What can you say about the temperature for the coming days if you know todays temperature? How good is the memory of temperature? More precisely: if today is warmer or colder than average for this time of year, what is the chance that a coming day is also warmer or colder respectively? It turns out that temperature has a memory of about a week, slightly longer in winter than in summer, and slightly longer at the coast than inland.

Warm and Cold Days 

We take the time series of the average temperature in De Bilt during the period 1991-2020 and determine for each day whether it is warmer or colder than average for this time of year. We will refer to these as warm and cold days. Figure 1 shows the cold and warm days for 2005 as an example. Next, we look for each warm or cold day (‘day 0’) whether a following day (‘day 1’, or ‘day 2’, etc.) is also warm or cold respectively. With only knowledge of the climate, the chance of a warm or cold day is simply 50 percent. The question is how much greater the chance is because temperature has a memory.