PurposeThere was a thread in the forums recently asking if there was a way to determine the elevation of cloud layers in the mountains(current and/or forecast). Generally speaking, yes, there is. In fact, you can determine more than just cloud information. You can find wind and temperature information also.
Hopefully this article will give you some basic techniques to head for the mountains perhaps a little more prepared for the weather that awaits you. However, I caution you that Mother Nature is unpredictable, weather forecast models are not 100% accurate, and mountains do indeed form their own micro-environments at times. So use this information to plan, but as always, be prepared for the worst.
Obtaining an Upper Air Sounding
Go to http://rucsoundings.noaa.gov/ to reach the page in the image above. We will use this interface to obtain an upper air sounding for a location.
You can use this interface to obtain the most recent sounding or forecast soundings for locations in the U.S. Unfortunately the forecast soundings time period does not go out very far on this site. But, the most recent soundings are reliable and they have a handy Java function we will use later.
To obtain a current sounding, select RAOBs in the initial data source block. Leave Start Time at Latest. Number of hours doesn't matter (unless you are trying to get forecasts). Then enter the identifier for the location you are trying to retrieve. You can find location identifiers at http://www.airnav.com/airports/. Generally speaking, any place with an airport will have an identifier. Finally, obtain the sounding by clicking Java-based plots.
Making Sense of an Upper Air Sounding
The image above is a plot of an upper air sounding for Great Falls, MT. This is a representation of the state of the atmosphere displayed in the vertical. The information used to make this plot is the raw information obtained from the release of a weather balloon.
At the top of the image we can determine that this sounding is for Great Falls, MT (TFX), and it is from 7 Jun 2006 at 1200 GMT. Note this info is repeated in the lower left.
Along the left margin you can see the pressure level in millibars. Note the pressure decreasing with height. Pressure levels generally correspond to certain heights but will vary with atmospheric conditions. There are some basic reference points though, as follows:
925mb - 2,000 ft MSL
850mb - 5,000 ft MSL
700mb - 10,000 ft MSL
500mb - 18,000 ft MSL
400mb - 24,000 ft MSL
You can also see heights generally corresponding to these pressure levels on the right side of the chart above the label Press Alt (Kft).
The thick red line and thick blue line in the middle of the chart are the temperature and dewpoint traces (red is temp, blue is dewpoint). These are plotted using the thin red lines oriented from bottom left to top right of the chart and correspondingly labeled with red numbers near the bottom of the chart in degrees Celcius. So for instance in this sounding, the temperature at 600mb is 0 Celcius and the dewpoint is -8 C.
Along the right side of the chart is a series of lines with other lines and pendants sprouting off of them. These are the winds at altitude. The tail of the wind line will point to the direction the wind is blowing from. On the sounding up is north, down is south, right is east, left is west. The barbs and pendants protruding from the wind line represent the speed. A half barb (line) is 5 knots, a full barb is 10 knots and a pendant (triangle) is 50 knots. On the example chart above, the wind at 15,000 ft is from 250 degrees (WSW) at 30 knots. Bear in mind these are steady state winds and will not include gusts.
Finding Cloud Layers
Once you obtain the sounding, you can click inside the image to activate Java tools. These are the lovely lines and numbers in the image above showing the height (mbs and ft) and the temp, dewpoint and winds at the level where you are holding your mouse pointer.
To identify cloud layers on the sounding, your first visual clue should be finding areas where the temp and dewpoint lines come close together. Clouds usually occur in areas where the dewpoint depression (the difference between the temp and dewpoint) is 5 degrees F (2-3 degrees C) or less. This also corresponds to a relative humidity of 70% or more. So to define cloud layers, moving vertically from bottom to top, we want to identify areas where the RH first exceeds 70% to determine the cloud base, then, proceeding vertically, goes back below 70% to determine the top of the cloud layer.
In the image above the base of a cloud layer has been identified at 15,974 feet, where the temperature is -4.9 C.
In the above image the top of the cloud layer has been identified at 19,094 feet, where the RH has dropped below 70% to 66%. The temp at this level is -11.9 C and the winds are from 255 degrees at 39 knots.
Above is a sounding for UIL, Quillayute, Washington from 7 Jun 2006 at 1200Z. Can you find the cloud layers?
The base of the first layer of clouds is at 3133 feet. Temp 4.8 C, RH 94%. Probably raining. Anything over 90% generally indicates precipitation.
The top of the first cloud layer is at 4,304 feet. The RH is down to 61%.
Proceeding upward, we find another cloud layer with bases at 20,981 feet. RH at 71%, temperature is -22.5 C.
Proceeding ever upward, we find the top of this second cloud layer at 23,816 feet. RH is still at 71% but drops sharply above.
Here is the surface observation from Quillayute around that time.
KUIL 071153Z AUTO 00000KT 10SM SCT030 10/08 A3007 RMK AO2 SLP182 T01000083 10122 20100 52001 TSNO
The observation reports scattered clouds (3 to 4 eighths of the sky covered) at 3,000 feet (SCT030). Pretty much what our analysis of the sounding told us. Note that it does not report the higher cloud layer we found. There is a reason for this. Notice this station says it is an automated station (AUTO). I can tell by looking at the remarks section (RMK) that this automated system is an AO2 system. Thus only equipped with cloud detectors that reach only to 12,000 feet. So this station is not capable of detecting the clouds we detected on the sounding up near 20,000 feet.
How Do I Use This?So how can you put your new found skills to use?
You can use a current sounding (if your excursion is happening soon and quickly) or a forecast sounding (for future, longer excursions) to get an idea of what conditions on your mountain of choice may be like.
The following site http://weather.admin.niu.edu/machine/fcstsound.html can be used to access forecast soundings. It is a little more user-friendly and reliable than the RUC site, but it doesn't have the Java tools. But you don't need those because you have skills now.
So you get to the site.
And follow the steps.
Step one, pick a model. I like NAM or GFS. Some models may be right some days and wrong others. Compare a couple. If you see two alike, they are probably right.
Step two, choose a forecast time. This is trial and error. You usually just have to poke, then look at the first image you get then adjust your time to what you really want.
Step three, decide data output. Leave it on Skew-T diagram.
Step four, decide on Metar Site. Pick a place close to where you are going to climb. For this example, our burly climber is headed up to do the Upper Exum on the Grand Teton. So, he has wisely chosen Jackson, WY to look at.
Step five, background color. Whatever floats your boat. White is easier to read.
So here is the forecast sounding for Jackson, WY valid at 1200GMT on Friday 9 June. Notice the sounding doesn't even start until around 750mbs or roughly 7,500 feet, this due to the high elevation at the Jackson Airport, so we know we are in the right place.
It doesn't do us much good to look above 600mb, or roughly 14,000 feet, since the Grand is 13,770 feet. So, proceeding up to 600mb, I don't see any areas of dewpoint depression less than 2 to 3 degrees Celcius, so it should be relatively cloud free. There are some higher clouds up around 400mb, but not of concern to us.
I also see a surface temp of around 9 C with winds from the WSW around 15 knots. It looks like summit temp will be about -2 C with steady state winds from the WSW around 20 knots. All in all, not a bad day to be trying to negotiate that step across at the end of Wall Street.