### #MWOMetMonday

2015-11-09 21:08:18.000 – Kaitlyn O’Brien, Co-Director of Summit Operations

We received a great question from one of our fans for today’s #MWOMetMonday!

Sometimes under certain atmospheric conditions, we actually report a warmer temperature on the summit than the valley. Why does that happen?

That seems a little odd, considering that the relationship between temperature and pressure is directly proportional. (Yikes! What does that even mean?)

Let’s take a closer look at what’s known as the ideal gas law:

PV = nRT

P = pressure, V= volume, n= amount of gas, R = gas constant, T = temperature

All that is necessary to take away from this is the fact that the P and the T are on opposite sides of the equation. If the pressure decreases, that means the temperature must decrease too. If the pressure increases, that means the temperature must increase. This is what’s known as a directly proportional relationship.

How does that relate to the air on the Rockpile versus the air in the valley?

If you start at the base of Mount Washington, and work your way up to the summit, the pressure will slowly decrease as you ascend the mountain. Under standard atmospheric conditions, the temperature will decrease as well, gradually becoming colder as you approach the summit.

But sometimes, we see temperature differences of 10 to 15 degrees warmer up top! When the temperature increases with height, that is an indication of an atmospheric inversion. Inversions are quite common and are easily spotted when looking at our Mount Washington Auto Road vertical temperature profile. In fact, there is an inversion present this evening:

On the left hand side, we can see that at the 4000’ site earlier tonight, the temperature was 47.5°F while the base site (1600’) was way colder at 33°F!

But what causes inversions to form?

There are several different reasons why an atmospheric inversion could occur. One of the more popular “flavors” of inversions is nocturnal inversions. Nocturnal temperature inversions are strongest on calm, clear, and cool nights. They generally begin to set up after sunset and are a result of radiational cooling of Earth’s surface. As the ground releases the day’s built up energy in the form of heat (known as longwave radiation), the air just above Earth’s surface begins to cool rapidly. Because air is a poor conductor of heat, these cooler temperatures at the surface are not transferred to the higher areas of the atmosphere without wind. So with calm conditions in place, there is little to no mixing, thus allowing the atmosphere to become very striated or layered, with distinct temperature differences noted across each layer. As Earth’s surface continues to release more longwave radiation, the air just above the surface becomes colder, creating an even stronger temperature inversion.

Sometimes sunrise is a great visual aid to see the effects of a strong nocturnal inversion that has developed the night before. The image below, taken in August of 2014, shows what I call a “pancake sun”. The layer of warm air at the inversion level below the summit is distorting the view of the sun as it rises, causing it to appear as flat as a pancake!

Kaitlyn O’Brien, Co-Director of Summit Operations

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