Overnight lows are usually achieved in the early morning hours when many of us are headed to work and school. That’s why it’s important to get them right, so you’re prepared for what awaits you outside your front door.
First, let’s establish something some of you may already know, some may not, and that’s how the cloud cover impacts the daytime highs. With the absence of cloud cover, we warm significantly allowing the daytime temperature to rise. With thicker cloud cover, it can act as a shield from that solar heat and keep us from warming.
There are other factors at play here. You can pull in colder air during the day even with full sun and that can keep us colder. Warmer air can also warm the surface which can help raise temperatures under the clouds or heat us up even more under a sunny sky. Those are aspects of the environment that meteorologists have to look at on a regular basis.
Now let’s examine how clouds impact the overnight lows. With the absence of clouds, the previous day’s heating is allowed to escape into space which allows the surface to cool faster. We call this radiational cooling. A deck of thick cloud cover can keep that warmth from escaping. Quite often that warmth radiates from the earth and bounces back to the surface.
This is why you can have morning lows in the mid-20s in one county and the 30s in another. Clouds may have thinned out or broken apart and allow heat to escape/
Just like in the daytime, wind and the direction it comes from can also impact those temperatures. Warm or cold air blowing in can certainly raise or lower morning temperatures whether there is cloud cover or not. Typically a northerly wind will keep the air cooler and a southerly wind will keep the air warmer.