Introduction.
	How the Sun makes the wind blow, and drives the Earth's large-scale atmospheric circulation.
	Circumpolar wind belts and areas of semi-permanent low and high pressure.
	The polar jet stream and New England's place in the three-cell circulation.
	Low- and high-pressure areas that cross New England.
	Airmasses influencing New England.
	Local breezes.
	Summing it up.
	Where to find high-quality, non-commercial weather information on the internet.

INTRODUCTION

The weather and climate at any given location in New England are controlled by physical forces of all sizes and time scales. Take a minute to look out the window, and then consider the following:

The Sun, 150 million kilometers away, provides the energy that drives the weather engine. Variations in the Earth's orbit on time-scales of years to millenia cause variations in the amount of solar energy reaching a given spot on the Earth's surface. The Earth's gravity, curvature, and daily rotation constrain the atmosphere to move within certain bounds, and give rise to phenomena such as jet streams (ribbons of fast-moving air thousands of kilometers long that steer weather systems), hurricanes, and blizzards.

Airmasses are bodies of air with horizontally similar characteristics. They are typically thousands of kilometers across, and usually have lifetimes of days to months. The great inland plains of the United States and Canada create airmasses that travel hundreds of kilometers before reaching New England. These airmasses are responsible for heat waves, cold waves, and sunny summer weather. The Atlantic Ocean creates other airmasses that bring hot, humid, hazy weather to New England in summer and cool, rainy weather to New England year round. The clash of these airmasses causes thunderstorms, tornadoes, high winds, and sudden changes in temperature, humidity, and wind direction.

Local effects such as sea breezes and valley breezes alter the temperature, humidity, wind, and cloudiness on spatial scales of a kilometer and time scales of a few minutes.

HOW THE SUN MAKES THE WIND BLOW

The Sun is an average, middle-aged star about 1.4 million kilometers in diameter. With a temperature of more than 10 million degrees, thermonuclear fusion in the solar core has been steadily turning hydrogen into helium for more than five billion years. It takes more than a million years for the energy generated in the solar furnace to climb up to the Sun's surface, but once it does, it escapes into space at the speed of light.

The Earth is a sphere, therefore light falling at low latitudes (near the equator) is more concentrated per unit area than light falling at high latitudes (near the poles). Light falling on low latitudes strikes directly, while light falling on high latitudes strikes a glancing blow.


Fig. 2. Solar radiation striking the Earth.
(Source: Lutgens and Tarbuck, 1998.)

The Earth's axis is not exactly perpendicular to the line between the Earth and the Sun, but is tilted at an angle of 23 1/2 degrees. In other words, the Earth is not exactly "vertical" in the Earth-Sun system. This means that for half the year, the northern hemisphere is tilted toward the Sun, and for the other half of the year, the southern hemisphere is tilted toward the Sun. This means that the angle with which sunlight strikes a given spot on the Earth's surface varies through the year. This is the mechanism responsible for the seasons.


Fig. 3. Earth-Sun relationship.
(Source: Lutgens and Tarbuck, 1998.)

The troposphere (the lowest part of the atmosphere) is heated from below by the Earth's surface, causing it to expand upward and become thinner. Strong upward vertical currents are established in the atmosphere above the equator.


Fig. 4. The layers of the Earth's atmosphere.
The troposphere is the lowest layer of the atmosphere, and typically extends from the Earth's surface to a height of about seven kilometers over New England. The troposphere is heated by solar energy absorbed by the Earth's surface and reradiated at infra-red wavelengths. Note that the temperature is highest near the heat source (the surface of the Earth), and drops off with increased altitude.
(Source: Barry and Chorley, 1998.)

The thin air is not as heavy as the cooler air to the north and south of the equator, thus a permanent area of low pressure is established on the equator. (Pressure is the weight of the atmosphere on a unit area of the Earth's surface, as in "pounds per square inch.")

The intense heat also drives a great deal of evaporation in the equatorial oceans, which causes a permanent band of cloudiness to encircle the Earth at low latitudes -- the Intertropical Convergence Zone. Tropical rain forests are found on land at these latitudes.


Fig. 5. The Intertropical Convergence Zone.
Note the bright, horizontal, semi-continuous band of clouds circling the Earth above the equator.
(Source: NOAA.)

The cold polar surface cools the troposphere. A kilogram of cold air fills a smaller volume than a kilogram of hot air (think of hot air expanding, as in a hot air balloon), so the volume of the polar troposphere shrinks and the air becomes thicker.

The polar troposphere's shrinking volume sets up downward vertical currents -- just the opposite of the situation above the equator. Air sinks above the poles.

The thick air at the poles is heavier than the warmer air at lower latitudes, thus a permanent area of high pressure is established on the north and south poles.

With high pressure at the poles and low pressure at the equator, a pressure gradient force is set up between the poles and the equator. This causes the cold polar air to flow toward the equator along the Earth's surface.

This is equivalent to the following:

1. Imagine a small aquarium filled with about an inch of water.

2. Pour some molasses into one end of the aquarium -- it sinks and forms a thin layer covering the bottom of the aquarium. The water is floating on top of the molasses.

3. Tilt the aquarium slightly so that the molasses flows along the bottom. Gravity pulls the dense molasses down the slope toward the low end of the aquarium.

4. Where the molasses had been (on what is now the high side of the aquarium) is now water. The water got there by flowing over the top of the molasses as the molasses slid down toward the low end of the aquarium.

The analogy suggests that as cold polar air slides along the Earth's surface toward the equator, warm air from the equator slides poleward aloft to replace it.

From here one can imagine a closed circuit:

1. Air sinks at the poles due to cooling (because only weak sunlight reaches the Earth's surface there).

2. High pressure sets up on the poles, and the cold air is pulled along the Earth's surface by gravity toward the low pressure at the equator.

3. Because of the intense sunlight and high surface temperature at low latitudes, air arriving at the equator is heated from below. The heated air expands and rises.

4. The heated air travels poleward in the upper troposphere and cools as it reaches the poles. From there it sinks vertically, completing the circuit.

The pressure gradient force (PGF) is caused by the difference between the weight of the air above the pole and the weight of the air above the equator. The polar air is heavier, so there is more weight per unit area on the Earth's surface at the pole than there is at the equator. Thus, the PGF points from the pole to the equator. Since "weight" is caused by the Earth's pull acting on the mass of the air, the PGF is a manifestation of gravity.

THE THREE-CELL CIRCULATION

The rotation of the Earth causes this simple circuit to change in two ways.

The pressure gradient force is counterbalanced by another force known as Coriolis or the Horizontal Deflection Force (HDF). The HDF causes flow to be deflected to the right in the northern hemisphere and to the left in the southern hemisphere.

Air moving from the north pole to the equator is deflected toward the west.

Air moving from the equator to the north pole is deflected toward the east.

The simple Hadley cell circulation (described above) is broken up into three small cells in each hemisphere. This is called (appropriately) the Three-Cell Circulation.

1. Permanent low pressure is on the surface at the equator and 60 degrees north. Permanent high pressure is on the surface at 30 degrees north and on the pole (90 degrees north).

2. Wind flows from the Polar High (90 degrees north) southward to the Subpolar Low (60 degrees north), and the HDF deflects it to the west. These permanent winds are called the Polar Easterlies. (Winds are named according to the direction they come from.)

3. Wind flows from the Subtropical High (30 degrees north) northward to the Subpolar Low, and the HDF deflects it to the east. These permanent winds are called the Prevailing Westerlies.

4. Wind flows from the Subtropical High southward to the Equatorial Trough, and the HDF deflects it to the west. These permanent winds are called the Trade Winds.

NEW ENGLAND'S PLACE IN THE THREE-CELL CIRCULATION

New England is situated between 40 and 45 degrees north: Right in the middle of the Prevailing Westerlies.

It is very difficult to discern the presence of a pervasive westerly (eastward) wind on the Earth's surface in New England. In fact, on any given day, the surface wind may be blowing from any direction. Note that this is not the case with the easterly (westward) "Tradewinds" (between the equator and 30 degrees north), which are extremely steady compared to the Westerlies.

Detecting the Westerlies on the ground in New England is accomplished by taking long-term averages. While the wind may blow from any direction on the compass at any given time, taking the average direction over a period of several days will begin to show a bias toward a westerly direction.

The longer the period over which one averages the wind direction, the more apparent the Westerlies will become.

Averaging over an entire summer will reveal a wind direction between 220 and 270 degrees: Southwesterly.

Averaging over an entire winter will reveal a wind direction between 270 and 320 degrees: Northwesterly.

The Westerlies become more apparent on a minute-to-minute basis as one moves upward into the troposphere. As one climbs to 3,000 meters above mean sea level (MSL), one finds that the wind over New England rarely blows from the eastern half of the compass. If one looks above 9,000 meters MSL, one finds that the wind almost always blows from somewhere between southwest and northwest.

The clearest manifestation of the Westerlies is the Polar Front Jetstream (PFJ). The PFJ is a virtually unbroken ribbon of eastward-moving air circumnavigating the Earth at speeds of up to 200 knots. (A more typical speed is 100 knots. 1 knot = 1.1 miles per hour.) Storm systems are associated with waves in the jet stream, and are steered from west to east across the United States and Canada by the PFJ. In New England, if the PFJ is overhead, it is usually at about 9,000 meters MSL -- somewhat higher in summer than in winter.

Meteorologists typically measure height in the atmosphere on a pressure scale. In other words, we speak of the height dimension in the atmosphere not as a function of linear distance, as in meters, but as a function of pressure. The mean surface pressure is about 1000 millibars (or one bar), and it drops logarithmically with height -- rapidly at first, then more slowly.

The average pressure at 1,500 meters MSL is about 850 millibars: 85 percent of surface pressure.

At 3,000 meters MSL, the average pressure is about 700 millibars.
At 5,500 meters MSL, the average pressure is about 500 millibars.
At 9,200 meters MSL, the average pressure is about 300 millibars.

Weather technicians make routine measurements of the atmosphere by reading surface-based instruments and by releasing instrumented balloons (called rawindsondes) into the upper troposphere. The surface observations are typically recorded once every hour -- and at many sites across New England. Balloon releases are only performed at a few special locations, and only twice a day (0000 and 1200 Greenwich, England time).

Results from a given set of observations are used to create as complete an image of the atmosphere as is possible for that moment in time. For those hours (twice a day) when balloons are sent up, a three-dimensional image of the atmosphere can be created. Horizontal slices can be taken through the 3-D image, and plotted on top of maps of North America. Such slices are typically taken at a set of standard pressure levels -- in other words, on a "surface" in the atmosphere that traces out a constant pressure at many different geographic locations.

Think of it this way: At any given geographic location, the altitude at which the pressure drops to 500 millibars (from the surface pressure value of about 1000 millibars) may be different. Imagine if there was a stick that reached from the Earth's surface up to the 500 millibar height at each balloon release location. Now imagine what would happen if you could very-tightly drape a cloth over those sticks -- like a giant tent. That cloth would trace out the 500-millibar "constant pressure level."

In addition to the familiar hourly surface charts, constant pressure charts covering all of North America at the 850-, 700-, 500-, 300-, and 200-millibar levels are routinely plotted and analyzed by meteorologists. These charts are used for detecting and predicting the future course of waves in the Polar Front Jet and the storm systems that accompany them.

But anyone who has watched the television weather report knows that lows and highs swing from west to east across the United States and Canada -- in other words, they move a long way in periods a lot shorter than a whole season. These "migratory pressure systems," trapped in the Westerlies and steered by the PFJ, are a fundamentally different phenomenon than the 3-cell's semi-permanent systems. Migratory pressure systems are the primary weather producers in New England.



Fig. 9. GOES infra-red photograph for January 16, 1998, 7:15 PM Eastern Standard Time.
This photograph was taken in the Earth's own emitted infra-red light, rather than sunlight reflected back to the satellite off the Earth's surface. In infra-red photos, grayscale is a function of temperature. Warm areas are dark; cold areas a light. Cloud tops are high up in the troposphere and are therefore cold. The ocean is warm, and is therefore dark in this picture. Note the organized areas of (white) cloudiness off the east coast, in the central United States, and in the west -- these are migratory pressure systems. A very mature cylone (low) can be seen in the North Atlantic between Nova Scotia and southern Greenland.
(Source: National Climatic Data Center.)

Migratory cyclones. The GOES photograph shown in Figure 9 (above) was taken in January, 1998, and shows several of the cyclones responsible for the "Great Ice Storm of 1998." A major cyclonic storm is just off the east coast. The lowest surface pressure is in the Gulf of Maine, southeast of New England. While the whole system was being steered toward the east by the PFJ, the counter-clockwise circulation around the system on the Earth's surface was toward the southwest in New England -- in other words, we had a cold, damp wind out of the northeast. The marine air from the Gulf of Maine dropped most of its moisture in the form of freezing precipitation over New England, New York State, and southestern Canada. As the PFJ carried this cyclone out over the North Atlantic, New England was granted a reprieve from the ice for a day or two, but soon the next migratory low-pressure system approached from the west. This pattern persisted for more than three weeks.



Fig. 10. Results of the New England ice storm, January, 1998.
(Photo by John Ferguson, National Weather Service, Burlington, VT Office.)

Like semipermanent pressure systems, migratory cyclones are not unique to the Earth. In the 1970's, the Viking orbiters documented the presence of similar storms on Mars.



Fig. 11. Martian migratory cyclone.
Recorded by one of the Viking orbiters in the mid-1970's, the rotation of this Martian cyclonic storm (approximately 250 kilometers across) is governed by the same physics as terrestrial cyclones. The spiraling clouds are composed of water and water-ice, just as the clouds in Earth cyclones. False colors have been used to enhance the storm's features.
(Source: NASA/JPL.)

Migratory anticyclones. Fair weather in New England results when a migratory high-pressure system moves in from the west. These high-pressure areas are usually formed in the western United States or Canada, and the Polar Front Jetstream steers them to us.

In many ways, highs (anticyclones) are the physical opposite of lows (cyclones). In addition to the opposite sense of the rotation of the two systems, some of these differences are:

1. The surface winds get very weak in the middle of anticyclones, but very strong in the middle of cyclones.

2. Anticyclones are associated with weak downward vertical motions in the atmosphere, while cyclones are associated with strong upward vertical motions.

3. The downward motions in anticyclones cause the atmosphere to dry out, and the clouds disappear. The upward motions in cyclones cause the atmosphere to cool and become saturated with moisture, and clouds are formed. Anticyclones may be identified on satellite pictures (such as the one shown in figure 9) as cloud-free areas.

4. The atmosphere is usually stable beneath an anticyclone, so temperature inversions may form and trap air pollution near the ground. The atmosphere is usually unstable beneath a cyclone, so air pollution is "mixed" upward and becomes dilluted.

There is a linkage between migratory lows and migratory highs that is similar to the simple pole-to-equator circulation described above. A given "parcel" of air is carried along by the flow converging into the center of a migratory low-pressure system near the Earth's surface. There, the parcel encounters strong vertical currents, and is lifted aloft into the middle and upper troposphere. When the parcel reaches the upper troposphere, it encounters air flow diverging from the system, and it is carried away by the Westerlies. Eventually, the parcel encounters an area of convergence in the upper troposphere and begins to sink downward into the center of a migratory high-pressure system. Once reaching the Earth's surface, the parcel is carried away from the high's center by diverging surface flow. Surface highs usually follow surface lows, as shown in figure 12 (below).



Fig. 12. Typical surface chart.
This NOAA surface chart (November 16, 2000, 10:00 AM EST) shows a situation that is somewhat similar to that shown in the fig. 9 GOES photograph. A low is centered in southeastern Maine and a high is centered over Mississippi. The high originated in the Rockies, and the low originated in North Carolina. Both were steered to their current positions by the Polar Front Jetstream. The influence of the high extends into the Great Lakes region. Another low is descending into the Great Lakes from southern Canada, and will probably become a factor in New England's weather in a few days.
(Source: NOAA.)

Airmasses originate in a characteristic source region. A source region is a large, relatively uniform area of the Earth's surface where a body of air can remain for an extended period of time. The airmass takes on the characteristics of the source region. For example, a cool, moist ocean area (such as the North Atlantic) will create a cool, moist body of air.

Airmasses are named according to a specific designation scheme that describes its source region. The airmass designation is broken into two parts:

1. A latitude designation:

E - Equatorial: These airmasses originate in the ITCZ and never affect New England.

T - Tropical: These airmasses originate in the region between about 10 degrees north and about 40 degrees north (seasonal variation is the cause of the approximate limits). Tropical airmasses affect New England during the spring, summer, and fall seasons.

P - Polar: These airmasses originate in the region between about 35 north and about 60 degrees north (seasonal variation is the cause of the approximate limits). Polar airmasses affect New England during all four seasons.

A - Arctic: These airmasses originate in far northern latitudes and only affect New England in the winter.

2. A surface-type designation:

c - Continental: These airmasses originate in land areas, are generally dry, and have wide daily and annual temperature ranges.

m - Maritime: These airmasses originate in ocean areas, are generally moist, and have narrow daily and annual temperature ranges.

The two parts of the designation are put together to create a shorthand method of describing every airmass. For example, a continental polar airmass is designated "cP." A maritime tropical airmass is designated "mT."

Table 1. General table of airmass characteristics.
All of these airmasses affect New England's weather, except Equatorial (E) -- which is confined to the ITCZ -- and continental tropical (cT) -- which is confined to the southwest United States and northern Mexico.

LONG NAME	SHORT NAME	DESCRIPTION	SOURCE REGION	TEMPERATURE VARIATION DAILY/SEASONAL
Equatorial	E	Very hot Very moist	ITCZ	Narrow Narrow
Continental tropical	cT	Very hot Very dry	Deserts and high plateaus	Wide Wide
Maritime tropical	mT	Hot Moist	Oceanic regions beneath subtropical highs	Narrow Moderate
Continental polar	cP	Cool Dry	Central regions of continents	Moderate Wide
Maritime polar	mP	Cool Moist	Oceanic regions beneath subpolar lows	Narrow Narrow to moderate
Arctic	A	Very cold Very dry	Polar regions	Wide Wide

Airmasses begin to modify once they move out of their source regions. For example, a cold, dry airmass (originating on a poleward continent) will slowly warm up and gain moisture if it moves out over an oceanic region.

Airmasses are moved out of their source regions and brought to New England by migratory pressure systems. The migratory high pressure area that follows a migratory low pressure area is usually associated with a "cP" airmass that originated in the western United States or Canada -- these are the cold, clear days in winter, and the cool, dry, sunny days during the other three seasons. Occasionally, a powerful mid-winter low departing New England will cause the jet stream to move an "A" airmass into the region -- these are the extremely cold, crystal-clear days in January. In high summer, the Subtropical (Bermuda) High may invade New England and bring an "mT" airmass along -- these are the hot, sticky days in mid-August. During all four seasons, a low pressure area in the Gulf of Maine (southeast of New England) may bring an "mP" airmass into New England from the north Atlantic -- these are cool, cloudy, rainy days.



Fig. 13. General schematic of North American airmasses.
New England's weather is affected by cA (or simply "A"), cP, mP, and mT airmasses. (See text for details.)
(Source: Lutgens and Tarbuck, 1998.)

Airmasses are areas of slowly changing weather. Generally speaking, airmass weather varies gradually from day to day, reflecting (1) the slow movement of different areas of the airmass over New England, and (2) the modification of the airmass. The high temperature one day will closely approximate the high temperature of the previous day. All facets of weather will generally follow a daily cycle.

While an airmass may be relatively homogeneous with respect to temperature or moisture over several hundred or even several thousand miles, the transition zone between one airmass and the next may only be one hundred kilometers across.

Fronts are usually associated with low-pressure areas, such as the system east of New England shown in figure 12.

A cold front marks the leading edge of an advancing cP, mP, or A airmass, and is depicted as a line with triangular "pips."

A warm front marks the back edge of a retreating cP, mP, or A airmass, and is depicted as a line with half-round pips.

An occluded front marks the combination of a cold front and a warm front, and is depicted as a line with alternating triangular and half-round pips on the same side of the line.

A stationary front marks the boundaries of two disimilar airmasses that are not moving with respect to each other, and is depicted as a line with alternating triangular and half-round pips on opposite sides of the line.

In figure 12, the cold front seen in the Atlantic Ocean, off the U.S. east coast, marks the leading edge of a continental polar airmass. The main body of the airmass is over New England, the Midwest, and the Great Lakes region.

Fronts are areas of active, rapidly changing weather. New England's cold fronts are often associated with thunderstorms and showery precipitation in summer; snow-squalls and sudden, gusty onslaughts in the winter. New England's warm fronts are often associated cool, rainy days in summer; widespread freezing rain or blizzards in the winter. New England's occluded fronts produce weather resembling a combination the weather produced by cold and warm fronts. New England's stationary fronts produce either weather resembling that of warm fronts, or none at all.



Fig. 14. General schematic of warm and cold fronts.
New England's weather is affected by both cold and warm fronts, as well as occluded and stationary fronts. (See text for details.)
(Source: Lutgens and Tarbuck, 1998.)

The sea-breeze/land-breeze (SBLB) circuit. The SBLB circuit is caused by the different rates at which the land and ocean surfaces are heated by the Sun.

1. During the daylight hours on sunny days, the continental surface heats up more rapidly that the sea surface. Eventually, a small area of relatively low pressure is established on the land side of a narrow zone up and down the coast. A small area of relatively high pressure is established on the sea side of the narrow zone. (This is akin to the "one-cell" hypothesized for the non-rotating Earth, above.) The wind begins to blow from the high pressure area over the sea toward the low pressure area inland. Once reaching the center of the inland low, the air rises to a height of about 500 meters, where it turns seaward, and eventually sinks in the center of the small offshore high. A complete circuit is established. The rising air currents in the middle of the landward low are often marked by puffy white cumulus clouds, while the descending air currents just offshore are cloud free. (Click here to learn more about cloud types.) The onset of a sea breeze actually reduces the vertical height to which air pollution is mixed and dilluted in the atmosphere -- thus -- sea breeze events are often associated with poor air quality in urban areas, such as Boston.

2. During the evening hours, the continental surface cools off more rapidly that the sea surface. Eventually, a small area of relatively high pressure is established on the land side of a narrow zone up and down the coast. A small area of relatively low pressure is established on the sea side of the narrow zone. The wind begins to blow from the high pressure area over the land toward the low pressure area offshore. Once reaching the center of the offshore low, the air rises to a height of about 300 meters, where it turns landward, and eventually sinks in the center of the small onshore high. A complete circuit is again established. The land-breeze phase of the system is generally weaker than the sea-breeze phase, and is usually cloud free throughout.

New England's SBLB circuit is the subject of a research project at the Climate Change Research Center. Click here to learn more.



Fig. 15. Sea-breeze/land-breeze (SBLB) circuit.
Coastal New England's weather is affected by sea breezes and land breezes. (See text for details.)
(Source: Lutgens and Tarbuck, 1998.)

The valley-breeze/mountain-breeze (VBMB) system. The VBMB system is caused by the different rates at which valleys and mountain tops are heated by the Sun.

1. During the daylight hours on sunny days, the valleys heat up more rapidly that the mountain tops. The warm valley air rises up the sides of the mountains, slowly cooling by expansion in the lower pressure aloft. The rising warm air often causes the formation of cumulus clouds, rain or snow showers, and even thunderstorms. (Click here to learn more about cloud types, including cumulus.)

2. During the nightime hours on clear nights, the mountains cool down more rapidly that the valleys. The cold mountain air sinks down the sides of the mountains, slowly warming by compression in the higher pressure below.

Fig. 16. Valley-breeze/mountain-breeze (VBMB) system.
See text for details.
(Source: Lutgens and Tarbuck, 1998.)

	NCAR Real-Time Weather Data website.
	New Hampshire State Climatologist.
	NWS Interactive Weather Information Network.
	NWS Internet Weather Source.
	Penn State University Northeast Weather Page.
	Plymouth State College Weather Page.
	SUNY Albany Weather Page.
	UM (Univ. of Michigan) Weather.
	Unisys Weather.

Barry, Roger G., and Richard J. Chorley, 1998. Atmosphere, Weather & Climate. Routledge, 409 pgs.

Gross, Grant M., 1993. Oceanography. Prentice Hall, 436 pgs.

Lutgens, Frederick K., and Edward J. Tarbuck, 1998. The Atmosphere. Prentice Hall, 434 pgs.

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This page is supported by AIRMAP, a joint project of the National Oceanic and Atmospheric Administration, the University of New Hampshire, and Plymouth State College. The AIRMAP webpage is here.

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This page was last updated on 10/04/2002.