Reflections on Water - Chapter 3 Dialog
Chris Nicodemus & Katrina Meserve
Image by Chris Nicodemus. Moisture and warmer air flow across the deeply chilled Ammonoosuc Valley and at the level of this inversion, moisture in the warmer air above has formed a low cloud deck below the mountains.
Q. Should we always expect less snow if global warming accelerates?
A. Not necessarily, the sensible weather here in northern New England in winter is determined by the lineup of the northern and mid latitude high pressure domes, the jet stream above the junction and the flow approaches the region. With global warming the junction of moist mid latitude air and dryer cooler northern air has shifted north but the significant winter snows occur on the northern edge of this junction, in a relatively narrow band. If the center of a low-pressure disturbance moving along the junction passes directly over or south of the region substantial snows will occur. The snowiest winters in New England are not the coldest we experience. When arctic air dominates, the moist southern air and the deep snows are shifted to the south. When storm after storm rides up the St Lawrence valley to our north through the winter, we have frequent snow to rain events, and the deepest snow builds up in the hills and mountains on the north side of the St Lawrence.
Stalled weather patterns to the east and the position the jet stream follows across the pacific establishes the pattern that can persist for many weeks through the heart of winter. Ocean surface temperatures and ocean currents influence the balance of airflow, but if arctic ice patterns and ocean currents shift dramatically from what they have been in our recent history, the balance of sensible weather in New England might change in ways no one can assuredly anticipate.
Q. Why is the north country’s weather different than southern New England’s?
A.The Mountains of New Hampshire and Western Maine separate the areas to the north and west from the coastal plain and the dominant ocean influence on sensible weather in that geography. To the northwest of the mountains moist flows from the great lakes updraft creating cloudy and damp conditions, whereas those same flows downdraft beyond the mountains toward the coastal plain and offshore clearing the skies but creating gusty winds at the surface. The clouds of northern New England dissolve as they approach the coast. In contrast when moist onshore flows of the ocean arise, the opposite pattern occurs, and the coastal plain and foothills can be hidden in low level clouds and dampness while north and west of the mountains warm sunshine can be found. Storms that track along a line offshore or just inshore on the coast create windy and moist storms and much of that heavy precipitation falls to the south and east of our area. The North Country valleys are also protected from the strong winds that accompany storms on the coastal plain. At the peak of sunshine in late spring and early summer the protected valleys of northern New England are very sensitive to solar heating and can experience a wider range of daily temperature than experienced to the south. That effect is less apparent in southern New England because of the marine influence that can limit the extremes of overnight cooling and daylight heating.
Q. How does water protect one’s garden from frost?
For those who live in the valley floors of northern New Hampshire and tend gardens, clear cloudless nights any time except perhaps in July can raise concern about the risk of frost. If the air is dry, radiational cooling into the dark night sky will allow the temperature to drop steadily until the water vapor in the air saturates at the “dew point” which is the temperature at which water starts to condense as dew on grassy surfaces and windshields. The temperature then hangs steady as water condenses and will only slowly lower as the total moisture in the air diminishes. The moist air thus buffers the temperature and prevents its continuing decline. Ground water, and the moisture in lush vegetation contribute to this buffering effect. However, if a valley is drained of surface water and devoid of lush vegetation, it likely will experience lower minimal temperatures and a shorter growing season.
Q. Why is it colder on long nights after a fresh snow?
An icy ground surface will hold air temperatures cool to the same temperature as the bulk of ice, but after a fluffy dry snow on a clear cold and dry night, the snow with its loose fluffy surface and small ice crystals in the snow can sublimate directly into water vapor as a gaseous form, a form of evaporative cooling also known as freeze drying. Such evaporation is net cooling to the air and that enhances the cooling associated with the radiation of heat out into the clear night sky.
Q. How does the ocean impact our sensible weather?
Image by EP Chow. Convective Squall line approaches from the west, a strong westerly blowing out from the leading edge and a line of heavy rain follows.
Northern New Hampshire is shielded from the most direct impacts of coastal storms, but the ocean feeds moisture over the mountains and enhances the precipitation we receive. Deep low pressure that forms over the ocean off the coast creates lower pressures and stronger circulations than typically form over land. The gradient between northern high pressure and deepening low pressure offshore can create low altitude jets of wind that dive toward the coast and produce the highest winds on the mountain peaks, especially Mount Washington.
References:
Layers of atmosphere: Layers of the Atmosphere | National Oceanic and Atmospheric Administration
https://www.noaa.gov/jetstream/wind_damage
Banner Photo by: EP Chow
