Mountain Weather
This page, like weather everywhere, is constantly under construction.
Images are courtesy of WeatherTAP.
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May 8th, 2001, a sattelite image.
An unusual day across the American Southwest...mostly clear everywhere!
Notice how the snow-capped mountains and dark green forests stand out in
Colorado.
This
is a large image (> 1
Mb) so it may take awhile to load on a dial-up connection.
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Snow cover in the Colorado mountains reaches its peak in March.
"Upslopes"
Upslope conditions occasionally occur, whereby moist winds from the east bump
up against the 12,000 foot wall
of the Rockies, causing rainy/snowy, foggy and generally miserable conditions
in the foothills but nice weather west
of the Condinental Divide. Upslopes are fairly common and are sometimes very
obvious on satellite images.
| Upslopes can happen in winter, as well. In March of 2003, the east slope got a record snowfall, with more than seven feet of snow falling at relatively low altitudes on the east side of the Front Ranges because a low pressure system stayed put for 36 hours, pumping enormous amounts of moisture against the Front Range. This animated image shows the action over a brief period of time. |
"The
Monsoon"
| At sometime during most summers--most frequently from mid-July to mid-August--a plume of moisture-laden air moves up into Colorado from the south, either from the Pacific or the Gulf. This is usually driven by a high pressure center to our east. The net result is beautiful clear mornings, and dark, rumbly afternoon thunderstorms which begin sometime after 3 p.m. Dewpoints in the mountains in June are frequently in the mid-twenties, and such low dewpoints usually indicate clear days. But when the monsoon arrives in July, dewpoints rise into the mid-thirties to forties. With so much (relatively) moisture in the air, the air becomes unstable in the mountains, and thunderstorms form. A good rule-of-thumb for hikers is that if cumulus clouds are forming over the high peaks by 10 a.m., you can expect a good chance of rain by mid-afternoon. Below is a series of screen shots from radar, taken on a day when the monsoon was very active. |
Typical July morning.
By noon, the storms are already brewing.
Corresponding noon lightning strikes...
And the mountains across the valley at noon...cumulus buildups already promise
afternoon storms.
and by 5 p.m.....sure enough...
it's raining again!
5 pm. lightning
July 8th, 2006: Radar image of a classic monsoon. This one brought the first
all-day rain since November '05.
Dew point, humidity and thunderstorms: summer
Many people who visit from the East don't understand how dry it is in Colorado, even in the mountains. They are surpsised sometimes to see sagebrush growing in open areas, and even cactii growing in relatively high places.
It's important for hikers to be able to read what's happening with weather. Being caught above timberline in a thunderstorm is quite litterally a life-threatening experience. There was a story in the paper awhile ago about an entire herd of elk being found dead above timberline; apparently all were killed by a single lightning strike!
In order for summer thunderstorms to form, at least two conditions have to be met: 1) the temperature aloft must be sufficiently cool and 2) there must be sufficient moisture. However, a high pressure center, centered on Colorado, can obviate thunderstorms for the day. Sometimes, I swear, it's a crap shoot!!
In the morning, the sun heats the surface. The surface heats the air immediately above it, and the air, being warmer and hence less dense, begins to rise. As it rises, it expands, since air pressure aloft is less. Because the air expands, it cools. The air will continue to rise as long as the rising air is warmer than the air surrounding it. If temperature equalization occurs, the air stops rising.
If the rising air cools to the dew point (the dew point is the temperature at which the air becomes saturated, when the relative humidity of the air is 100%), the moisture in the air begins to condense. The process of condensation releases heat. That heat warms the air a bit as it rises, helping it to continue to rise. This is the process which fuels those huge, billowing cumulonimbus thunderstorm clouds. The cloud may rise as high as 60,000 feet, bur rarely does over the mountains. Nevertheless, as budding thunderstorms move over the very warm eastern plains, they may continue to grow into real monsters.
In my limited experience, I can say that if dewpoints are in the 20's in summer, thunderstorms are unlikely. The probability of afternoon thunderstorms increases as dewpoints rise into the 30's. Dewpoints in the 40's usually guarantee afternoon and evening thunderstorms. But not always. I guess that's why we pay meteorologists! <grin>
The low humidities also present interesting challenges for food storage. Bread dries out rapidly. And brown sugar, if not vigorously protected, will turn into a rock-hard brick. On the other hand, containers holding cold drinks do not "sweat" like they do back East.
AUTUMN:
Mountain wave clouds:
Much as water tumbles and rises as it goes over rocks in a mountain stream, the atmosphere does the same as it crosses the Rockies. When conditions are right, stationary mountain wave clouds form. The satellite shot, below, shows a stationary wave just to the east of the Rockies. We could see the western edge of the clouds from our deck (picture).
The western edge of the cloud.
The Front Range, in yellow, with a mountain wave cloud to the east.