Planetary Weather

As you probably know, the planet Earth isn't the only planet with weather. Any planet with an atmosphere has weather. This concept is paramount to understanding the possible weather on exoplanets, planets that orbit distant stars.

What causes weather? There are several reasons for changing weather on a planet with an atmosphere. The tilt of the planet's axis to the ecliptic is very important. As the planet orbits its star, this tilt exposes the star's rays in a variable way to the surfaces of the planet. We see this with Earth. If you live in the northern hemisphere, the summer months are hotter because the tilt exposes the Northern areas more than the Southern areas. The Earth's tilt is 23.5 degrees, which causes a huge difference in the amount of heat received from the sun. You'll notice this in how far above the horizon the sun travels in different seasons. The lower the sun is at midday the less heat hits the surface.

Venus is only tilted 3 degrees, but because of greenhouse effect, the surface is heated to 286 degrees F. Venus' dense atmosphere has lots of carbon dioxide to trap heat from the sun. However, Venus does have seasons, but they're shorter than Earth's: 55-58 days versus 90-93 days on Earth.

The other main aspect that drives weather is the rotational spin of the planet. This drags the atmosphere along as it spins, causing a rotation effect, which in turn causes a coriolis effect. Combined with the fact that the rotation effect on the atmosphere is not even as one goes form the equator to the pole, rotation causes the storms, such as hurricanes and tornadoes.

The Earth spins at approximately 1609 kilometers an hour. Jupiter on the other hand, spins much faster, 43,300 kilometers per hour. That's the reason that Jupiter's atmosphere is replete with massive rotational storms, including the great red spot, which is enormous and has been storming for hundreds of years.

Obviously, the size of a planet's atmosphere also affects its weather. The larger the atmosphere, the more powerful the winds will become. Also, a larger atmosphere can retain more heat from the star. Atmospheric composition also affects this. Some gases retain more heat. Carbon dioxide is a good example of this.

However, even a planet like Mars, which has a very light atmosphere, shows evidence of weather. Mars often suffers little tornados and dust storms. It's also possible that it has dry ice precipitation.

Titan, the largest moon of Saturn is very cold but has a substantial atmosphere with methane being one the principle constituents. This results in liquid methane rains.

The trouble with exoplanets is that they orbit stars that may not be similar to our sun. Some K-class and definitely M-class stars (our sun is a G-class) have much lower luminosities forcing the planet to orbit closer if it's in the habitable zone. This could result in tidal locking, which means that one side always faces the star, causing it to be heated to ridiculously high temperatures while the opposite side is frozen. However, winds will cause the heat to be more evenly distributed, especially at the terminator areas between the hot side and the cold side. However, the winds could be very fierce, making life difficult.

We have a lot to learn about planetary weather systems. We'll have to know more if we ever try to go to these exoplanets around distant stars.

Thanks for reading.