In previous blogs, we've noted how there are a few ingredients that affect temperatures. We're all familiar with the affects of sunshine and its resultant heating. In meteorological terms we like to call this the "diurnal cycle", since it corresponds to the daily sunrise and sunset cycle. Here's a graph of what we're talking about:
|Hourly temperature graph for Deer Park, WA.|
This shows the temperature at Deer Park, Washington for an "average" winter day, one that wasn't affected by clouds, precipitation, wind, etc. With time starting on the left, the temperature warms during the day shortly after sunrise. The temperature then reaches it peak around 3pm after continuous solar heating. The temperature then rapidly plummets after sunset, continuing to cool through the remainder of the night until the sun rises again and the cycle repeats itself. Sunrise and sunset for this time of year are around 730 am and 400 pm.
On the past few nights, we've seen some temperatures that haven't "behaved" like the average diurnal cycle. The image below shows Deer Park on the 7th through the 9th.
|Hourly temperatures for Deer Park, WA 12/7-12/9 2013|
On the morning of the 7th (left part of the graph), temperatures started off around -5°F, and then gradually warmed to 13°F in the afternoon under sunny skies. And then the temperature quickly plummeted back to 0°F by 6pm. But then a strange thing happened; the temperature warmed overnight, and actually reached 13°F again at sunrise on the 9th. In other words, the temperature at 8 am on the 9th was the same as during the previous afternoon. How could this happen?
Let's take a step back and discuss how the sun actually warms us. The suns rays don't warm the air directly. Instead, the suns rays largely pass through our atmosphere and reach the earth's surface, where it is absorbed by the ground, roads, buildings, etc. This energy then warms the air above it.
|Earth-Atmosphere Energy Balance|
After the sun sets, there is no incoming solar energy. But the earth's surface still radiates it's energy (i.e. it cools). If the skies are clear and the air is dry, this process is very efficient, and the cooling is rapid. But if the skies are cloudy, the warmth that is radiated by the earth is trapped, and this acts to warm the air, even at night.
|Nighttime heat radiation|
A similar event took place last night. Here's the temperature trace at Deer Park again, this time for the 10th and 11th.
As before, temperatures reached their high around 3pm (25°F on Tuesday), cooled quickly to 10°F by 7:30pm, and then warmed overnight. All of this warming could be solely attributed to the cloud cover. There is no "warm wind", warm front, or anything like that.
All of this leads to the question: In the winter, at our latitude, what's better, a sunny day and clear night, or a cloudy day and night? If you're not a fan of cold temperatures, then as nice as a sunny day is, it's probably better to hope for the clouds. The prevalent low clouds and fog typically found in the Inland Northwest during the winter may seem dreary at times. But it keeps our temperatures on average much milder than other locations at our latitude (e.g. Great Falls, Duluth, and Caribou, ME). The clouds have the most apparent impact on our low temperatures. See the chart below for details.