Chaos and cosmos

I continue with my series on considering plausible, practical climate scenarios relevant to individuals and communities.

This one is about how our weather is organized, which has been one of my more popular science lectures. It is essential to this series because when we think about future weather in a warming world, it is mainly going to be organized in the same way.

The organization’s building blocks are the Sun, Earth’s orbital tilt, Earth’s rotation, and the distribution of land, oceans, and especially mountains — physical geography.

Seasons in the Sun

At the beginning of a course on climate, we learn that our surface temperature is determined mainly by heat from the Sun and the concentration of greenhouse gases. These gases are both naturally occurring and from our energy waste. The most important ones are water, carbon dioxide, and methane.

That’s climate. I want to focus on weather, which is how we feel the climate.

We experience seasons and the rhythm of days getting longer as we progress from winter to summer. Then, in summer, it turns around, and the days get shorter. The length of day changes, and it goes from colder to warmer to colder again.

The seasons occur because Earth is tilted on its axis. In the northern hemisphere, as the Earth orbits the Sun, darkness reigns over the North Pole during the month of December. Then it’s light 24/7 once June arrives.

More energy is received at the equator than at the poles. The atmosphere and the oceans move heat around to even out this difference. Some of this mixing of heat is caused by storms — weather. The contrast between a pole and the equator is greatest in winter, so we get large winter storms one after another.

A critical point for this article is that the winter pole is dark and gets cold. This remains true as carbon dioxide increases.

The first building block is that the heating in the tropics and the cooling at the poles are fundamental. Also fundamental, the atmosphere and oceans transport heat to smooth out the differences. So, in the northern hemisphere, weather systems often move from south to north.

As the world turns

These jet streams
align with Earth’s rotation. Their location is determined by the contrast between heating between at the equator and the poles. Jet streams are important for steering weather systems. (Image credit: NOAA.)

The next building block might surprise you. It is the importance of Earth’s rotation. In the same sense that we are aware of the seasons, we are familiar with the rotating winds of hurricanes, tornadoes, and winter storms. All of these storms have spinning winds, and most of the storms we encounter have spinning winds.

The rotation of the Earth naturally aligns winds from west to east, which leads to the formation of jet streams. The jet streams steer many weather systems, so storms usually move west
to east. In the U.S., you can follow storms from the Pacific in winter, essentially breaking like waves as they encounter mountain ranges in the western U.S.

Have you ever noticed that hurricanes are all about the same size? They tend to be a few hundred miles in diameter. Tornadoes are smaller. Winter storms are larger. One of the fun facts about fluids — and air and water are fluids — is that motion organizes preferentially in different-sized features.

The rotation of the Earth appears different to people at different latitudes. At the equator, the air does not appear to rotate around a person standing on the surface. If you were at the pole, rotation would be all you see. You don’t have to get far from the equator for rotation to be important. Hurricanes, for example, form some distance from the equator but not too far north (or south). The rotation has to be just right.

Earth’s rotation is an organizational building block because it promotes or obstructs the formation of different types of storms in different places and during different seasons. If there is a little bit of rotation to kick things off, it often helps a storm get started. If that little extra rotation aligns with Earth’s rotation, then a storm is more likely to grow.

All storms transport heat, but the organizing influence of rotation helps keep different types of storms in different places.

Mountain high

I learned in grade school about the rain shadows of mountains. It is likely to be wet on one side of the mountains, the California coast, but not on the other side, the Nevada desert. But this drying effect is far from the only way mountains affect the weather.

More important to the organization of weather is that as air goes over a mountain, it changes its rotation and can start to spin. You can understand this by watching water flow over rocks in a stream. You can see little spinning vortices and organized whirlpools.

Mountains and even small ridges are often the source of a little rotation that might kick off a storm.

Land versus ocean

Winter storms form downstream, east of the Rocky Mountains, and move to the north and
east transporting heat. (Image: geography.name.)

Again, appealing to our experience and intuition, we know land heats up more than water in the summer. That’s why we go to the lake shores and seasides. In winter, land cools down more than water, which is why people in Michigan have historically fled to the Gulf of Mexico.

The temperature contrast between land and water is one of those imbalances in energy that the atmosphere seeks to smooth out.

If you think about the big Nor’easters on the U.S. East Coast, they occur when cold air over the continent meets warm air over the Gulf Stream in the western Atlantic Ocean. Though the storms appear to be hugging the coastline, the Gulf Stream is more influential in determining their track.

Here, we see the emergence of storm tracks, places where we are more likely to see “weather.” The storms’ motions are influenced by both Earth’s rotation and the imperative to transport heat; they move from the west and south toward the north and east. In the eastern U.S., we have identified the role of the mountains to the west and the Gulf Stream to the east. We have a confined region of particular types of storms.

Order from disorder

Here, we see order emerging from disorder. Different types of weather features are present in different places during different seasons. In Michigan, we do not see the same kinds of storms in winter that we see in summer. On the East Coast of the U.S., we don’t see hurricanes in January — yet, anyway. Weather is not random, and using the building blocks of how weather is organized will help us set reasonable expectations regarding a warming climate.

I plan to make the next entry my last in this series, and I will explain how to combine all of these articles, in a guidebook, perhaps, for plausible climate storytelling.

(Lead image: Seasonal cycles are not to scale. Credit: National Geographic.)