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2006-05-06 15:03:40.000 – Jim Salge,  Observer

Fair weather clouds above the snowfields…

An unfamiliar sound woke me from a deep sleep this afternoon, and honestly in my delirious state it took me some time to figure out that the cog railway had made its first trip to the summit this season! The summer season is truly here!

The picture at right shows the clouds that have been above the slopes of Mount Washington the past few days. Though these clouds have been nothing more than summer-like stratocumulus clouds, they tell a lot about the state of the atmosphere. Here’s fair-weather clouds 101:

First off, the presence of these clouds tell us that there is a very sharp contrast in temperature between the valleys and that at the level of the clouds. To verify this, we merely check our “Auto Road Vertical Temperature Profile” located in the weather page. At the time of the picture was taken, the valley temps were in the mid 70s, and the summit was in the low 40s. Over 30 degrees in 5000 feet a very sharp change indeed!

This temperature gradient allows the warm air in the valley to rise due a process called instability. Simply, warm air weights less than cold and will rise. However, as it rises, it slowly cools, until it condenses and forms the clouds. Instability however is the same process that forms massive thunderstorms, so something else must be limiting the growth of these clouds. That ‘factor’ is the high pressure overhead, constantly knocking the clouds back down.

Perhaps the neatest thing about these clouds is what happens to them right before sunset. Once the valleys cool off in the evening, the air stops rising, and the sky instantly clears!

For hikers and mountaineers, these are the type of clouds that you want to see while in the mountains in the summer. There is a definite reason why these are called ‘fair-weather’ clouds, and usually indicate that fair weather will hold through the day, without a risk of storms.

Jim Salge,  Observer