Elevation and Temperature: Tricky Relationship
By Heidi Sonen & Roscoe Shaw
The day before Thanksgiving, a foot of snow fell at Wintergreen but hardly a dusting stuck to the ground in warmer Crozet. Elevation obviously plays a huge role with temperature, but it’s not simple. Heidi and I recall first year meteorology classes where we learned the basics in gory mathematical detail.
The riddle to unravel is, “If cold air sinks, then why is it colder in the mountains?”
This is not easily explained and not always the case. Let’s start with the “cold air sinks” part. Yes, cold air is heavier. Many people mistakenly believe that hot, humid air is “heavy.” If that were the case, hot air balloons would be “cold air balloons” instead and have freezers on them, not heaters.
So, on a clear, calm morning, cold air will settle downhill into the valleys in a process called “cold air drainage.” Often, Crozet might be 10 degrees colder than the top of Buck’s Elbow. So, it’s not always colder high in the mountains
Cold air drainage is like pouring Hershey’s Chocolate into milk. The chocolate settles to the bottom. When the wind blows, it is the same as stirring the chocolate and milk together. But if you sit the glass down, eventually, the chocolate will settle back to the bottom.
The second and more complicated part of this explanation is that pressure and temperature are proportional. When you go uphill, there is less air above you. The air on top of you weighs less and so the pressure is lower. Because temperature is proportional to pressure, lower pressure means lower temperature. This basic relationship was discovered by Robert Boyle in 1662.
One strange result of this is that when you fly in an airplane, the bitter cold air outside, often 30 below zero, must be air conditioned to make the cabin temperature comfortable. The air that is 30 below zero outside has almost no air pressure. When you pressurize it and bring it indoors, it would be about 190 degrees! The increased pressure causes a dramatic increase in temperature.
All these factors come into play when forecasting winter storms here in Virginia. Cold dense air can get stuck in the valleys causing freezing rain. Other times, like this Thanksgiving, the air up in the mountains is much colder due to the lower pressure. Modern computer weather models do a nice job of calculating the temperature at different levels and the evolution of the temperature profiles as storms pass.
Heidi and I love to play “Guess the Temperature” when we drive somewhere through the mountains. We do the math in our heads in great detail. It’s what nerdy meteorologists do. Usually, despite our supposed expertise, the kid shouts a number from the back seat and wins the contest.
November Recap
November was another cold month in a cold wave that has now lasted most of the past year and a half. The low of 13 on November 19 was astonishingly cold for this early in the season. Overall, the month averaged 43 degrees, which was 4.5 degrees below normal.
The first half of the month was dry, but three wet days late brought the total for the month to a respectable 3.19 inches.
Rainfall:
- Crozet Mint Springs 3.19”
- Old Trail 3.35”
- Greenwood 2.79”
- Univ. of Va. 3.95”
- Waynesboro 2.33”
- White Hall 3.04”
- CHO Airport 2.79”
- Nellysford 3.70”
