Spring across the Higher Summits
2017-03-26 17:30:59.000 – Adam Gill, Weather Observer/IT Specialist
Spring at high elevations will always come at a later time than in the valleys. The summit of Mount Washington will still have snow on it after many of the plants in the valley have started to green up. There are several factors as to why that is!
First of all, being at a higher elevation, the temperature in the atmosphere cools with height. This is known as a lapse rate and the average lapse rate is 6.5°C per kilometer or about 3.5°F per 1,000 feet, and can be as much as 5°F per 1,000 feet! The summit sits about 5,000 feet above the base of the autoroad and almost 6000 feet above North Conway so we will usually be between 15-30°F colder than the valleys! As you can imagine, this cooling with elevation will cause us to see an early winter and a late start to spring and summer. So when it is 60°F in the valley, make sure to check out the summit conditions page to make sure that you are properly prepared for all the weather you will encounter.
Another reason is because warming in the upper atmosphere is delayed and takes a little bit for the warming surface to mix to higher elevations. Below are a few charts depicting the annual temperatures at 925mb(~1,500 feet), 850mb(~5,000 feet), and 700mb(~11,000 feet). In February, the average temperature begins to increase at 925mb because it is closest to the ground and will be influenced by solar heating first. At 850mb, it does not see a significant increase in temperature until late in February into early March. Finally, at 700mb, the warm up does not really get going until mid-March.
Due to every winter being slightly different, I do not expect to see significant warming across the higher summits until the snow in the valley is all melted. Currently, some of the solar radiation during the day is being reflected back into space from the snow cover and much of the remaining energy is going into melting the snow. Once all the snow is melted, then that solar radiation will go into warming the air and ground and spring really starts to get going in the valley. Eventually that heat will make it up to summit level and start melting the snow. We can still see warm temperatures at higher elevations during strong storms that bring warm air quickly from the south before it has time to cool off but as a whole, we will be spending much of our time below freezing until the snow pack really dwindles over New England.
A few more interesting tidbits that come along with season change is the wind that we see up here. Below is an image of the annual variation at 850mb and during the winter season of October to May for us the winds are fairly strong but June to September has a more significant lull allowing for the Cog and Autoroad to open to visitors.
Adam Gill, Weather Observer/IT Specialist