Highs Blow, Lows Suck

A while ago, Steph came across this saying written on a meteorological t-shirt.  She asked me what it meant so I decided that I would answer it here..... it just took longer than normal for me to get around to it!  Anyways, on to the topic!

The creation of high and low pressure in the atmosphere is all about the convergence and divergence of air high in the atmosphere.  When air "converges" into a given air column, the mass of air in that air column increases over time.  When you add mass to a column, the amount of force, or pressure, that it places on the surface below increases as well, creating an area of high pressure.  Similarly, when air "diverges", the mass of air in that column decreases over time and reduces the amount of pressure on the surface below, creating an area of low pressure.

An example of how convergence and divergence in the
upper atmosphere creates high and low pressure systems at the surface.

In addition, there are vertical motions that take place with convergence/divergence.  When air converges (diverges) in the upper atmosphere, it has no where to go besides down (up).  Therefore, there is sinking (rising) motion associated with high (low) pressure systems.  At the surface, this results in divergence (convergence) under high (low) pressure systems - hence the saying "highs blow, lows suck."  In high pressure systems, air is forced down from upper levels to the surface and spread out (the blowing).  Meanwhile, in low pressure systems, air is pulled up do to the upper level divergence, pulling the air into the column at the surface (the sucking).  However, there is much much more to pressure systems that just this since this concept does not take into account one factor.... rotation and the jet stream!

To understand how air flows around areas of high and low pressure, you need to under stand the Coriolis force.  As air flows around the globe, the Coriolis force turns the flow to the right in the northern hemisphere and to the left in the southern hemisphere, creating a curved flow.  (to understand how the Coriolis force does this, I suggest you watch this video:

 

As a result, the divergent flow under a high pressure system is turned to the right, creating a clockwise rotation.  This kind of rotation is called anti-cyclonic.  Meanwhile the converging winds under a low pressure system are also turned to the right, causing a counter-clockwise or cyclonic rotation.  The opposite is true in the southern hemisphere where winds are turned to the left.  High pressure systems there are cyclonic and low pressure systems are anti-cyclonic.

Now in the upper atmosphere, the jet stream flows around areas of high and low pressure creating the ribbon-like appearance we are use to seeing.  As the jet stream flows curves around high pressure, the winds increase in speed, becoming what we call "supergeostrophic" (faster than the normal geostrophic value) while at the base of the trough, the winds decrease in speed or "subgeostrophic" (slower than the normal geostrophic value).  Therefore, the air must accelerate from the bast of the trough to the crest of the ridge and decelerate from the crest of the ridge to the base of the next trough.

Now, let's imagine that this flow is actually like bumper-to-bumper traffic with cars crossing the "ridge" faster than cars in the trough.  Travelling from the first trough to the next ridge, the distance separating the cars increases as their speed increase, or they diverge.  But between the ridge and the trough, the cars begin to pile up as the faster cars in the back slam into the slower moving cars in front.  As a result, they converge.  The divergence between the trough and next ridge decreases the pressure at the surface thereby strengthening a surface low pressure system or weakening a surface high pressure system.  Conversely, the convergence between the ridge and the trough increases the surface pressure, thereby weakening a surface low or strengthening a surface high underneath.

Obviously, there are some other factors that influence high and low pressure systems (including jet streaks!), but that would make for a very lengthy article that could be a bit boring to read if you're not that interested.  So with that I will cut this article short, and maybe dive back into it at a later date if you are at all interested!