Reflections on Water - Chapter 3, Sensible Weather

Chris Nicodemus & Katrina Meserve

Many of us living in northern New Hampshire love the weather. If we didn’t, we would choose a quieter region where the weather was not so changeable. Water is a key component determining our “sensible weather” here in New England and this relationship is the subject of our next Reflections chapter. Better understanding of the geographic and atmospheric forces influencing our local weather may allow changes in sensible weather to be more easily anticipated by our readers and hopefully more interesting to witness. Growing seasons are short in northern New England, with mountain valley frosts on clear still nights possible until all the snow has left the mountain tops in late May or sometimes early June. The humidity of valley moisture protects late summer crops from frost, but in a dry summer late August or early September frosts can occur. 

A planet that can sustain the diversity and sheer volume of life including humanity currently on this earth, requires a balance and an equilibrium. If that balance changes, so too will the life that is supported and can thrive. Life on the planet as we know it has reached its present state during this most recent period of “cycling glaciation” known as the “Quaternary period” of the last 2.6 million years. The two most recent glaciations peaked 20,000 and 300,000 years ago but there were numerous additional cycles in the Quaternary and evidence suggests that continental ice has persisted over Antarctica for at least 30 million during which time and well prior that continental plate has been located at the south pole. A billion years of organic deposits including coal, oil, and carbonate rich lake and sea bottoms were biologically created using carbon metabolized from greenhouse gases over time and have been being sequestered while continents moved, oceans formed, and sea levels fluctuated. Nevertheless, there have been multiple ice ages and warm intervals that have occurred through this period while carbon dioxide levels were high but declining. Cycling glaciation can occur in periods with relatively elevated carbon dioxide. This implies that fluctuating CO2 is not the only cause of global climate change, however it is a contributing factor. Polar ice in both hemispheres serves as a thermal buffer cooling midlatitude landscapes and ocean waters and generating the range of seasonal cycles and the plethora environments that support the range of life that now accompany humanity on this planet. 

Life on the planet can certainly survive a return to the warmer glacial-free era but no one can reliably predict exactly what will happen, or how quickly the process of deglaciation will take if the melting continues. It is certain however, that all coastal plains, including coastal cities, and low-lying islands around the world will be inundated and disappear beneath the sea if the current sequestered continental ice on Antarctica and Greenland are returned to liquid water. If we as a civilization choose to release a billion years of sequestered carbon dioxide in a relative instant of geological time, the consequences will be disruptive and severe. This is both a challenge and a learning opportunity that will serve as a future judge of the wisdom of humanity, but it should not be ignored. It is fair to say that the science of climate change is still evolving and likely includes factors that are beyond human control. 

In thinking about sensible weather, and how to meaningfully relate this complex science of applied physics and chemistry to a general audience is a challenge. Popular attention often is focused on storms, calamities and beach weather as the “sensible weather” of interest, but perhaps thinking about high pressure systems who’s interactions spawn, feed and extinguish the storms might be an instructive way to understand the subject.  

 Because the earth is a sphere with an annual revolution around the sun in a plane that is tilted off perpendicular and the planet is also spinning a full rotation each 24 hours, the atmospheric gas cloud hovering over the earth’s surface is impacted by a series of motion related forces that generate the weather. Meanwhile, solar heat bathes half the spinning planet continuously in a 24-hour sweep that includes two polar regions one experiencing continuous low angle light and the other permanent twilight or darkness that shifts back and forth with the seasons. Air rises in response to the solar heat and shifts poleward in each hemisphere with maximal solar energy occurring at the latitude directly aimed at the sun for each day. On the two equinoxes in March and September the sun is directly over the equator but at a hemispheric summer solstice, the sun is directly over latitude 23 degrees north or south per the calendar. The atmosphere moves with the surface of the earth below but held loosely by the force of gravity and not firmly anchored thus also sliding some relative to the surface. 

The shape of the spinning sphere of the earth, sea, and atmosphere contains angular momentum that produces the “Coriolis effect” responsible for the rotary movement of currents in both the atmosphere and ocean. The total distance of any location on the earth's surface moves each day around its center of rotation varies with latitude. The circumference at the equator is almost 25,000 miles whereas the circumference of the circle of rotation diminishes to zero at each pole. Thus the relative speed and angular momentum of a water or air at the equator (~1000 mph) is very fast relative to a similar sample at mid latitude or the motionless sample at a pole. A continuous circulation of air rising and shifting poleward occurs daily as the sun passes westward. Because of the diminishing planetary circumference with increasing latitudes, with poleward movement the moving air’s eastward velocity and angular momentum carries the air to the east relative to the slower moving land surface below. This three-dimensional Coriolis circulation of air rising and moving north and eastward and then descending and shifting south and westward in a planet of our size produces three bands of distinct airmasses in each hemisphere, one in the tropics, one in the midlatitudes, and one in polar latitudes. The airmass in each band rises and moves poleward with daily heating and shifts eastward before settling back down and southward and deviating to the west. Near the poles the polar airmasses rotates around a vortex that oscillates close to the center of rotation of these “polar caps”, but off set by the tilting planet and changing available solar energy. “Sensible winds” vary by altitude and are the product of angular momentum, pressure changes from heating, evaporation, and condensation of gas and water and the pull of gravity in infinite variations.  

At the coldest upper-most level of the “troposphere” (lower atmosphere) at an altitude varying from 30 to 50,000 feet) the rising eastward moving air accelerates in wavering bands near the polar side edges of the rotating tropical and the mid latitude airmasses. These strong westerly (ie flowing eastward) “jet streams” of wind oscillate north and south, sometimes merging and changing intensity influenced by the nature of land and water below as they circle the planet. The jet streams thus mark the fluctuating overlap of the three intermingling airmasses in each hemisphere. In the northern hemisphere, the tropical, mid latitude and polar airmasses each rotate with rising air following a southwesterly flow moving to the north and east slowly deviating to the right forming “ridges of high pressure” that then swing south eastward adjacent to a “trough of high pressure” of the adjacent airmass to the north that has dipped to the south in front of the flow as it as it travels east. In the southern hemisphere there is a band of open ocean uninterrupted by continental land masses the encircles the planet and creates the “roaring 40’s” band of strong jet stream driven winds that are more relentless than the northern hemispheres equivalent whose mid latitudes include two continents and two oceans. The mid latitude jet stream marks the boundary of northern and mid latitude airmasses and in eastern North America the mid latitude air flow contains moist southern air originating in the south Atlantic and Gulf of Mexico. When the Jet stream flows bend south chilled northern air is carried back south which sinks and continues to shift to the right. The easterly “trade winds” flow at the surface near the tropics on the southern edge of the midlatitude airmass and on occasion a less dominant easterly equivalent occurs along the bottom edge of northern airmasses especially when the airmass is being blocked by a midlatitude ridge further east. Back door cold fronts that occur in northern and eastern New England and on occasion locally interrupt summer heat waves reflect this pattern of stalled high pressure.  

Meteorologists use the term “ridge” to describe the curved top of a flowing airmass rising and curving northward from the midlatitudes and “trough” to describe the bottom of U shape southward bulging northern airmass sliding under warmer southern with the interweaving pools of air in a pattern overlain by the ribbon like Jetstream riding above the junction. The solar energy vaporizes surface water during the day which rises and forms convective clouds but then cools and condenses at night. When convective conditions favor storms, with evaporation, convection, condensation and the gradients associated with these changes in temperature and pressure, and the differences in temperature and humidity of northern and southern airmasses can trigger local convective rain storms as in a summer thunder shower or in fall and winter the afternoon cloudiness and “spitting” local rain and snow showers that often occur after what started as a sunny day here in northern New England. Typically, these clear up after sunset. When ridges and troughs become very deep, the jet streams swing south and then back north in a tortuous path and the center of the high pressure can become stationary or even retrograde. In some cases the airmass can become cutoff from the global flow and sit stationary rotating over a large area. These blocking patterns can define a prolonged pattern of weather that can persists for an entire season. The southern edge of a stalled trough may produce persistent clouds and rains while a stalled ridge may produce drought conditions at its center.   

In our region, low pressure circulations form along the eastern edges of troughs. Northeast winds to the north and southwest winds to the south with rising convection on the southern side create a spinning vortex around a center of lowering pressure that then is amplified by convection in an ongoing energized cycle that forms a counterclockwise low pressure circulation. The jet stream speeds the rotation passing along midlatitude’s northern edge and the jet streams can also dip down to lower altitudes accelerating surface winds as the pressures drop. These are the ingredients of our northeastern storm systems that are responsible for much of northern New England’s stormy weather, especially in the cooler seasons. In contrast, intense solar heat in spring and summer often creates local convection with strong updrafts, thunder and lightning, tumbling precipitation and strong down draft winds blowing out to the east of the local approaching storm cell.  

By watching the ridgelines and mountain tops of northern New England, one can see when warm moist southern air starts to arrive on a southerly flow riding over cooler drier northern air. Thin high-level clouds appear first in the high layer of moist southern air moving in from the southwest and riding over northern air. Mountain ridges next are touched by the southwest flow above the still air or light northerly flow at the and as moisture increases lower altitude clouds form where the south winds are pushed up approaching the upland, and often vaporize again as the air dips down north of the mountain. 

Northern New Hampshire’s sensible weather is the product of our mountain ridges and valleys and the abundance of water in all directions. Our glacier modified mountainous terrain explored over the prior two chapters sits close to the Gulf of Maine and Atlantic Ocean to the east, the St Lawrence Valley to the north, the Great Lakes to the west, and Mid-Atlantic coastal waters to the South. Northern and southern air masses interact regularly shifting back and forth channeled by our ridges and valleys. In winter, the precise path of the surface low pressure and the degree of cold in the northern airmass dictates the sensible weather at a given location. Up drafting moisture laden air cools and condenses as it rises over ridgelines but then compresses and dries on the downwind side beyond. The Ammonoosuc and Connecticut valleys are shadowed by the mountains to the east while the Androscoggin valley is on the updraft side of the same mountains when low pressure approaches from the west. 

When winter storms track along a line to the north and west of our region in Quebec, the south/south westerly flows dominate at the height of the storm. Snow at the leading edge of warm moist air flowing from the southwest often turns to rain in warming down draft locations to the north of the mountain ridge and moisture dries. Deep snow might accumulate in an updraft location like Franconia Notch, while just a few miles to the north at lower altitude there may be little precipitation with some rain and little accumulating snow. Strong southerly jet stream associated winds can bend down as they cross the mountains and lower to the surface the Ammonoosuc Valley and produce the strongest winds experienced in the region all year. If the southern air is warm enough the further down draft warming can bring snow eating winds that clear the valley of its snow base, especially in open locations exposed to the south and southwest winds.  In contrast when a storm circulation center passes to the south and east, cold north easterly winds picking up some moisture from the Gulf of Maine and encountering the moist southerly flow on the circulating low pressures eastern fringe pushes up against the eastern slopes of Coos County and the White Mountains bringing maximal precipitation to the northeast slopes with heavy deep winter snow. In this setting the valleys along the western edges of the White Mountains see relatively less precipitation and if the storm system is relative mild, down sloping easterly winds can turn frozen precipitation to rain. For Western Coos and Northern Coos counties, it is the period after the low pressure has already passed by to the east that the western flows on the back side of the storm bring cold moist air enhanced by the St Lawrence and Great Lakes that brings persistent accumulating snow that can be deep but is light and fluffy, while south on the coastal plain the storm has ended. 

On occasion a trough line from northern cool air pushing south under rising moist southern air will result in a period of prolonged light showery cool rain or snow. Tornadoes rarely occur in northern New England but can occur when cumulus convection interacts with high altitude winds and develops a local rotation in the updraft under the convection.  Hurricanes rarely reach northern New England intact but moisture laden remnants of tropical storms that move from the south into the Ohio Valley typically can track across New England on a south westerly flow and clusters of thunderstorms can drop extreme quantities of moisture when the flow approach local mountain ridge lines.  

The remnants of hurricane Beryl in the overnight hours of July 11, 2024, moved across northern New England long a trough line and cause significant flooding in north east Vermont and Coos County. The accompanying photo shows the screenshots from the Windy and Wunderground weather apps captured at the height of the storm around 1 AM. The Windy surface winds screen shot show on the left show south – south-westerly winds flowing up across central and northern Vermont and New Hampshire and across to central Maine. There is a blue band of calmer winds stretching from east of Moosehead Lake in Maine just north of Coos county, below Sherbrooke Qc and towards the NE corner of New York state with brisker northeasterly flowing surface winds stretching from northern Maine into the St Lawrence Valley. The remnant moisture laden convective thunderstorms cells from the Hurricane have lost all rotation but show moisture rich heavy rains stretching from NE Vermont across Coos and into Maine. The Wunderground weather radar map at from the same moment indicate that no rain is falling in the band on the down draft southern edge of the Ammonoosuc Valley along the range of the Mountains from Moosilauke to Mt Washington. The warm air reaches down to the ground and is delineated as a surface warm front at the southern edge of the precipitation field. The updraft western edge of the northern range White Mountains running up the center of Coos County in this event is perfectly positioned to ring maximal water content out of the moist southwest flow. 

Remnants of TS Beryl 1 AM July 11, 2024 Crossing Coos County on Windy and Wunderground Apps. Surface winds left panel.

Tropical storms form over warm ocean waters near the interface of the tropical and midlatitude airmasses when clusters of thunderstorms develop a closed cyclonic circulation as it draws northeasterly winds in from high pressure to the north. These storms intensify with the heat of the water. In 1938 a strong Hurricane catapulted along the western edge of a ridge from east of Florida into the Connecticut River Valley bringing Hurricane force winds, massive rains and flooding into northern New England. Jet stream winds can shear storms apart with time, but also add to their strength if the storm moves quickly.. The Hurricane of 38 remained fully organized as a tight tropical circulation moving so quickly that the relatively cool waters off New England had little time to weaken the storm. Most Hurricanes do not ride similar high-altitude winds but rather weaken as they approach southern new England. With increasing global temperatures, this balance may change in the future.  

References: layers of atmosphere: Layers of the Atmosphere | National Oceanic and Atmospheric Administration 

Banner Photo by: EP Chow