Looking eastward into Salt Lake Valley on a snowy morning.
I’ve lived in a mountain valley for a few months now, and I’ve recently started noticing an interesting weather phenomena; I begin my morning commutes on the valley floor, where it can be rather chilly well into the late-morning hours. However, by the time I make it up to my office at the top of the valley, I notice it usually feels warmer. At first I thought maybe it had to do with surface heating, but it seemed a bit unlikely that the ground would warm by that much after a mere 10 minute drive. Instead what I’ve been experiencing is a phenomenon that’s known as a valley cold air pool, or (VCP), for short.
The effects of nighttime radiative cooling on the environment. Source: Hong Kong Observatory
Essentially, longwave radiation after sunset carries warm air from the surface up into space, cooling the surface and lowest layers of the atmosphere. This process is accelerated on nights with clear skies given that there’s even less of a barrier stopping the radiation from escaping out into space. As a result, the pre-dawn hours of the day tend to also be the coldest, and it’s not until sunrise that the re-introduction of solar radiation allows for temperatures to bounce back up. And while this process is fairly straight-forward in areas like the Great Plains, they tend to get a little more interesting once we get into more mountainous areas.
In places like the Rockies, the radiative cooling processes that occur at night also come into contact with topographical hurdles, including high elevation and valleys. Given that cold air is denser than warm air, air that cools at higher elevations will become heavy and naturally sink to lower elevations. In cities like Salt Lake and Portland, this sinking patterns leads to cold air descending from the mountains and into a valley floor that’s already radiatively cooling. By pre-dawn hours, the end-result is colder temperatures within the valley when compared to temperatures along its ring. And this is exactly what I’ve been experiencing since moving to this kind of environment!
Certain areas of the country experience more notable cold pools than others, and are therefore studied in greater depth by atmospheric scientists. In some cases, research has been done that tries to measure the height of cold pools and even their evolution, as has been the case by researchers up in Mt. Washington, New Hampshire, and down in the Yampa valley of northwestern Colorado.
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© 2019 Meteorologist Gerardo Diaz Jr.