How accurate is the 3.5 - 5 degree F temperature drop per 1000 feet rule of thumb?
Related Q (especially its link to inversion): What is the relationship between altitude gain and temperature decrease when mountaineering?
This question is concerning the commonly used rule of thumb that an increase in elevation of 1000 feet will, on average, decrease the temperature by 3.3 to 5 degrees F (3.3 for moist air, 5 for dry). Some sources differ slightly in the exact numbers, but 3.3 and 5 are what I have seen most often.
TL;DR
I have read that local conditions on a mountain can trump this rule of thumb. Trying to understand this better, I found some examples of temperature increasing going up. I am wondering if this is a common occurrence or a rare one.
How closely can we trust this rule of thumb? Said another way: How often does reality deviate sufficiently from this rule of thumb to reduce its usefulness?
My specific example
I camped on Cascade Mountain in the Adirondacks at about 1500 to 2000 feet (460 to 610 meters) higher elevation than nearby Lake Placid. The temperature at Lake Placid was 37 F (3 C). One of the forest preserve stewards told me the next morning that she expected the temperature on the mountain to be below freezing.
This is not a huge elevation difference and perhaps not a great sample, but I am hoping to use it to help understand the conditions I was in and better prepare for future.
The mountain was wet below the tree line, so I would go with the lower change estimate.
Temp difference: 3.3 x 1.5 = 4.95 = approximately 5 F and 3.3 * 2 = 6.6 F, so...
Estimated temp: 37 - 5 = 32 F to 37 - 6.6 = 30.4 F
Estimate: 30 to 32 degrees F at my elevation. 27 to 29 F (about -1 to -3 C) if we use the 5 degrees per 1000 feet estimate.
Since it did not feel below freezing and I never noticed anything frozen or frosted, am I correct to assume that the rule of thumb was not accurate in our case and that it was not actually at or below freezing? I still am not sure how often the inaccuracies/inversions actually happen and am wondering if I can assume that it was the case in my situation.
In other words: what is the likelihood that the rule of thumb is not accurate? Is the likelihood high enough that we should always take the estimate with a grain (or more) of salt, or is the likelihood low enough that I should trust the estimate more than my gut based on how I feel?
This post was sourced from https://outdoors.stackexchange.com/q/19703. It is licensed under CC BY-SA 4.0.
1 answer
TLDR: The rate is accurate unless you have an inversion, in which case it is completely inaccurate as the temperature will actually increase above a certain altitude.
With increasing height, air temperature within the troposphere and mesosphere drops uniformly with altitude at a rate of approximately 6.5 degrees Celsius per 1000 meters - known as the environmental lapse rate. However, sometimes this normal overall decline is reversed. A point or layer at which the temperature increases with height is called inversion or inversion layer.
Temperature inversions frequently occur in anticyclones, but are also common in depressions when air in the middle troposphere subsides.
Given the right conditions, the normal vertical temperature gradient is inverted such that the air is colder near the surface of the Earth. This can occur when, for example, a warmer, less-dense air mass moves over a cooler, denser air mass. This type of inversion occurs in the vicinity of warm fronts, and also in areas of oceanic upwelling such as along the California coast in the United States. With sufficient humidity in the cooler layer, fog is typically present below the inversion cap. An inversion is also produced whenever radiation from the surface of the earth exceeds the amount of radiation received from the sun, which commonly occurs at night, or during the winter when the angle of the sun is very low in the sky. This effect is virtually confined to land regions as the ocean retains heat far longer. In the polar regions during winter, inversions are nearly always present over land.
At Fairbanks the inversion strength peaked on Saturday morning, right under the ridge axis, with a temperature difference of 60°F between the surface (-19°F) and 3000' above ground (+41°F). This is essentially a tie with the strongest inversion ever measured by radiosonde in Fairbanks; the record inversion was just 0.1°C stronger in December 1956. Here's the so-called skew-T diagram.
Deep Cold: Interior and Northern Alaska Weather & Climate
Do note that it is normal for temperature to decrease with an increase in altitude, so most of the time the estimated rate will be correct.
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