In our last weather class, we were looking at Earth’s greenhouse effect and how it keeps the planet warm enough for life as we know it to exist. That was a part of our general look at Earth’s energy balance. With that in mind, this week’s lesson looks at the difference between land and water and how they “manage” the energy coming in from the sun.
Before diving into that topic, I’ve had a few questions over the last week, asking if we are heading toward one of the coldest Aprils on record. The quick answer: No. Using just Winnipeg’s data, I calculated the average maximum, minimum and mean temperatures for the first half of April, dating all the way back to 1938. I then compared these values to this year’s numbers. It turns out, and the table here shows, that there have been much colder starts to April.
While we have not seen record cold, it has still been a cold first half. Looking back, it was about the 10th coldest in the last 80 or so years. Now, on to this week’s topic.
The lay of the land
In its simplest form, Earth is composed of two different surfaces: land and water. These two surfaces are not arranged evenly around the planet, but instead are broken up into an irregular arrangement of continents and oceans. The differences in the ways land and water absorb and store incoming solar energy, and the irregular distribution of those two surfaces, are what greatly influence our planet’s weather.
There are five main differences between land and water that we will examine this week: evaporation, transparency, specific heat, vertical movement and, finally, horizontal movement.
The first main difference is evaporation. When solar energy strikes water, some of that energy is used to turn the liquid water into water vapour. This energy is absorbed and is stored in the water vapour as latent heat. You can feel this happen when you wet your skin and then let air blow across it. That part of your body feels cooler as heat is being absorbed from your skin to allow the evaporation to take place. Over the entire Earth it’s estimated nearly 85 per cent of all evaporation takes place over oceans, because the oceans are large and, of course, are all water — and land surfaces are not. This means that over water, a large portion of the sun’s energy is being stored as latent heat.
Our second main difference is something called transparency. We all recognize that if we call something transparent that usually means we can see through it or that it lets in light. When we compare water to land it becomes pretty obvious that water is much more transparent than land. Over land, the sun’s energy can’t penetrate the ground, so it gets absorbed at the surface. This allows land surfaces to heat up rapidly during the day and also to cool rapidly at night. Over water, the sun’s energy can penetrate as deep as 60 metres. This means the energy is being spread out and absorbed over much, much larger areas. This results in a slower warm-up during the day and a much slower cool-down at night.
The third main difference, known as specific heat, is tied fairly closely to the previous difference. Every substance has its own specific heat value, and that value basically tells us how much energy it takes to heat that substance up. When we compare land and water, we find that, given the same volume of each substance, water takes four times as much energy as land does to heat. So, if we combine this fact with transparency, we see it will take a lot longer to warm up a body of water than it will to warm a land surface. We can see this happen every fall and spring. On a cold fall day you will find that a body of water is much warmer than the land surfaces around it, while in the spring, the land areas warm up quickly, while the water struggles to melt the ice.
Our final two differences are movement, in both the vertical and horizontal directions. This one is pretty simple to picture and understand, but it has very large implications for our planet’s weather. When we look at land, for the most part, it doesn’t move. Soil at the surface that warms up quickly doesn’t then move underground, taking its heat with it. Nor does one field get up and move down the road. Land pretty much stays where it is. Water, on the other hand, is almost constantly in motion; the sun warms the top layers the most and these top layers can then be transported downward with wind and currents, warming the deeper layers and, in essence, creating a larger heat storage area.
Even bigger than the vertical movement of water is the ocean’s currents, or horizontal movements. These currents can take energy that’s being stored in one part of the ocean and move it to another part. Some of this energy can then be released, moderating the temperature of the area.
In our next weather class we’ll build on this knowledge by looking at how all this affects wind and general atmospheric circulation.