Last weekend’s cold, windy end to a run of nice spring weather was due to this low pressure cell, moving into the Great Lakes from the US midwest. It dumped upwards of 8″ of lake effect snow on parts of the Chicago area and gave the GTA a reminder of the winter of 08-09, with blustery lake effect snowy days from the cold NW flow on its back side. We went from shirt sleeves and BBQs on Sunday, to boots, mitts and gloves on Monday.

We don’t always get to see a nice tight rotation like this over us. Pressure got down as low as 992 hpa and it dropped about 50mm of rain out of the warm sector (leading east edge) before flipping over to woolly time (the low is much larger than what is shown – this radar site’s range is only able pick out the core of the low. Usually we see only air motion in one direction as the radar samples a portion of the low away from the core.

Northern hemisphere low pressure cells are counter-clockwise rotating cyclones, where high pressure areas rotate clockwise and are anti-cyclones.

If you watch the video and think about it a minute, you can see how the leading edge of the low draws warmer air from the subtropics , and the back side pulls down colder arctic air. The mass of colder air being slowly pulled down produces the drier “cold front” rotating around the center of the low, pushing up against the warmer air dragged up from the south.

Ahead of this cold front the air is moist and turbulent, leading to the active weather we experience. As the cold front rotates past us, the drier air gives rise to clearer skies (drier air) and cooler temperatures.

Air being pulled into the low (low pressure because the air at the center is moving upward) causes the strong winds we experience, as the low and fronts pass. In contrast, Highs pull dry air down from the upper atmosphere. This air being drier, doesn’t have water to condense and form clouds, hence the clear, but ofter cooler days.