After taking a week off from Weather School to take our monthly look back at the previous month’s weather, and to peer ahead to see what type of weather we may see over the next couple of months, it’s time to finish up our discussion about the seasons. We need to take a look at the “march of the seasons” and the extremes we encounter in day length during the year.
Day length is measured by the interval between the sun rising and setting each day. The two extremes of day length (shortest and longest days) occur in December and June and are referred to as the solstices. On Dec. 21 or 22, we experience the winter solstice and the sun/Earth angle has the sun directly overhead at 23.5 S latitude. It’s at this time that the Northern Hemisphere experiences its shortest day, which, in southern Manitoba, is about eight hours and six minutes in length. It’s also on this day that all locations north of the Arctic Circle experience 24 hours of darkness.
On June 20 or 21, we experience the summer solstice. At this time the sun is directly overhead at 23.5 N latitude, giving us our longest day at about 16 hours and 21 minutes in length. Areas north of the Arctic Circle then experience 24 hours of daylight. Now, usually you would expect the shortest days to be the coldest and the longest days to be the warmest during the year, but this is not the case. In a few more lessons we will see why the warmest and coldest days usually occur about one month later.
Our last two important dates in the “march of the seasons” are the vernal and autumnal equinoxes. These are the dates when the sun is directly over the equator, giving approximately 12 hours of daylight to all places on Earth. The vernal equinox occurs on March 20 or 21 (March 19 this year, due to 2020 being a leap year) while the autumnal equinox occurs Sept. 22 or 23.
We will revisit the sun’s journey from south to north and back again in a later Weather School lesson, as this simple process helps to drive much of the weather we experience here in Canada. For now, we need to take a look at the composition of our atmosphere.
When looking at the Earth’s atmosphere, there are a couple of ways of breaking it down. If we look at the composition of the atmosphere, we would find that there are two layers — the heterosphere and the homosphere. The heterosphere begins at about 80 km up and extends right out to the beginnings of space. This part of the atmosphere makes up only about 0.001 per cent of all our atmosphere’s mass, and as the name suggests, the gases in this part of the atmosphere are not evenly distributed.
In the homosphere, which is everything below 80 km, we find the vast majority of our atmosphere’s mass. While the density of gases in this layer drops off fairly rapidly as you leave Earth’s surface, the mixture of gases stays relatively uniform or homogenous — with the exception of water and ozone. The reason for the rapid drop-off in density as we move upward is that gravity is pulling down on the air, compressing it near the Earth’s surface. In fact, once you travel upward to a height of around 5,000 metres, nearly half of the atmosphere’s mass is below you. At a height of 11,000 m — where most jets fly — over 75 per cent of the atmosphere’s mass is below you!
Looking closer at the homosphere, we find that it is almost entirely made up of three gases: nitrogen (78.08 per cent), oxygen (20.95 per cent), and argon (0.93 per cent). The remaining part of the atmosphere (0.03 per cent) is made up of gases such as neon, helium, carbon dioxide, methane, and ozone — along with several other trace gases.
The second way of describing the makeup of our atmosphere is using temperature. Using this method, we can break the atmosphere into four distinct layers. Starting at the top of the atmosphere and working our way down to the surface, the layers are the thermosphere, mesosphere, stratosphere and troposphere. For those of us who follow weather, or paid attention in science class, the bottom two layers are probably the most familiar and are the layers that come into play when trying to understand the weather.
Next week we’ll continue our look at the atmosphere by breaking down these layers and discover why most of our weather is confined to just one of these layers, the troposphere — stay tuned!