Actually, it depends upon a couple of factors and as well as the time of day.
For example, when you're looking at the seven-day planner, we're predicting next Tuesday will have a high of 81 degrees and a low of 50.
Those numbers will likely change over the next week, but we get those numbers from a program called Computer Model Guidance. The model can predict the highs and lows for each day from 16 days out.
A number of factors play into the model's prediction. It depends on the air mass and several qualities in the air. Those factors include air pressure, density, and moisture in the air. The jet stream also plays a factor. It drives how warm and how cold it will be each day.
The time of day also plays a factor. We can predict how warm and cold it will be at any given hour, but time can be critical. For example, the warmest part of the day is around 3 o'clock in the afternoon, and the coldest is right before sunrise.