In summer, when hundreds of hikers summit daily, conditions are often mild, winds light, but thunderstorms with hail and gusty winds can develop rapidly and catch hikers and tourists off-guard. In winter, a popular time for skiers and mountaineers to hit the slopes, temperatures often drop below -30 degrees F, rime ice accumulates over 10 inches per hour and winds exceed hurricane force every two days and 100 mph every fourth day on average. The combination of extreme cold and winds in the winter produce wind chills between -60 and -100 degrees F, often the coldest observed in the world. Fog envelops the summit an average of 62 percent of the year, making it difficult to see your next step. Nearly 100 inches of precipitation (rain and snow liquid equivalent) falls on the summit on average each year, including an average 281 inches of snow. Researchers have long recognized that these variable and extreme conditions, combined with easy accessibility to the summit, make Mount Washington an ideal spot for studying the processes that create the weather and climate we experience, and in particular how mountains interact with the atmosphere to modify these processes.
It has long been a part of the mission of Mount Washington Observatory (MWO) to advance our understanding of mountain weather and climate on Mount Washington and the surrounding White Mountain region. Since the establishment of the Observatory in 1932, observers and staff have engaged in research spanning a wide range of topics that includes air pollution, studying how cold air pools and flows in mountain valleys at night, testing methods of measuring snowfall in high winds (still a formidable challenge), identifying the cloud properties that create icing on aircraft, understanding how clouds form and testing wind turbines and jet engines in extreme icing conditions. More recently in 2012, the Observatory continued its commitment to research by creating a Director of Research position shared with Plymouth State University’s (PSU) meteorology program. I have filled this role since 2012, actively working with MWO colleagues and PSU students. Research capacity was bolstered again in August 2018 with the creation of a Research Specialist Observer position. Taylor Regan (MS in mechanical engineering) fills this role and splits her time between taking observations, maintaining the summit facility and performing research with my team.
The biggest current research project at MWO is understanding the changing climate of Mount Washington and the surrounding region. Mount Washington and Mount Mansfield in Vermont, two representative high-elevation peaks, have witnessed warming trends that are much slower than surrounding low-elevation weather stations in the northeast US (~0.3 vs. ~0.6 degrees C per decade, respectively). What makes this observation intriguing is that most other mountain ranges around the world are experiencing warming rates faster at higher elevations than nearby lower elevations. Furthermore, nearly all climate model projections indicate higher elevations will warm faster during the 21st century. Why is the northeastern US bucking recent global trends and climate models?
The answer may lie in the variable air masses the higher elevations experience that the lower elevations do not. As early as 1941, MWO scientist Victor Conrad recognized that for part of the year the summit of Mount Washington experiences an air mass different from the air mass lowest to the ground, called the boundary layer. In 2005, MWO scientist Andrea Grant and her collaborators estimated that the summit is above the boundary layer, in an air mass called the free troposphere, roughly 50 percent of the year. The boundary layer and the free troposphere are two distinct air masses, often with large differences in temperature, moisture, and clouds. (If you have ever had the pleasure of hiking above treeline and noticed cloud tops below you, then you had hiked into the free troposphere.) It makes sense, then, that if the summit of Mount Washington spends roughly half of the year in an air mass different from the low elevations, this could cause a different warming rate—often referred to as elevation-dependent warming.
Dr. Adriana Bailey (researcher with the National Center for Atmospheric Research), Georgia Murray (Senior Researcher at the Appalachian Mountain Club) and I are the first to propose this idea and aim to put it to the test. A key first step is to develop a robust method to diagnose the type of air mass at the summit at any given time. From other research, it is evident that no single meteorological variable measured on a mountain will always tell you the air mass type. However, several variables are good indicators some of the time, and combining these variables may help piece together a more complete timeline of summit air mass type.
In 2016, the research team tested the idea that several weather variables must be measured to state definitively what air mass is at the summit most of the time. In a project funded primarily by PSU, the team set out to capture the changes in boundary layer depth from prior to sunrise to just after sunset. During two days in August and September, they measured vertical profiles of water vapor at intervals along the Auto Road from the base to the summit by driving a water vapor analyzer up and down the mountain in a pick-up truck. Kelsey, Bailey and Murray performed these measurements about eight times on each of the two days. These along-slope measurements supplemented the continuous measurements of temperature, humidity and wind at the summit and at six sites along the Mount Washington Auto Road. Meanwhile, PSU meteorology students launched weather balloons from the base of the Auto Road to obtain temperature, humidity and wind profiles over the valley away from the direct influence of the mountain slopes.
This approach to determine boundary layer height worked—the team was able to use the along-slope data to determine with confidence when the boundary layer was at the summit. The most interesting result, however, occurred on August 19 when comparing the along-slope weather data to the free air data collected by the weather balloons. The height of the boundary layer over the valley was 400 feet lower than its height along Mount Washington during the morning. Why? Local effects caused by the mountain explain this difference. The wind at the summit was 30–40 mph from the west in the morning, which means that the wind was likely pushed upslope from a lower elevation to the summit. Indeed, the morning weather balloon temperature and humidity profiles confirm that the air at the summit originated from about 400 feet below the summit and that the air was from the boundary layer. The presence of a cloud covering the summit was also an indicator of upslope flow. In contrast, the weather balloon profiles indicated that free troposphere air over the valley was at the summit elevation.
Between 11:00 am and 1:00 pm, the summit wind decreased to only 5–15 mph and was not strong enough to push boundary layer air to the summit. As a consequence, the transition layer between the boundary layer and the free troposphere, called the entrainment zone, descended upon the summit. Because the lower free troposphere is associated with warmer and drier air, the summit temperature warmed a couple of degrees and the humidity varied wildly between near-saturation (boundary layer air) and very dry (free troposphere air). The period of time in the entrainment zone might have continued if it were not for the strong August sunshine warming and expanding the boundary layer upward past the summit elevation by about 400 feet between 1:00 and 5:00 pm. The observations suggest that for this entire period (11:00 am–5:00 pm), the boundary layer height on the mountain and over the valleys was about the same because of the light winds.
We are documenting dozens of cases each year when the summit is in the entrainment zone or the free troposphere—air masses that have different temperature, humidity, wind and cloud properties from the boundary layer. These cases support high elevation exposure to different air masses as a possible cause for the summit to be witnessing a slower warming trend than the surrounding low elevations that are always in the boundary layer. If true, the implications are varied and important: First, all mountains of a certain prominence above surrounding valleys may spend a significant amount of time above the boundary layer, and therefore have a different temperature trend than surrounding lower elevations. Second, the height of the boundary layer as measured by the twice-daily weather balloon launches at nearly 100 National Weather Service (NWS) offices cannot be used to infer the boundary layer height in nearby mountain ranges under many atmospheric patterns. Instead, mountains must be outfitted with a vertical profile of weather stations in order to determine definitively the height of the boundary layer on the mountain slopes. Third, since most poor air quality days in the White Mountains occur when the mountain slopes are in the free troposphere, future changes in the frequency of free troposphere exposure have important health consequences for humans and sensitive alpine ecosystems. Lastly, if the amount of time the summit spends in the boundary layer is reduced in the future, the summit could see its slow warming trend change. It would most likely result in faster warming given that the free troposphere is warming in general. This could have huge ramifications for the White Mountain region winter recreation industry, local mountain economies and the mountain ecosystems that, up until this point, have benefitted from slower high elevation warming.
Flooding and avalanche risk are also huge safety, ecological and economic concerns in a warming climate. The spring melt-out of the mountain snowpack has resulted in flooding on all of the main rivers coming off Mount Washington over the last century. And with warming winters, we have seen recent floods in the middle of winter, sometimes exacerbated by ice jams. Backcountry skiing, hiking and ice climbing have increased rapidly over the last 20 years, and as more people ski Tuckerman Ravine and summit the Presidentials, more risk getting caught in avalanches. To assess and mitigate these risks, MWO Director of IT Keith Garrett, PSU Research Technician Dan Evans and PSU Applied Meteorology graduate student Liz Jurkowski and I are developing, deploying and testing inexpensive snowpack sensing stations to measure the highly variable snow depth, snowpack temperature and snow water content at different elevations on Mount Washington.
The new sensing stations are equipped with twelve thermistors (resistance-based thermometers), an ultrasonic snow depth sensor and a temperature and relative humidity sensor for the air above the snowpack. The thermistors are placed every 20 centimeters from the ground surface to 2 meters above ground, and one in the soil 10 centimeters deep. The snow depth sensor is located about 2.5 meters above ground and hopefully will remain above the snowpack all winter to provide snow depth measurements. Data from these instruments are being logged by new, inexpensive microcontrollers, powered by small lithium batteries and a solar panel.
This past fall, the team deployed six of these stations on the slopes of Mount Washington between Pinkham Notch and the base of Tuckerman Ravine to monitor the snowpack throughout the winter. The researchers will check the stations every 2–3 weeks, swap batteries, measure snow water content for hydrologic interests and document snowpack layers for avalanche prediction. These data will be sent to the NWS’s Northeast River Forecast Center (NERFC) to improve flood forecasting and to the Mount Washington Avalanche Center (MWAC) to inform the public of avalanche risk. Currently, on-the-ground observations of these snowpack variables are sparse at best in the White Mountains. The NWS flies aircraft over the region a few times during the winter and spring to estimate snow depth and snow water equivalent, but these measurements are only along narrow transects and have high uncertainty. The ground measurements this winter and spring will help validate these airborne measurements.
If testing of these snowpack sensing stations goes as well as expected this winter (2018–19), the long-term plan will be to add communications equipment to each station that will automatically send data in real-time to the researchers, MWAC, and NERFC. Furthermore, funding will be sought to build and deploy more stations on other mountain slopes.
Boundary layer: Most people live in the boundary layer. The boundary layer is typically the lowest 1000-3000 meters of the atmosphere, depending on the time of day, season and weather pattern. This layer feels the direct effects of the daytime heating and nighttime cooling of the Earth. The air is typically turbulent because of the wind interacting with the Earth’s rough surface, such as trees, hills, mountains and buildings. As you learn on a plane, the most turbulent part of the flight is during take-off and landing when the airplane is in the turbulent boundary layer.
Free troposphere: The free troposphere resides above the boundary layer. The wind in the free troposphere is relatively smooth and horizontal. Generally, there is not any turbulence, hence flying in an airplane is smooth above the boundary layer (sometimes marked by low clouds). The air is usually drier than the boundary layer. The bottom part of the free troposphere is often warmer than the upper part of the boundary layer.
The entrainment zone: The entrainment zone is a thin layer between the boundary layer and the free troposphere. It is a transition layer that is often marked by high turbulence and rapid changes vertically in temperature, moisture and wind. Usually, the temperature increases, moisture decreases and wind increases with height through this layer.
About Mount Washington Observatory
Founded in 1932, Mount Washington Observatory is a private, nonprofit, member-supported institution with a mission to advance understanding of the natural systems that create the Earth’s weather and climate. Since 1932, the Observatory has been monitoring the elements from its weather station on the summit of Mount Washington, using this unique site with year-round facilities for scientific research and educational outreach. There are many ways to become involved and assist the Observatory—through membership, corporate support, volunteer opportunities or participating in the nation’s premiere hiking event the Seek the Peak Hike-a-thon that takes place in July. Learn more about our science, education and life on the rock pile, as well as how to become involved at mountwashington.org and seekthepeak.org.