Summer 2019 Research Projects

Observers have measured temperature and humidity every day since 1932 using a sling psychrometer. The exact location these measurements are taken have changed over the years whenever the Observatory location changed. But does it matter for the fidelity of ourclimate record?

Interns Ben Charles and Austin Patrick took the first step toward answering this question. They sought to determine whether or not the variability of and average temperature and humidity during a 5-minute period at multiple locations were different between sites. To do so, they first needed to see if the mobile instrument they were using (a handheld Kestrel 4000) is accurate enough to use for this study. They compared measurements from the Kestrel 4000 to the calibrated instruments at the summitby placing the instruments side-by-side for 5-minutes at a time during different times of day and under variable weather conditions. They also comparedtemperature, humidity and wind speedmeasurements from two Kestrel 4000 instruments to determine whether the Kestrel 4000 is accurate and precise enough to use for this research project.

Ben and Patrick analyzed data collected over three weeks and concluded that the two Kestrel 4000 instruments and the methods of taking the observations resulted invalues withlarge average and variability differences that make it an inappropriate mobile instrument for this particular study.Some changes in the methods for taking the measurements may account for some of the differences and will be explored in future research.

Interns Ben Charles (left) and Austin Patrick (right) sling Kestrel 4000 instruments alongside Observer Ian Bailey (center) making an official hourly temperature and humidity observation with a sling psychrometer.

Mount Washington weather does not vary consistently with the El Nino-Southern Oscillation, which originates in the tropical Pacific Ocean. So does Mount Washington weather respond more consistently to other dominant large-scale patterns, such as the North Atlantic Oscillation (NAO)? The NAO describes the variability of the strength of the dominant pressure centers in the North Atlantic Ocean: the Icelandic Low and Azores High. For many low elevation sites in the Northeast US, a negative phase of the NAO often means colder and/or more snow because the jet stream dives south into the Mid-Atlantic region. A positive NAO often means warmer and/or rainy conditions as the jet stream retracts to the north. The NAO is a strong pattern during the cold season (November-March). In the warm season (April-October), the NAO becomes weak as the jet stream retracts toward the Arctic.

This research project correlated NAO phase with Mount Washington temperature and precipitation for the cold and warm seasons. Overall, there were not strong correlations for temperature or snowfall with the NAO when using the raw NAO index values; only up to ~6% of their variability was shared. However, these correlations became significant and strong (>65% of their variability was shared for the cold season) when the seasonal NAO index values were binned into 0.5-unit bins. The strongest correlation was for cold season temperature (76% variability shared) such that negative NAO values correspond with colder temperatures for the cold season. Improvements in seasonal forecasting of large scale patterns like the NAO will help improve local forecasts on Mount Washington and other locations. In addition, climate model projections of the NAO during the next 80 years will help inform decision-makers and planners when making business decisions and decisions about infrastructure and land-use.

A diagram showing the jet stream pattern and weather conditions associated with the negative (left) and positive (right) phases of the North Atlantic Oscillation pattern. The Icelandic Low (L) and Azores High (H) are weaker during the negative phase and stronger during the positive phase. Source: NOAA: https://www.climate.gov/sites/default/files/NAO_Schematic_0.png

Skier and winter recreationalists dread those episodes of warm, rainy conditionsin the middle of winter. Furthermore, mountain ecosystems rely upon relatively consistent cold winters with a thick insulating snowpack.Recentresearch has examined how winter temperature and precipitation has changed in the Northeastwith a focus on the lower elevations. But weather and climate can vary substantially between the low elevations and the higher elevations of the Northeast. This research examines how “melt events” are changing at Mount Washington.

Intern Ethan Rogers studied how these melt events have evolved from 1935-2019. He examined four key metrics: 1) number of hours above 32F, 2) number of melt-degree-hours(the cumulative number of degrees above 32F each hour), and 3) the number each winter and 4) duration of melt events. He focused on melt events during the months that are climatologically below freezing: December-February.

Results revealed that although there is no significant change in the number of melt events each winter, they are becoming warmer and longer, especially in the last 40 years. The average number of melt-degree-hours per winter has nearly quadrupled from about 100 to nearly 400 over the 85-year period. The average number of hours above 32F has doubled from ~50 hours to just over 100 hours. Futureresearch will examine these same metrics using dewpoint temperature – which is a stronger indicator of melt.

The number of hours each winter (DJF) that were above freezing at the summit of Mount Washington from 1936-2019. The red line is a 10-year running average.