| Climate at Hubbard Brook: |
(the following summary was excerpted from the Hubbard Brook guidebook)
Although the climate of Hubbard Brook varies with altitude, some major features include: (1) large and rapid changes in weather, (2) broad ranges in daily and annual air temperature, and (3) uniform monthly precipitation (about 100 mm/mo; Figure 3). Major air flow over the forest is either (1) continental polar air from subarctic North America (the predominant direction), (2) maritime tropical air from the Caribbean and Gulf of Mexico from the south or southwest, or (3) maritime air from the North Atlantic out of the east or northeast. In spite of the proximity of Hubbard Brook to the ocean (116 km), the climate is predominantly continental. The diverse character of the air masses which influence central New Hampshire produces climate which is highly variable from week-to-week, month-to-month and year-to-year.
Annual precipitation averages about 1,400 mm, of which about one third to one quarter is snow. Approximately 111 separate storms occur each year, or about 2 storms per week. A snowpack usually persists from mid-December until mid-April, with a peak depth in March of about 1,020 to 1,270 mm, having about 250 to 300 mm of water content. In the summer months, big rain storms occur about once a week and in the winter, precipitation occurs almost every other day.
Winters are long and cold. January averages about -9ºC, and long periods of low temperatures from -12ºC to -18ºC are common. Even though temperatures are low most of the time, occasional midwinter thaws result in elevated streamflow (Figure 3). Short, cool summers are the rule. The average July temperature is 18ºC.
The average number of days without killing frost is 145; however, the growing season for trees is considered to be from 15 May, the approximate time of full leaf development, to 15 September, when the leaves begin to fall.
The estimated annual evapotranspiration (ET) is about 500 mm, determined by difference between precipitation and streamflow. This calculation is a reasonable approach for Hubbard Brook because of the apparent minimal deep seepage and annual removal of summer soil-water deficits by autumnal rains and spring snowmelt.