Most were not here, or perhaps, some had forgotten. But six days
ago, while most of campus was relatively deserted, a -15°F wind
chill reading was recorded at Logan Airport. Yet, the hottest
temperature on record (67°F) for any January 8ths happened
yesterday.
What's going on? How did this happen? There is
actually a very straightforward explanation for this and it centers
on two words: high pressure.
As you have learned in class, there is clockwise flow (in the
Northern Hemisphere) around a high pressure. So on January
3rd, when a strong high pressure system was located to our
southwest, this induced strong northerwesterly flow in our region.
Do you see that from the surface map taken from that chilly morning?
In essence, the cold air from Canada was "pulled" into our
region. Fast forward 5 days, and the surface map looks like
this:
At this time (day of record high), with the same high
pressure system remaining strong and pushing off to our east, the
clock-wise flow around this high pressure allowed for southwesterly
wind. This allowed strong mixing all along the eastern half of
the United States. In other words, southern states shared the wealth
of warm air to its northern counterparts. For instance, with a
record high of 67°F yesterday, there was only a 2°F difference
between Boston and Atlanta.
To better convince you of this strong mixing, we now
plot the temperatures over the continental US.
In the absence of such latitudinal mixing, you would
expect the Northern States to have about at least a ~20°F
temperature difference than its Southern States. However on this
day, if you draw a line from Ohio 1000 miles south, the line will only
be going through a couple of contours about a 10°F difference.
So in short, we can thank this strong high pressure
system for bring in the cold, but also the record warmth of yesterday
as well.
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