A thermograph record, which takes hourly air temperature readings throughout a 24-hour period, shows that there is a rhythmic rise and fall of air temperature during a day. This phenomenon is called the daily cycle of temperature.
Sometimes it is also called the diurnal march of temperature. The difference between the highest and lowest values of temperature recorded at a locality is referred to as the diurnal range (or daily range).
The temperature curve for a cloudless day shows one low point about sunrise and another high point in mid-afternoon or late afternoon. After attaining a maximum between 2 P.M. and 4 or 5 PM, the temperature then begins to fall until sunrise the following day.
After reaching a minimum about sunrise, the temperature curve climbs until late afternoon.