THE VERMONT WEATHER BOOK by David Ludlum (Vermont Historical Society, 1996)
CONTINUED from PAGE 1:
During April, with the axis of the main westerly jet stream remaining quite far south at 38°N, the storminess of late winter continues. An area of high cyclonic frequency lies off the Middle Atlantic coast, and some of its disturbances affect the Green Mountain area. The main storm tracks of April resemble those of March: the St. Lawrence Valley path has frequent travelers whose frontal systems trail south over New England and bring quick changes from warm to cold to warm, and from dry to wet to dry.
The Hudson Bay High is now the dominant feature of weather maps over eastern Canada; its high-pressure ridge, which extends over the ocean, continues to block the movement of storm systems from the west and along the Atlantic coastline. This pressure alignment often feeds cold airstreams from Labrador and Newfoundland southwest into New England and is directly responsible for the periods of backward spring weather so characteristic of the region.
A center of maximum anticyclonic activity in April is situated over South Dakota; airstreams from this source that arrive in Vermont are modified by their trip over bare ground and do not have the sharp crispness of the air flow from the highlands east of Hudson Bay.
The thermal onrush of the new spring season outdoes the increase in solar elevation. Mean temperatures rise from 12 to 14 degrees in various parts of Vermont during April. At Burlington, for instance, the mean maximum rises from 46° on April 1 to 60° on April 30, and the mean minimum climbs from 27° to 38° with a freezing night being an exception by the month’s end. Newport in the northeast has a mean of 40.2°, while Vernon in the southeast is 44.9°, and Bennington in the southwest, 45.8°. In April temperatures have ranged from a maximum of 97° at Vernon in 1976 down to -12° at Bloomfield in 1923.
A mean temperature of 45° is the threshold of growth for much of nature’s plantlife. A harbinger of spring is the appearance of pussy willows, which require a few days with warmth in excess of 45° to get started. The “pussy” is not a flower, but a bud, said to resemble a number of small kittens climbing the stem. Though pussy willows may be brought indoors and honored as a decoration in April, they must give way in May to more colorful productions of nature.
The precipitation pattern begins to change from a winter to a spring type in April. By the beginning of the second week at Burlington, the chance of snow drops below 50 percent and by the end of the month well below 25 percent. Convective-type showers and thunderstorms increase, which swell the precipitation totals well above the winter amounts. The mountain stations continue the wettest with average amounts about 4.50 inches, and valley stations show substantial increases: Vernon to 3.84 inches and Bennington to 3.56 inches.
“April showers that bring May flowers” play an important role in nature’s calendar. After the general storms of winter with steady precipitation over wide areas, the rainfall pattern becomes more localized, with brief showers over limited areas predominating. The increase in the sun’s heat causes the surface air to warm, while the air aloft remains relatively cold. This sets in motion the process of convection whereby a parcel of warm air rises in the same manner that a hot air balloon does. In rising, the air cools at a steady rate until it reaches its saturation temperature, or dew point. Its moisture condenses into a visible cloud. If conditions continue favorable, the cloud will grow and turn into a cumulus shower cloud or even a cumulo-nimbus thunderhead. April is usually the first time in the season that sufficient warmth is available in the atmosphere to start the local cloud-making process, which is why showers and April have become synonymous.
