Before forcing our way into our next weather school topic, here is a quick look at global temperatures for April. As we all know, April across central North America was not warm, but globally it was another very warm month. According to the Copernicus Climate Change Service, this April came in tied with April 2016 as the warmest on record. Sometimes we tend to forget about the world and only focus on our little part. If you take a look at this week’s weather graphic which shows global temperature anomalies for April, you can see that most of the planet is red or warmer than average, with the exception of our part of the world — lucky us. The warmest region, compared to average, was across a large portion of Siberia, where temperatures were as much as 12 C above average! Places as far north as Churchill experienced daytime highs above 30 C in late April. We have to remember, there is always cold air over the northern part of our globe; if a usually cold region is experiencing really warm temperatures, then some other region will be seeing colder temperatures.
Contents under pressure
Now, on to weather school. Last week we introduced the four main forces that drive our winds; gravity, air pressure, Coriolis, and friction. This week we will go into a little more detail on these forces and hopefully come to a better understanding of just what creates wind.
If you remember back to last week’s article, we discussed that without gravity there would be no weight to the atmosphere, and without that weight there would be no air pressure. So, in our discussion we will leave out gravity and focus in on the forces of pressure gradient, Coriolis and friction, and how they interact together to create our winds.
Pressure differences exist in our atmosphere due to the unequal heating of Earth’s surface. Cold air is dense and will exert greater pressure than warm air. We show or measure the strength of a pressure gradient by isobars or isolines. These are lines on a map that connect areas of equal pressure. The spacing between the isobars tells us the intensity of the pressure gradient force — close spacing indicates a steep gradient, while wide spacing is equal to a lesser or more gradual gradient.
If we were to take a simple look at the Earth, this would mean that the polar regions would be regions of high pressure since they are cold and the equatorial regions would be areas of low pressure since they are warm. The air would then move directly from the north toward the south at right angles to the isobars. We all know that this is not always the case; the question is, why not? The answer lies with our next two forces, Coriolis and friction.
First let us look at the Coriolis force. First of all, I am not going to get into an argument about whether this is a real force. We could spend a whole article discussing that point. Instead, for simplicity’s sake, let’s just agree that it is a force. Basically, the Coriolis force arises from the fact that the Earth rotates. Anything that flies or flows across the Earth’s surface (wind, airplanes, ocean currents) will be deflected from a straight path because of that rotation.
In the Northern Hemisphere, things are deflected to the right of their intended path, and to the left in the Southern Hemisphere. The Coriolis force is weakest at the equator and is strongest around the poles. The amount of deflection from the Coriolis force is also controlled by the speed of the object that is moving; the greater the speed, the greater the deflection.
So, on our simple Earth, the air flowing from high pressure at the North Pole toward low pressure at the equator would be deflected to the right, eventually ending up flowing parallel to the isobars. While this type of airflow is what we would typically find in the upper atmosphere, we really don’t see that near Earth’s surface. Why? Well, we have our last force to deal with that is not really felt in the upper atmosphere, but can have a significant impact near the earth’s surface: friction.
Friction between the wind and Earth’s surface varies depending on the type of surface over which the wind is blowing. Typically, frictional forces will affect winds up to an altitude of about 500 metres. Friction decreases the speed of the wind and therefore reduces the effect of the Coriolis force. Now instead of the wind blowing parallel to the isobars, it blows at an angle across them. The net effect of this is that winds blowing from areas of high pressure tend to spiral outward, then spiral inward as they move toward low pressure. The image I’ve included with this week’s article nicely shows these three physical forces that produce wind.
Next week we will continue our look into wind and start to explore how these three forces produce the general patterns of winds we see blowing across the Earth. After that, I think it will finally be time to start our yearly look at thunderstorms — the good, the bad, the beautiful and, sometimes, the ugly.