule of thumb you'll use. From 1500 onwards, the temperature falls continuously until a little after sunrise. The lowest temperature of the day occurs at about sunrise plus 30 minutes. At that point, once again, we get a balance between incoming and outgoing radiation — but this time it's the minimum, not the maximum. Let's talk about what we call the Diurnal Variation, often abbreviated DV. Diurnal Variation is the difference between the daily maximum and minimum temperature. It is greatest with clear skies and little wind — those conditions let the sun heat the surface strongly by day and let the Earth radiate heat away freely at night. DV varies with a number of factors, but in temperate latitudes it is about plus or minus 6 degrees about the mean. So if the mean temperature for the day is, say, 15°C, the maximum might be around 21°C and the minimum around 9°C. Now, cloud cover by day changes that picture. By day, some of the solar radiation is reflected back by the cloud tops. That means less solar energy reaches the surface, so the maximum temperature — which we call T Max — is reduced. So a cloudy day will have a lower peak temperature than a clear day under the same conditions. I've covered the key points: the radiation balance at 1500 local time giving maximum temperature, the minimum at sunrise plus 30 minutes, the definition and typical value of Diurnal Variation, and how cloud cover reduces T Max by day.