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Boston Weather: 32.0°F | Fair

Weather Confusing Ice

Confusing Ice

By Greg Lawson
STAFF METEOROLOGIST -- This morning’s sleet and freezing rain mix will shift to rain and slowly end by late this afternoon or early evening.

Many people often confuse hail and sleet (and sometimes freezing rain). The confusion is understandable as hail is relatively rare and does look roughly similar to coarse sleet, however, hail and sleet are very different in nature.

Sleet and freezing rain are winter weather phenomena. Their formation depends on the vertical structure of the lower portion of the atmosphere. Frontal passages in winter storms often force less dense warm air above the denser cold air. This can form a layer of warmth between the clouds and ground which can melt falling snowflakes. If there is enough cold air between the warm layer and the surface, then the water can refreeze as ice pellets or sleet; if not, then it generally becomes supercooled and freezes instantly on impact; this is freezing rain, the main culprit of ice storms.

Hail leaves its precipitating cloud as ice rather than being transformed into ice on its descent. Hail is most commonly found in the spring and summer and is associated with large cumulonimbus clouds, the very tall “anvil” clouds which can spawn tornadoes. Even in the warmer seasons, these clouds are tall enough so that their tops are often well below freezing. Very strong upward velocities exist in these clouds, strong enough to overcome the weight of ice pellets.

Hail forms as a continual freezing, falling, partial melting, rising, and freezing cycle within the clouds until the ice pellets formed are heavy enough to overcome the strong upward velocities. By this time they are large enough to last as ice on their descent to the surface.

Extended Forecast

Today: Sleet or freezing rain changing to rain. Strong southeasterly winds. High of 38°F (3°C).

Tonight: Rain ending and skies clearing. Winds shift to the northwest. Low of 32°F (0°C).

Wednesday: Partly cloudy. Breezy. High of 50°F (10°C). Low of 33°F (1°C).

Thursday: Partly cloudy. Windy. High near 52°F (11°C). Low near 37°F (3°C).

Friday and Beyond: Potential storm on the way ...