I was reflecting on this winter’s weather, and I thought it would be interesting to take a look at some of our stronger wind events and the conditions that led to them.

 

High wind events in the winter tend to have some similar features, and much of this can be explained by a little bit of physics.

 

First let’s take a look at some surface maps.

 

We’ll start off with our 2nd highest daily peak gust.  This map show the surface weather analysis for January 5th, where we recorded a gust of 129 mph from the west.

 

 

The black lines on these maps designate lines of equal pressure as determined according to values reported by surface weather stations, so as you cross black lines, pressure is either increasing or decreasing.  These lines are called isobars.  Pressure values are given in millibars, or hectopascals (1 millibar = 1 hectopascal).

 

 

So immediately, you should notice high pressure over Indiana and Illinois decreasing sharply as you look to a strong low pressure area northeast of Maine.

 

Pay attention to the location of the highs and lows (H’s and L’s) as we look at these next couple of maps.

 

Here’s the analysis of our big wind day this winter, February 16, when we recorded a 141 mph gust from the northwest.

 

 

The low over the Canadian Maritimes bottoms out at 960 mb, wow!

 

And the last surface analysis map we’ll look at is just from a couple weeks ago when we recorded a gust of 124 mph, also from the northwest.

 

 

And wouldn’t you know it, another cyclone sitting over the Canadian Maritimes!  This time the low isn’t quite as deep, only down to 972 mb this time.

 

Ok, what’s going on here?  Anyone else starting to notice that these maps look pretty similar?

 

In all three cases, we see significant cyclones over the Canadian Maritimes, and high pressure pushing in from the west.

So why are the winds so high, and why is the location of the highs and lows so important?

We need to understand a few things in order to explain this.

First, what causes wind?

Wind is caused by differences in pressure, and wind is the movement of air in an attempt to balance out these pressure differences.

So if this makes sense, the greater the difference in pressure, the faster the air moves, and the greater the wind speeds.

We describe the rate of change in pressure over a given area as the pressure gradient.  So the closer together the black lines are, the greater the pressure gradient, and the greater the wind speed.

 

Air is trying to move from high to low pressure, but as the earth spins, the air doesn’t quite follow this trajectory because of the rotation of the earth.

 

It’s like trying to play catch on a merry-go-round.  If your friend is sitting in the center, and you are sitting on the edge, and you’re both spinning in circles, if you were to try to throw a ball directly towards your friend, the ball would appear to deflect away from the direction you were spinning.

The same thing happens with winds, and for this reason, winds flow roughly parallel to the black lines.

But as I’m sure you can imagine, it’s a little more complicated, and it involves a little more physics.

As you may have noticed, the pressure lines are not very straight; in fact, they’re quite wavy and curved. Understanding the effect the curvature has on these forces is important.

We’ll employ another analogy to help us understand what’s going on.

Think about performing the classic “Around the World” yo-yo trick. You’re spinning the yo-yo in a circle over your head, by your feet, and eventually back to your hand. There is always a centrifugal force that pushes the yo-yo outward, but is always at least partially held in place by tension from the string.

What is important in this analogy is to understand the role that gravity plays. When the yo-yo is above your head, gravity is opposite the direction of the centrifugal force, so there is less tension needed to balance out the forces. But when the yo-yo is by your feet, gravity is in the same direction as the centrifugal force, so the force of tension provided by the string needs to be much higher.

As these forces increase while entering the lower half of the yo-yo’s trajectory, it cause the yo-yo to move faster by your feet than when it is overhead and the forces are dampened.

This same type of thing happens in the atmosphere, except different forces are involved.

When air circulates around high pressure systems, the centrifugal force pushes outward in the same direction as the pressure gradient force pointing from high to low pressure. When these forces stack up, wind speeds ramp up.

We see this happening to some extent in each of these cases. Deep low pressure systems create significant pressure gradients accompanied by a high pressure ridge pushing in from the west. This combines to create exceptionally strong winds on the backside of the low.

Next time you see high winds in the forecast, take a look at a forecast map and check out what’s going on!