Common sense tells us that temperatures usually warm during the day and cool at night. I say “usually” because in meteorology there are some exceptions to that general rule. Many factors come into play such as cloud cover, wind speed and direction, snow pack, length of day or night, warm air advection, and cold air advection. This article will explain these factors in more detail as to how they affect temperatures.
It’s real easy to picture those cold, crisp, and bitter nights during the winter when you can look up at the sky and see the bright stars. The lesser clouds there are at night, the cooler it is. Clouds act like a blanket at night and trap some heat from radiating into the atmosphere. When there are clear skies, however, very efficient radiational cooling can occur. Cooling is often slowed down at night where there are windy conditions because winds mix up the atmosphere. Heat therefore cannot radiate straight up into the atmosphere. The air will remain mixed. Snow pack also speeds up cooling at night. Finally, the longer the night, the more potential exists for an area to cool. In conclusion, the perfect conditions for colder nights are clear skies (no clouds to act like a blanket), calm winds (no mixing), and snow pack depending on the time of year.
Everything is really flipped around when talking about warming during the day. Whereas clouds keep temperatures warmer at night, they block out the sun’s radiation during the day and keep temperatures cooler. For efficient heating, there has to be ample sunshine. The sun’s angle will also play a role in how fast an area warms up. The higher the sun angle is, then the faster the warming process. For optimal heating during the day, winds need to stay calm. Once again, calm winds provide for minimal mixing of the air. The sun’s radiation warms the ground, and then the heat rises up and warms the air. Like at night, snow pack during the day leads to cooler conditions. The longer the day is, the more potential exists for warming. In conclusion, the perfect conditions for maximum warming are sunny skies, calm winds, and a high sun angle.
There are a couple processes which I have not talked about yet that affect heating and cooling. The first process is called warm air advection. Warm air advection can occur during the day or night and usually occurs after the passage of a warm front when winds shift to the south. This can actually lead to rising temperatures during the middle of the night! This is because strong southerly winds “advect” in warmer air to prevent cooling.
The last process that I’m going to cover is called cold air advection. This process can occur at any time and usually occurs after the passage of a cold front with winds shift to the north. Many times throughout the year, forecasts will call for falling temperatures through the afternoon. That is due to cold air being “advected” into an area on the backside of a cold front.
Temperature forecasts can be rather difficult at times. As you can see, there are many processes that meteorologists must look at when making a forecast.