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Reading the winds on the continent. - 01.30.06

Knowing what the wind is saying is the fundamental task for the weatherwise person. Find out about continental wind patterns from Doc Weather.

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Fig.1

The patterns of winds on the continent are best described by using a wind rose. The wind rose indicates the prevailing positions from which the wind will blow throughout the year in a given location. The chart below shows the wind rose for Philadelphia, PA. In Philadelphia the most common prevailing wind at all times of the year is from the southwest. The wind rose shows this in the long line to the southwest extending beyond the outer circle. It is longer than any other line in the diagram. The next direction that prevails is from the north / northwest. These lines just reach the outer circle. The next prevailing direction is due west, that just reaches beyond the second circle. This is followed by the northeast line, which reaches the second circle. Very little prevailing winds come out of the south or the southeast.

Fig.2

To understand this wind rose it is useful to look at the mean tracks of storms across the Midwest into the Philadelphia area during the storm season. In image 2 we can see that no matter where one of the continental storm tracks gray arrows has its origin it tracks through Philadelphia from approximately the same direction. The Great Lakes track across the northern tier of states, brings warm maritime air from the Pacific zonally or horizontally across the top of the US during the winter. Storms (marked L in the chart) in the northern hemisphere have a counter-clockwise circulation. A storm tracking into Philadelphia would have a circulation that would place the prevailing wind to the southwest as the storm approached Philadelphia from the northwest. This is depicted in the upper track.

A storm approaching Philadelphia from the Great Plains would also produce a southwest wind as it came into the area. This is depicted in the lower line. The circulation around a transiting low-pressure area will predominantly bring southwest winds Philadelphia no matter what direction it comes from across the continent. All storm tracks lead to the northeast corridor. To see why this is so it is useful to look at the dominant high and low pressure areas and their circulations during the winter months.

Fig.3

In image 3 the large low-pressure area over Hudson Bay is the dominant feature of the continental circulation during the winter. This is due to the fact that the ground around this enormous body of water is much colder than the water. Water retains its heat much longer than land. As a result, during the winter the Hudson Bay area is a consistent source of rising air. Rising air produces low-pressure. The low-pressure circulation is counter clockwise so the prevailing circulation around this semi permanent low is from the northwest in the Philadelphia area (dark arrow). This gives us insight into the northwest element of the Philadelphia wind rose.

The circulation is so immense around this low in the winter that it pulls air from the Pacific Ocean into the continent when it is very active. This maritime air, as we have seen, will steer lows across the northern states and then funnel them into the northeast giving rise to the prevailing southwest winds as the fronts pass the east coast to the north of Philadelphia.

Fig.4

Occasionally the Hudson Bay low is influenced from the east by the presence of a strong high-pressure area over the Maritime Provinces or Greenland. This high-pressure area or block is known as the Greenland high. A block here prevents the passage of the warm maritime air across the northern states. The Hudson Bay low starts to pull down cold air from the western and central Canadian Plains. The cold air pushes the storm jet down into the High Plains states. When this happens, storms track south, farther to the west and penetrate deeper into the south on the continent. Usually when the Greenland high is active a trough forms over the east coast. This is illustrated in image 4. Storms track low across the Gulf Coast states and then as they approach the East Coast they run into the trough. We can see that the eastern leg of the trough roughly parallels the East Coast. This is due to the fact that the high-pressure from Greenland often blankets the whole western Atlantic during these events, making the jet run parallel to the East Coast. The trough usually settles against the offshore high and has little incentive to move east across the high-pressure ridge. In the chart we can see a low traveling into the East Coast around the eastern side of the trough. The circulation of the low brings the prevailing wind still to the southeast as the low approaches the East Coast.

Fig.5

In image 5 however, we can see the situation when the trough is placed so that the eastern leg is out over the ocean. This happens when the block is strong to the north but weaker to the southeast over the ocean. The trough drifts to the east. The chart shows a detail of a low moving north along the eastern leg of the trough running parallel to the coast. The circulation around the low puts the wind out over the ocean on the leading edge of the low. The wind then shifts to the northeast as the low moves up the coast approaching Philadelphia from the south. This is the famed nor�(tm)easter that brings wet snows and blizzard conditions to the New England and northeastern sections of the eastern seaboard. It also gives an insight into the strong elements in the wind rose from the northeast.

Fig.6

In image 6 the Greenland block is placed into a context of the whole Atlantic Ocean. The placement of the Greenland block far to the west over the Maritime Provinces is, in certain years, a regular event during the winter. The usual pattern is that the high that is normally over the Azores off of the west coast of Africa, shifts to the north Atlantic. The high latitude high-pressure ridge then oscillates between Iceland and Greenland in approximately ten-day intervals. That is, for a few days the high is over Iceland then it starts to drift to the west at a high latitude ending up over Greenland. The continental jet stream on the US is then pushed back towards the west as the Hudson Bay low slips to the west because of the Greenland block. The Greenland block may stay in position for a week or so and then the whole cycle starts again near Iceland. When the block is in place the polar jet brings cold into the eastern seaboard. When the block is not in place the polar jet pulls air off of the continental US and shunts it out to sea.

These patterns are the major weather makers for the eastern third of the country. The placement of these blocks and troughs varies throughout the year in the context of the placement of the eclipse points and the subsequent eclipse grid that is generated by them.

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