Weather and Zonal Flow

Meteorology and weather forecasting would be an entirely different proposition if the planet on which we live did not rotate about its axis. The wind patterns would be completely different, blowing directly from high pressure to low pressure.

The rotation of our planet causes the winds to be deflected to the right in the northern hemisphere, and to the left south of the equator. This apparent force is called the Coriolis force and is equal to 2 times Omega V sine Phi, where Omega is the rotation of the Earth, V is the wind speed and Phi is the angle of latitude.

The equation above shows that the Coriolis force becomes more important the further the wind is from the equator, since the sine of the latitude at the equator is zero. Hence the whole equation, and the Coriolis force, at the equator equals zero.

This generally west to east flow of the wind is called the zonal flow. Near the ground the winds are deflected across the contours, or isobars, towards the low pressure, due to friction.

With the semi-permanent high pressure to the south, on the northern edge of the Hadley cell, and low pressure to the north, these westerly zonal winds are the usual winds that blow across the Atlantic and bring the rain-bearing weather fronts across the country.

When this zonal flow persists during December through to February, we tend to have a wet, mild winter.

The air masses over the north Atlantic are continually on the move as warm tropical air pushes northwards in one area, whilst a cold plunge of arctic air forces its way southwards somewhere else.

When this happens the zonal wind can often be deflected from its normal west east orientation. The winds then, at all levels in the atmosphere, twist around and can move through 90 degrees or more, so that instead of blowing west to east they blow north to south.

Zonal flow is very important to the natural variability of the weather that we get in this country. Without it we would not get the rainfall that we need to grow our crops, and live the life that we do without the massive extremes in the weather that we see around the world.