Why is it hottest a couple hours after noon? Why is it coldest a couple hours after dawn?
At any given point on earth, solar noon is when the sun is most directly overhead. So why is it not also the hottest part of the day? In the absence of weather fronts to influence the temperature, the hottest part of the day is typically a couple of hours after noon.
In a similar fashion, the coldest part of the day should be right before dawn but it's not. In the absence of weather fronts, the coldest part of the day is an hour or two after sunrise. Why is that?
The hottest part of the day is later in the day
When the sun rises, it starts to heat the atmosphere. This heat is cumulative and builds throughout the day. The surface of the earth also absorbs some of that heat which influences the atmosphere near the ground. Even though solar noon is when the sun is most directly overhead, heat is still building up in the atmosphere after noon passes. Because of this, the hottest part of the day tends to be a couple hours after noon.
A similar concept is also at play when it comes to the seasons. Even though the start of summer occurs at the summer solstice near the end of June, the hottest part of the year tends to be around the end of July. This is because heat is still accumulating in the atmosphere even though the point where the sun is most direct has passed.
The coldest part of the day is just after sunrise
While heat accumulates during the day, it dissipates after the sun sets. Additionally, the heat that the surface of the earth has absorbed dissipates. While heat in the atmosphere has dissipated as much as possible just before sunrise, heat in the ground is still dissipating. If the temperature has fallen enough, dew or fog is common in the morning. The change of state from vapor to liquid requires heat absorption that can also influence the atmospheric temperature. This, combined with other atmospheric factors causes the temperature to drop just after the sun has risen.
Why this matters in engine tuning
Changes in temperature are just one factor that influences air density changes and density altitude changes. If you combine changes as a result of weather fronts bringing wind or moisture, tracking weather changes is important to engine tuning. Maintaining an ideal air/fuel ratio is key to getting the most power and consistency out of your engine.
This is a very basic explanation of what's going on. If you would like to learn more about this concept, please read the sources listed below.