It’s Wintertime, when lots of people grumble a lot and some people feel like dancing around in the brisk air. Most of my family are lizard people who like it when they can sit in the shade and sweat. I am, in this respect as in quite a few others, a sheep of a darker tone among my family. I like cold weather. My favorite day of the year is the first day I can wear a sweatshirt. I find hot weather oppressive. I get annoyed when I see a weather forecast for a week in January or February when it’s going to be warm, because I feel like I’m being cheated out of a week of Winter. So, this seems to be a good opportunity to talk about the weather.
Credit: Mathias Krumbholz, Wikimedia Commons
First off, let’s start off with a little review about climate change, since we’ve already had an article on it (Article #20). The first thing is that you can tell absolutely zip about climate change from what’s happening outside at any particular time or in any particular season. It wouldn’t matter if it was averaging 15 degrees below normal or 15 degrees above. What is going on outside in any given year is not a relevant data sample. Global warming is just that—global. The data that is used to determine climate change is taken from all over the world, not just our little corner of it. Secondly, one year of data is just one year of data. It might be a warm year, or it might be a cool year. It might be wet or dry. There might be lots of hurricanes (or tornadoes or wildfires) or fewer than normal. To make determinations about climate trends, multiple years of data are necessary. The determination that the climate is warming is based on decades of temperature measurements made at thousands of different locations. So don’t go outside and think, “Well, this is obviously a sign of global warming”, or, if you are one of those folks who choose to ignore the mountain of objective data and refuse to believe that the climate is warming, you also can’t go out in a snowstorm and say, “Global warming, my eye! It’s been snowing for a week!” What tells the story are trends in weather and weather-related events over time, and that story is clear that the climate is warming and it is negatively affecting multiple different aspects of our lives.
But let’s think about weather (what’s happening now) instead of climate (what happens over time) now. Why is it winter at all and why is it sometimes warmer, sometimes colder, sometimes raining, sometimes sunny, sometimes windy, and sometimes calm.
There are a lot of misconceptions about why the seasons change. Many folks, based on common sense (which is often neither sensible or common), think that, because the Earth’s orbit around the sun is not a true circle, but actually an ellipse (an oval-shaped path), that the seasons change because the Earth is closer to the Sun in the summer and further away in the winter. As far as the distance is concerned, the opposite is actually true—the Earth is actually closest to the Sun in January and farthest away in July. The distance is not a factor in determining the seasons. The only real effect that the elliptical orbit has is that the Earth rotates a little faster when it’s closer to the Sun and a little slower when it’s farther away. Unless you use a sundial to keep time, that difference in rotational speed will not be noticeable. The real reason that the seasons change is because of the tilt of the Earth’s axis. The Earth rotates on its axis once every 24 hours, which is why our day is 24 hours long. A day on Venus, on the other hand, lasts for over 5,800 hours (243 Earth days), while a day on Jupiter is only 10 hours long. Anyway, the Earth’s axis, which is the line through the center of the Earth around which it rotates (sort of like the axle around which a tire rotates) is tilted, in respect to the plane of Earth’s orbit around the sun. The axis of rotation also slowly “wobbles” as the Earth orbits the Sun, sort of like if you took a pencil and stood it on end on a table with your finger on the eraser, then moved your finger around in a circle. This means that Earth’s axis is sometimes tilted away from the Sun and sometimes it is tilted more toward the Sun. It is the tilt of the axis that determines the seasons (as well as the length of daylight in a day). When it is summertime in the Northern Hemisphere (where 2/3 of the Earth’s land, including the United States, is located), the axis is tilted toward the Sun and we get more direct sunlight (the sun is higher in the sky) and for a longer time. Because of that, we absorb more heat energy from the Sun and it is warmer. When the Northern Hemisphere is tilted toward the Sun, the Southern Hemisphere is tilted away, and they (places like Australia, southern South America, Antarctica and Southern Africa) are having their winter, because they get less direct sun and are having shorter days. I’ve always thought that it must be weird in Australia to have Christmas in the early summer. Places near the equator are always warm, because the equator is never tilted away from the sun. The tilt is also why the sun never sets in the mid-summer in the Arctic and never rises in the mid-winter. You can use a flashlight and a globe to see this for yourself. Just aim the flashlight at the globe and pretend it’s the sun, then spin the globe beneath it. Tilt the globe toward the sun and then away and see the difference in how the light hits it.
So that explains seasons, but what about weather? Why does the weather change from day to day, or even minute to minute, as it seems to for people who live where I do? Like everything else we’ve been talking about in these articles, the answer to that question is energy. Weather occurs because of uneven heating of the Earth’s surface, which means there are places where there is a lot of energy and places where there is less. Because some places are cloudy, they get less sun on a given day. Some places have a lot of water, so they heat up more slowly than land (we talked about this effect when we article about the properties of water in Articles 11-14). In any event, the Earth heats up unevenly, which makes some areas warmer than others, and water heats up and cools off more slowly than land does. Because of entropy (remember entropy, the tendency of ordered, high-energy systems to move toward disordered, low-energy systems? See Article #10), warm air moves to areas of cooler air, carrying energy (heat) with it. When air moves, it makes currents, which we feel as wind. More importantly, air also moves up and down in the atmosphere. Warm air rises and cold air sinks. Usually, depending on where the warm air came from, it is moist, because warm air can hold more water vapor than cold air can. As the warm air rises and meets the cooler air, some of the water vapor in it starts to condense, making clouds. If the warm air is quite a bit warmer than the cool air, the warm air can rise pretty fast, causing those really towering, billowing clouds that can indicate weather like thunderstorms. If enough moisture in the air condenses to make drops that are heavy and large enough to fall, we get rain. If the air in the clouds and between the clouds and the ground is below freezing, the water vapor forms ice crystals and we get snow. If there is a thin layer of warm air between the cloud and another layer of cold air near the ground, the snow will melt as it passes through the warm layer and refreeze when it hits the cold layer, eventually hitting the ground as sleet (sleet is just frozen drops of water, whereas snow formed as an ice crystals in the clouds and stayed as crystals all the way down). If there is a thicker layer of warm air between the clouds and the ground, but it’s still below freezing at ground level, the snow will melt as it passes through the warm layer and stay liquid until it hits the ground, where it re-freezes. This is what happens when we get freezing rain and all sorts of bad things happen.
Satellite image of Hurricane Katrina. Credit National Oceanic and Atmospheric Administration
When there are rapid, big swings in temperature, there is a good possibility of severe weather. When a really warm air mass hits a much cooler air mass, the warm air can rise very fast. Anytime the warm air rises, air has to come from somewhere else to fill the space left by the rising air. Air currents will move in at different altitudes and at different speeds. When currents at different directions and speed hit one another, they can cause vortices (plural of “vortex”, or spinning structures) to form. If the warm air that is generating our local thunderstorm is rising slowly, we can have relatively gentle breezes. If the air is rising very fast, it can speed up the vortices formed by the horizontal winds and we can get damaging winds. If it rises fast enough, the vortices are pulled vertically and can start to spin faster and faster, sucking more air up from below. This is due to a phenomenon called the Coriolis effect, caused by the rotation of the Earth. You see this effect often, as water swirls around a drain or if you look at the clouds on a weather map, which often appear as swirls. If the air is moving fast enough, these swirling air patterns can generate so much energy that they turn into tornados, which can extend from the storm cloud to the ground. Tornados are not uncommon in the South and Midwest in the summertime, because there are often large, warm air masses from the Gulf of Mexico that move north and run into colder air masses dropping down out of Canada. They can, however, happen anywhere that two air masses of differing temperatures collide.
There are incredible amounts of energy that can be generated by storms, even if they don’t turn into tornados. As the water vapor in the air masses condense and the air masses move against each other, a static charge builds up. The faster the air masses are moving, the more charge is built up in the clouds. The electric charges built up in the clouds separate, with the positive charges moving toward the top of the clouds and the negative charges building up in the bottom of the clouds. As the bottom of the cloud becomes negatively charged, the surface of the Earth beneath the cloud builds up a positive charge. When the charge builds up enough, the negative charge from the cloud will start to move toward the ground. When that charge gets close enough to the ground, the positive charges from the ground meet up with it, forming an electrical arc of incredible power. A lightning bolt can contain over a billion volts of electricity and generate enough power to run a house for over 50 years. The power of a lightning bolt can be equivalent to more than a ton of dynamite, all released in microseconds. There is so much energy in a bolt that it heats the air nearby to a temperature hotter than the Sun, turning the air into a plasma and generating a shock wave that can travel for many miles. We detect that shock wave with our ears, hearing it as thunder.
I first learned a little about weather in my sixth-grade science class, when we were allowed to randomly pick up short books about different topics to study for a week. The book I randomly picked up just happened to be about weather. I started out thinking it was going to be kind of dull, but it was really fascinating, and it has served me well all these years. I could look at a weather map when I was 13 years old and know what all the swoopy lines and stuff meant. I also became a life-long lover of weather. I love storms. I like to stand outside and watch the lightning and feel the wind and just experience the raw power and glory of nature. It’s just one more instance of how just a little bit of knowledge can make life so much more rich and interesting. By the way, we’re having a thunderstorm as I write this. It’s early February, and we’re having a thunderstorm. That’s just not right.