View Full Version : Undercast conditions
01-19-2011, 02:02 PM
I have been thinking about it lately... what kind of weather conditions can cause the mountain to be undercast? And can this vary enough to make some of the smaller mountains to be undercast as well.
Good question - because if we non-weather folk knew the answer we would know when to keep hiking to get great pictures.
01-19-2011, 06:41 PM
You need high moisture in the lower levels (below mountain tops) and drier air aloft.
If you got good at reading model soundings you could forecast when it will happen. Soundings are a vertical profile of the atmosphere's temperature, humidity and wind speed.
01-20-2011, 04:44 PM
Are the soundings something that are on the webpage or can they be found online somewhere else? Or do we have to build our own weather observatory?
01-20-2011, 05:16 PM
The key thing to look for to predict and undercast is a temperature inversion along with ample moisture. They are more frequent and predictable in the winter but with that being said they are still hard to predict. The main reason is around here (and most parts of the country) model soundings are all you can rely on, which aren't the most relyable sources but adequate for big picture stuff. If you are at a university or near a Met. office (NWS) that does soundings, you have a better chance at predicting them.
They can occur at all levels but in the White Mountains, they usually average between 2500 and 5000 feet. But occasionally they drop just low enought that we are the only peak above the clouds and other times they drop low enough that almost every summit in NH is exposed while valleys are covered. They vary greatly. More information on temperature inversions can be found here: http://en.wikipedia.org/wiki/Inversion_(meteorology) I was going to link to an NWS site that has a better explanation but the server is currently down. But you can also google "temperature inversions" and get some more sites.
As far as soundings go, model soundings can be found here: http://weather.niu.edu/machine/fcstsound.html or by using google and searching "model soundings". Most forecasters are taught to use Bufkit which is available for Berlin, NH (which is our closest model package) here: http://www.erh.noaa.gov/gyx/wxgraphix3.html. If you want other stations, you will have to google them and get those seperate packages. If you are looking for soundings from actual balloon launches at NWS, they are here: http://w1.spc.woc.noaa.gov/exper/soundings/. You are going to have to do some research on your own on how to read these soundings since it takes semesters worth of instruction to really know what you are looking for. But for undercast conditions, you want to focus on how to look for, identify, and read inversions.
Good luck and have fun.
01-20-2011, 08:46 PM
The main take away should be that forecasting an undercast is very difficult. Even under ideal conditions it might not happen, or the cloud deck will be too high. The big problem is that there isn't much of a market for undercast forecasts...for obvious reasons.
With that said you could become proficient with the right conditions, model soundings and your own intuition to become an expert at forecasting them. There are plenty of resources on the web to teach yourself about soundings and you can ask any questions on the forum for more help. There are many trained meteorologists here.
01-21-2011, 05:21 PM
A subsidence inversion always seemed like the best indicator. Subsidence inversions are formed when high pressure builds aloft and air sinks which causes adiabatic warming and thus a temperature inversion. As others have mentioned there must be a high amount of moisture in the lower levels of the atmosphere. You can look at the forecast soundings and see the inversion develop well above mountaintop level, strengthen as it descends, and effectively cap any rising air below mountain top level.
01-21-2011, 07:57 PM
But what is the kicker? Subsidence inversions happen all the time (like every night in the summer during periods of fair weather) and the vast majority don't produce an undercast. And when does a valley fog event become an undercast or vice versa...and what's the difference?
It would be cool to re-analyze those pirate ship days when only Mount Washington and only a few other ships were sailing on the high seas.
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