On some days there is a large difference between high and low temperatures. On other days the difference is relatively small.
Why?
Some locations in our area have a large change between day and night, while others do not.
Why is there such a difference?
Consider the temperatures in Olympia, Washington this past week. Two days ago, the temperature rose to 85 degrees and then dropped to a chilly 49 degrees Fahrenheit — a 36-degree swing (see below), but today it only rose to 72 degrees Fahrenheit.
Not far away in Hoquiam, near the WA coast, the situation was completely different. Most days had a very small temperature range (or daily variation) with highs around 65F and lows in the upper 50s. But one day was completely different, with temperatures rising from 85 to 56. Why?
Moving east of the Cascades, Pasco in the Tri-Cities had a day that reached 108 degrees, with a low of 63 degrees. The temperature range was 45 degrees F.
But if you REALLY want to experience concrete-cracking daily temperature swings, where should you go? This map of typical July daytime temperatures gives you an idea:
Eastern Oregon is world class. Consider Baker Oregon (below). One day the high was 109 and the low was 58. Temperatures hovered around 51F. That’s great.
How can we account for all the differences in daily temperature ranges?
It has a lot to do with humidity – the amount of water vapor in the air. Clouds have a big impact. And so does the influence of the sea.
The effect of the ocean is very mild. The surface of the water has a lot of thermal mass and does not change temperature rapidly. Here in the Pacific Northwest, the summer is quite cool (usually in the low 50s), so the coastal area does not warm up much during the day and does not cool much below the water at night.
Clouds also limit diurnal temperature changes by reflecting the sun’s warm rays during the day and blocking the cooling effect at night (clouds act as blankets). Over the past week, coastal areas have had a lot of clouds and maritime influence (see today’s satellite image). No wonder the temperature range is so small! But one day, there was an easterly (easterly) wind blocking the maritime influence — the temperature range was large at Hoquiam that day.
But why does eastern Oregon and much of the interior Southwest have such a wide range of temperatures? Yes, they’re protected from the cool ocean by the Cascades. But they have a secret weapon: dry air.
Water vapor is very, very active in the infrared part of the spectrum. It can absorb infrared radiation emitted from warm surfaces and re-emit it back to the ground. In this way, it acts like an atmospheric blanket.
You think CO2 acts like an atmospheric blanket? Water vapor is even better. Desert areas are notoriously cool at night…contributing to the large diurnal range. Remember how it wasn’t cool in western Washington a few days ago when it was so wet and sticky?
There are a number of other factors that affect the daily temperature range that I could mention… like the urban heat island effect… but I’ll save that for another time.
