I received one question asking if there was some sort of formula or rule of thumb to know, for example, if it's 75 in Seattle but I want to go camping at 5,000 feet, what can I expect for a temperature up there?
There's no slam dunk answer -- mountain weather conditions can typically be their own beast since topography can create its own little microclimates. (For example, it's more likely to rain in the mountains when the air is unstable and we have just expect "scattered showers" in the lowlands.)
But, there are a few gauges you can at least try to get an initial feel for the temperature. On a normal, run of the mill partly cloudy day, you might use the "Environmental Lapse Rate", which is the rate under standard atmospheric conditions. That roughly equates to a drop of 3.5 degrees for every 1,000 feet you go up in altitude, up to about 35,000 feet. If it's a sunny, dry day, the lapse rate is a little greater, and if it's completely overcast and rainy, it's a little lower (about 2.7 degrees/1,000 feet).
But like I said, there are many more factors involved. For example, an inversion could mean it's actually warmer at Snoqualmie Pass than Seattle. On those hot, dusty east wind days, the mountains get a warming wind from Eastern Washington and it can be just as hot at a few thousand feet as it is in Seattle.
One easy place to see current temperatures up there is at this link from the Northwest Avalanche Center
It shows, for example, that on Tuesday morning in Olympia, it was 62 degrees. But at White Pass at 4,480 feet, it was 61, and it was 58 at 5,780 feet. So the lapse rate isn't really working well there today, as we had a bit of an inversion this morning.
The Simple Way To Get Forecasted Temperatures:
You can always just check this site from the National Weather Service, which has a simple mountain forecast.
The Complicated, Daring Way, But Perhaps More Helpful
But, if you want to be really daring -- and I mean Really daring, you can try this site from the Storm Chase machine at Northern Illinois University:
(A word of warning, this is about to get complicated. Unless your a real weather geek, or up for a challenge, this might be beyond what you need.)
Anyway, if you're going to take the plunge, go to that Web site and leave most of these buttons in their default location, but step 2, you can choose how far out you want the forecast to be in hours. It defaults to 12, maybe try it at 24 or 36 hours, but be sure to notice what time the forecast is valid for. (On the final chart, it'll say at the top, and "Z" time is 7 hours ahead of PDT and 8 hours ahead of PST, so 12Z Wednesday is 5 a.m. Wednesday in Seattle.)
Under Step 4: change the location to KSMP -- that's Stampede Pass, and a good general gauge for the Cascades.
Then say "Go make the sounding"
What you get is a forecasted "sounding" -- basically guessing what a weather balloon would find if it were launched from that location at that time.
There's lots of squiggly lines on this, but the only one to be really concerned with for hiking purposes are the two vertical lines in the middle of the page -- the left one is dew point ,the right one is temperature. So the right one is the one you want. The scale on the bottom is temperature in Celsius (roughly double and add 30, or exactly: multiply by 1.8 and add 32) and the scale on the left is altitude in meters (multiply by 3.1 to get feet)
Here is one from today:
So, if you want to see what the forecasted temperature at roughly 4,500 feet, find the 1,500 meter level on the left, follow that to the right until you reach the bar on the vertical bar on the right.
Now the hard part. Those numbers on the bottom are the temperature in Celsius, but you don't follow it straight up. Instead see those angled blue lines -- that's the temperature legend line. So once you've reached the vertical line on the right, you follow it back down and left diagonally along the blue line until you reach the number at the bottom.
(See! Told you it was confusing. Welcome to our world of wacky weather charts.)
We found the height we wanted (1487m), followed it to the right (the red line) until we hit the vertical bar on the right. Then followed that back down and left along those slanted blue lines until it hit the temperature on the bottom.
So in this example, that corresponds to roughly 8 degrees C, or 46 degrees. I know, complicated. But if you're lost, feel free to e-mail me. Again, this is just a gauge. You can also just check the National Weather Service mountain forecast :)
What Do We Mean By Freezing Level?
A reported "freezing" level is the altitude where it is forecasted to remain below 32 degrees for the entire forecasted time period. Thus, even though there are some mornings where you have frost on your lawn, we might report a freezing level of 4,000 feet since that is where it's supposed to stay at freezing all day.
A "snow" level is the altitude at which anything higher is expected to receive their precipitation as snow. It's slightly lower than a freezing level (around 500 feet) because it can snow at a couple degrees warmer than 32.