Pleasant Winter Days on the Summit

By Karl Philippoff

The summit of Mount Washington is known to be the “Home of the World’s Worst Weather” due to high winds, fog, and cold temperatures. However, after a particularly rare day earlier in February, I received a media inquiry about how unusual it is to have relatively pleasant weather conditions on the summit during meteorological winter (December – February), the time when the summit usually experiences its stormiest weather with respect to winds and fog.

Now what constitutes a ‘pleasant’ weather day during the middle of winter can certainly change depending on the context, especially what weather one is used to. For example, one of my family members who lives in Hawaii would probably not find any day on Mount Washington during the winter ‘pleasant,’ as temperatures in the 60s are now considered ‘chilly.’ But I thought using a broader definition, with respect to Mount Washington, might provide allow these benign days to be put in their proper context.

For this exercise, I primarily used data on sunshine minutes, temperature, and two flavors of wind speed to characterize a tranquil day as they are relatively easily searchable within our database and provide a workable answer to the question. Since we only began recording sunshine minutes since January of 1944, this did limit the range of data from which to pull from, but not significantly. The criteria I ended up using were 1) a peak daily wind gust below 45 mph, 2) an average daily wind speed below 20 mph, 3) temperatures above 5F above zero, and 4) sunshine minutes making up more than 80% of possible sunshine minutes during a particular day. These criteria fit well with winter averages in the following ways: 1)The daily peak gust criterion is roughly what our average wind speed is between December and February, and so would apply only to much lower than average wind days, and 2) The average temperature criterion is roughly the summit’s average daily temperature in January and February (and slightly below the summit’s average daily temperature in December). I decided that the sun and wind criteria (seen in the table below) were more important for overall comfort levels for outdoor recreators than the temperature being greatly above average, as bright, sunny days are often perceived as warmer in most contexts, and that the winds and temperatures measured on the summit are usually the coldest and windiest conditions experienced anywhere on the mountain on any given day which would not be what recreators experience if they were only ascending to the top of Tuckerman or Huntington Ravine.  In addition to the above criteria, I could have subset them to only be during daylight hours, but for a quick dive into our records, the daily criteria seemed to be adequate. And finally, I also decided to vary the to be more or less generous on what a ‘pleasant’ day might look like to see how much each individual criteria would change the number of days included.

Other than the temperature criterion, there will be about 7-8 days that meet the peak wind and average speed criteria laid out above, with just over two weeks of sunny days (>80 % of possible sunshine minutes) throughout meteorological winter. However, the coincidence of sunny, relatively calm, and warm days is, not surprisingly, very low, only about 1-3 days per winter season, and relatively insensitive to the exact criteria used with the difference from the most generous (Temperatures > 5F, Sunshine percentage > 70%, Average wind speed < 30 mph, the peak wind gust < 45 mph) to the most restrictive (Temperatures > 15 F, Sunshine percentage > 80%, Average Wind speed < 20 mph, and Peak wind gust < 35 mph) only varying by about 2 days per winter season.

As one further side note, this research did highlight a particularly remarkable stretch of calm weather on the summit between January 31^st, 2005 and February 6^th, 2005 when there were 3 consecutive days and 5 out of 7 days that met the above criteria (Temperature > 5 F, Sunshine > 80%, Average wind speed < 20 mph, Peak wind gust <45 mph) when the summit was sitting under a particularly large and broad high pressure system that minimized pressure gradients and provided sunny conditions for days at a time.