How the Wind Goes Around--Part 2, Coriolis Effects-Text

Earth Science Essentials

by Russ Colson

   

Lecture Recap

Image from lecture

 In the lecture we learned that winds travel

 

The Merry-go-around effect

One might expect that wind will blow directly from high pressure to low pressure areas, but, because the Earth is spinning, this does not occur. The Earth's spin causes the wind to bend due to a Coriolis Effect.

You feel this effect on a spinning merry-go-round when you try to move inwards or outwards on it. You feel an apparent force pulling you to one side.

Winds on a spinning Earth also respond to this effect.

 

The coriolis effect is really a consequence of Newton's laws of motion, a body tends to continue in a straight line unless some force acts upon it.

It seems that the ball on the spinning carousel is bending to the right or left, but it's really just trying to go in a straight line and the carousel is spinning beneath it, as shown in the series of pictures below.

Initial direction of marble on spinning table

Nest position of marble on spinning table, showing curve to the left when spin is clockwise

 continuing curve to the left on spinning table, showing that the marble is actually maintaining its original travel direction

 

This results in winds curving to the left of the direction that they would otherwise go (the 'downhill' direction) for the case of a clockwise-spinning disk.   Thus winds bend to the left in the southern hemisphere and to the right in the northern hemisphere.

 

Winds curve to the left with clockwise sping of the Earth, and to the right with counterclockwise spin.

 

As seen from the analysis above, the coriolis effect is not really a force, but we can think of it as a force.   The forces acting on a molecule of air include the pressure gradient, the coriolis force, and friction between the air and the land.   If we ignore the effect of friction, the forces are in balance when the wind travels parallel to the isobars as shown by the vectors below.

 

Illustration of the balance of forces acting on moleculres of air, ignoring friction

This means that, ignoring friction, winds will travel nearly parallel to isobars.   Assuming that friction is nearly zero works well for modeling winds in the upper atmosphere.   At the surface, friction is a significant factor.

 

Winds around Low and High pressure centers

 Wind gets deflected to the right in the northern hemisphere due to the coriolis effect.   This causes winds to spiral counterclockwise around low pressure and clockwise around high pressure as shown below.   Notice that the downhill direction is always perpendicular to the isobars and from the higher pressure isobar to the lower pressure isobar.   Also, 'to the right' means to the right of the downhill direction, not to the right of you, the observer, as you view a picture in a lesson text!

 Illustration of how winds getting bent to the right in the northern hemisphere results in a countclockwise rotation around a low and clockwise around a high

For the following puzzles, consider the map below from the northern hemisphere of Earth.   Also take note that by convention "the direction of the wind" means "the direction the wind is coming from."

 Isobar map with low and high pressure

 

Backing and Veering Winds

We can not only think about what the wind direction the wind is coming from at a particular place at any one time, but we can think about how wind direction will change at that place through time.   How the wind changes through time tells us whether the low pressure is passing to our north or to our south.   Whether the low passes to your north or south makes a profound difference in terms of the kind of weather that you experience.  

 

Consider the illustration below of a classic winter storm in temperate latitudes of Earth.   At these latitudes, storm systems tend to travel from west to east with the prevailing westerlies and the upper level winds.   However, the exact trajectory can vary from one storm to another.  

 

Consider the two possible paths of the storm shown below.   One path takes the storm north of our location (our location is at the dot) while the other path takes to storm south of our location.   One path will bring the heavy snow and wind over us.   The other path might lead to some freezing rain and showers followed by warming (generally much less severe winter weather).  

 Illustration of a classic mid-latitude cyclone showing precipitation and possible directions of propogration--for northern hemisphere

For the following puzzles, assume that the isobars are concentric, centered on the low pressure center.   Also, ignore friction for these puzzles.

 

As the low progresses toward the east passing north of the dot through time steps 1, 2, 3, and 4 the wind will change direction in which of the following sequences?

Wind shift puzzle when that storm tracks north of your location

Now consider the case in which the low passes to our south.

Wind shift puzzle when the low tracks to the south of our location

Often the approach and passing of a major winter storm at a particular location is accompanied by a backing wind.   Notice that with both veering and backing winds, the wind starts out from the east or southeast, even as the storm approaches from the west!

 

Last updated May 24, 2023.   All text and pictures are the property of Russ Colson.

.