Having just talked about solar energy, it’s logical to talk about atmospheric moisture next. Without moisture there are no clouds, and thus no precipitation. Since water vapor is a greenhouse gas, the presence of moisture in our air moderates the temperature of our atmosphere, and prevents wild swings between hot and cold.
We can take an educated guess on what conditions might be like on Earth if there wasn’t any moisture by studying Mars. Astrophysicists believe that at some point during the formative years of our solar system, Mars had a thick atmosphere close to that of Earth, and surface water. We see evidence of this today in the form of dried riverbeds and channels in Martian rock formations. It also had a magnetic field produced by its core that protected it from solar storms, just like Earth.
Mars’ inner core cooled quickly around 4.2 billion years ago, causing its magnetic field to weaken, no longer protecting it from the Sun’s most harmful effects. Over the next few hundred million years, Mars’ atmosphere was blown away by solar storms, causing the planet’s temperature to drop and moisture to evaporate into space or freeze solid on the surface. With no moisture in the air and a thin atmosphere that can’t retain heat, Mars now experiences wild daily temperature swings of 170 degrees or more, even at the equator during Martian summer.
That is an extreme example, but it shows just how important water vapor is to the Earth system. And there’s a lot of water on our planet: 75% of the Earth’s surface is covered in it, in some places many thousands of feet deep. The water in our rivers, lakes, and oceans isn’t even all of it: about 2% of the world’s water is frozen in ice sheets and glaciers.
While this doesn’t sound like a lot, if all of this frozen water melted and ran off into our oceans, it would be enough to raise sea levels by over 200 feet.
Moisture in our atmosphere is constantly cycling, what we call the Water Cycle. The sun’s energy heats the ground and water, causing surface water and moisture to evaporate. That moist air rises and cools to a certain point where it condenses and cools, creating clouds. As moisture condenses further it becomes precipitation, which can no longer be supported by the rising air. That precipitation falls to the ground, and the process begins again.
Most of what we call weather is generated by this continuous cycle. For rain and snow, you need clouds, and for clouds you need rising air and moisture. In general, the strength of this cycle determines high and low pressure, and the difference in weather conditions between the two regimes.
Near an area of low pressure, there is less pressure pushing down on water molecules allowing for increased evaporation. This is why clouds, rain and snow are associated with low pressure systems. On the other hand, an increase in pressure keeps water molecules closer to the surface making clouds harder to form, leading to the tranquil, sunny weather we experience under the influence of high pressure.