Dear Tom,
When does the warmest temperature on a given day usually occur?

Len Whorlass, Lake Geneva, Wis.

Dear Len,

On average, Chicago experiences its highest maximum temperature around 3:30pm. This is true for most loctions in the Midwest. Chicago weather, as changeable as it is, does have some averages that are quite reliable and can be depended upon to produce fairly regular results. The time of occurrence of daily maximum temperatures is one of them.

It should be stressed though, that 3:30pm is only an average, and it occurs at that time mainly in the summer when the time of max heating is largely dependent on incoming solar radiation. In July, a day’s maximum temperature is reached between 2pm and 5pm about seven days out of ten. During winter, the time of max temperature is less influenced by solar heating and more dependent on air mass regime which is why it’s not too uncommon for a day’s high temp to be observed in the middle of the night. In January it is only three days out of ten that the day’s high is reached between 2pm and 5 pm.