When we look at weather forecasts, one of the most noticeable parts will be the forecast of the maximum and minimum temperatures. But what time in a day is usually the air temperature the highest and when is the lowest?

On a typical sunny day, when the sun reaches the highest position at around noon, the amount of incoming solar radiation is also the greatest. Should it be the time with the highest temperature in a day?

Actually not. Among the received solar radiation, only a small portion is directly absorbed by the atmosphere and the rest is mostly absorbed by the land and sea. After the ground absorbs solar radiation, it then heats up a shallow layer of air above it by conduction, which in turn heats up the air further above by convection and radiation. As the whole process takes time, the daily maximum temperature usually occurs in the afternoon. In other words, the changes in air temperature lag behind the solar radiation variation.

In simple terms, if we ignore the complex weather factors, while the heat energy of the atmosphere is continuously radiating out to the outer space, the temperature will keep rising as long as the incoming energy exceeds the outgoing energy. The temperature peaks when the two reach a state of equilibrium, then it starts to fall until sunrise while the outgoing energy exceeds the incoming energy. Thus, the daily minimum temperature usually occurs at around sunrise. When solar radiation returns after sunrise, the temperature will go up again. (Figure 1)

The occurrence times of the daily maximum and minimum temperatures in a place may also vary due to other factors such as seasons, the weather of the day, topography and the surrounding environment. In high density urban area, the rate of temperature rise (fall) due to heat absorption (dissipation) is usually slower than that of the rural area, so the daily maximum temperature in the urban area generally appears later than that in the rural area. On average, the temperatures peak at around 3 pm at the Hong Kong Observatory Headquarters in urban Kowloon in summer, about one hour later than at Ta Kwu Ling in the rural northern New Territories (Figure 2). The elevation angle of the sun will change with seasons and geographical locations, which will also affect the occurrence times of the daily maximum and minimum temperatures. Furthermore, certain weather conditions or changes (such as thunderstorm and heavy rain, passage of a cold front, subsiding air ahead of a tropical cyclone) may cause the daily maximum and minimum temperatures to occur at other times in a day.