Predicting Temperature at Altitude

[DriCamp]

Hello, I’m new to BPL. I recently planned a dayhike to the summit of Mt San Jacinto, noted that the nearby city of Idyllwild (around 5K feet) was predicted to have a noon temp of 58F the next day, so did two calculations for temp at the peak, which is under 11K ft. I expected to peak around noon. Used multipliers of 3.6F per 1000 ft (enviro lapse rate) and 5.4F (dry adiabatic lapse rate) which estimated 36F to 26F at the peak.

That’s cold for So Cal, so I packed a fleece, a down vest, and a watch cap–all of which sat in my pack as ballast all day long! I hiked all day comfortably in a LS tee and a wind jacket. Took a thermometer with me, and at the peak it read 60F in shade.

Not only were my calcs off, they caused me to heavy up my daypack! What am I overlooking with my prediction?

The day started with ascent through misty cloud, had rained the night before. Was shaded and moist to the peak, at which point the high clouds moved off and the sun was shining brightly onto low clouds below. Windy at the top, but nowhere near the freezing temps I’d calculated.

How can I make a better prediction next time? And not carry so much warm gear if it’s not needed?

I read https://backpackinglight.com/forums/topic/25964/ but didn’t post there because I don’t know BPL norms for resurrecting threads.

[DriCamp]

And while the question is hanging, I’ll add this: I’m using “Saturated Adiabatic Lapse Rate” of 2.7F if raining, “Environmental Lapse Rate” of 3.6F if cloudy, and “Dry Adiabatic Lapse Rate” of 5.4F if clear and sunny. Does that sound right?

[Jon Fong / Flat Cat Gear]

Aren’t there a lot more variables like windspeed, barometric pressure, and a host of other variables that would prediction difficult?  My 2 cents.

[Ben H.]

Great topic!… I don’t have the answers :( I once got zapped the other way. I took my 5 year old on his first backpacking trip along the Sespe river in early January (a while ago… he’s 14 now). The weather station on the ridge was mid to upper forties all week with no change in weather predicted. I figured the river bed 500′ down has got to be warmer than that. What I learned is that the sun hardly ever gets down to the river. It’s shielded by the ridges. I woke up the next morning very cold with the Sespe frozen over. We would have been fine at 30F. We had to have hit the low 20’s if not the teens. Checked the weather station 500′ up hit a minimum of 46F.

[Todd T]

You expected 58F below…what did it turn out to be?  Maybe the whole forecast was off, not (just) your calculations?  On the other hand…inversions do happen.

[Dan]

I find that the NWS actually has forecasts for high elevations if you go directly to their website and click around on the map. Forecasts are not limited to cities and towns.

[DriCamp]

Dan: Maybe that’s the answer…let someone else (NWS) do the predicting; perhaps a good prediction requires more variables than I can reasonably gather. Great tip. To wit:

https://forecast.weather.gov/MapClick.php?lat=33.8145&lon=-116.6792

Interestingly the forecast is for 8.6K feet, but that would have told me what I needed to know!

Todd: At noon I was on the mountain without cell reception, but when I descended in the evening, the actual evening temps for Idyllwild matched the evening predictions, so maybe the noon prediction was reasonably accurate, too. I don’t know what an inversion “looks” like, but there were distinctly two layers of clouds, one around 4-5K (which I drove and then hiked through) and another somewhere above 11K. Of course, if that’s indicative of an inversion, I wouldn’t have seen it from below.

[Dan]

I don’t know how NWS decides to localize their forecasts, maybe they coincide with stations, which can be anywhere. I was wandering around off-trail in a southern Colorado wilderness area last year, and saw something in my peripheral vision. On closer inspection, I found this. Maybe a weather station? I don’t really know, but it was well above 11,000′ for sure.

[Rex Sanders]

Recently I stumbled upon a commercial web site that compares U.S. weather forecasts from many sources, private and NWS. Shocked to discover that the “best” high and low temperature forecasts were within a reasonable range (+/-3 F?) about 2/3 of the time. Averaged across big cities. With a long record of weather stats.

I know from living on the left coast that forecasts are not that good without hundreds of stations upwind. Luckily, the weather in most of California is kind of boring most of the time.

San Jacinto weather can be full of surprises.

Adiabatic lapse rates are useful, sometimes, under the right conditions. Plus taking into account global climate weirding.

Weather stations pop up in unexpected places, like random inaccessible ridge-tops in California’s second largest state park. Not all of them feed into NWS.

— Rex

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