HIGH pressure dominates but is it all calm?
High pressure is known for calm, clear conditions, with little wind, cold frosty and foggy nights especially when there is little cloud. Pretty unexciting weather. However, HIGH pressure is not as unexciting as all that. Anticyclones can sometimes be surprisingly windy especially round the edges. We spend a lot of time learning about LOW pressure, with associated storms and gales and torrential rain but understanding the inner workings of HIGH pressure is important to get the full picture of mid-latitude weather.
Pressure gradient force: winds from high to low
Coriolis force: deflects to right by coriolis force
So… buckle up for the ride and let’s get super-geostrophic! Wind blows from HIGH pressure to LOW pressure. The wind speed and direction is the result of two forces: the pressure gradient force (PGF) is the difference between high and low pressure and sets up the strength of the wind and the overall direction which is for winds to blow directly from HIGH to LOW pressure. Coriolis force (or Coriolis Effect) is a result of the spin of the Earth and deflects resultant winds to the right of their intended path in the northern hemisphere. Here are some video links to review these forces before proceeding with super and sub-geostrophic winds. Skip below these videos if you already know about PGF and Coriolis.
The winds do blow from high to low… but get pushed to the right by that Coriolis fellow!
The pressure difference between high and low pressure determines the speed of wind. Winds do blow from high to low due to the pressure-gradient but are deflected to the right by another force called the Coriolis effect! Below is a chart showing upper winds at 850hPa (1500m) blowing round the same HIGH pressure shown on the synoptic chart at the top of the post. Note the relatively high wind speeds circulating round the HIGH in the north of Scotland, the North Sea and across France and Biscay especially. Winds obviously blow faster across the ocean but remember this is an upper wind chart so is above the boundary layer of most frictional forces upsetting the wind. In any case, none of these locations is associated with a trough… it is all anticyclonic super-geostrophic wind. So why is the wind blowing so strong when there is no LOW for miles?
Given the same isobar spacing the wind speed aloft round high pressure ridges is often greater than the wind flowing around troughs and low pressure. This is surprising because we associate gales and windy weather with “storms” and low pressure systems. The chart above illustrates super-geostrophic winds circulating around the Azores high across Europe. These look pretty strong at 850hPa (1500m), the level above frictional effects of the surface. The chart also shows the trough of low pressure over the Mediterranean where, given some of the locations with similarly spaced and even tighter isobars, the wind strength is not especially any greater and perhaps even less than that circulating freely around the HIGH.
Wind is a result of pressure differences across the planet surface. Wind wants to blow from high to low pressure. This is called the pressure gradient force. Due to the spin of the Earth winds in the northern hemisphere are deflected to the right of their intended path. The two forces, pressure gradient and coriolis force, actually balance out to produce a theoretical wind that flows parallel to the isobars called the geostrophic wind, shown above. Unfortunately, isobars are almost always curved so the geostrophic wind hardly ever actually blows.
Assuming a constant isobar spacing. Around troughs of LOW pressure the wind is sub-geostrophic. This means it blows less than the expected geostrophic wind. In the chart above the wind is shown as a black arrow. In addition to the coriolis force, the centrifugal force acts to “push” the wind away from the low centre and is acting in the same direction as the coriolis force. Note that the resultant wind is pointing slightly away from the LOW towards the HIGH, which is of course not possible because the wind would be moving into and against increasing pressure. As the pressure gradient force cannot change, the coriolis force must weaken to allow the wind to return parallel to the isobars. This means that the wind flowing around troughs of LOW pressure has reduced force acting on them given the same isobar spacing of a similar HIGH. These winds therefore blow slower than geostrophic wind and are called SUB-GEOSTROPHIC.
Here is the HIGH pressure situation. This time the centrifugal force is acting with the pressure gradient force to push the wind into low pressure. As the pressure gradient cannot change the coriolis force must INCREASE to pull the wind back parallel to the isobars. This means that the wind flowing around ridges of HIGH pressure has GREATER forces acting upon them than winds flowing round lows with equivalent isobar spacing. These winds therefore blow faster than geostrophic wind and are called SUPER-GEOSTROPHIC.
Usually, of course, low pressure cyclones and depressions exhibit tighter isobar spacing than HIGH pressure and so resulting wind speeds round LOWS are most frequently higher than the HIGH pressure feeding them. Nevertheless, assuming the same pressure-gradient force, winds exiting anticyclones can produce higher wind speeds than those entering depressions.