I’m avoiding having to walk in the rain and I’m watching Wunderground and the rain cloud traveling from north to south over me seems to disappear from the system right south of me as I’m waiting for it to stop raining.
It looks like the rain is just traveling right over me and then poof. What’s happening?
In: Planetary Science
Weather at the ground level is created by bubbles of air that are blown around by higher level winds, and also a ton of very complex things.
Imagine something like when you were a kid and your science teacher put soap water on a plate and then blew bubbles in it so you had those big half-sphere bubbles moving around. If you blew on them you could move them around the plate.
Air can hold a certain amount of moisture in it and this amount is determined by pressure and temperature. Lower temperature and lower pressure air can hold more moisture in it than higher temp and pressure air. This is why clouds are in the sky, the air farther from the ground is less dense and colder than that on the ground.
If you have a high pressure bubble that holds a bunch of humid air and a low pressure air bubble moves into it some of the moisture will move out, but lots of it will hit the bubble wall and suddenly the air will cool and the pressure will drop to the point that the air can no longer hold the moisture in it, causing it to drop out in the form of rain. This is why relatively smaller storms often look like arcs, the moisture that isn’t being pushed out is hitting the bubble of colder air, cooling and dumping out.
A big chunk of the moisture in the higher pressure area moves out as the high pressure bubble is pushed away and is replaced by the low pressure bubble, so you get storms mostly on the boundary between the two bubbles.
There are a whole bunch of other causes of typical storms, another big one is mountains, as these air bubbles move against mountains the air is forced up calling cooling and pressure drops, which is why mountain ranges typically are very moist (temperate/tropical) on one side and dry (desert) on the other.
To answer your question as to why storms seem to disappear so quickly it’s because the bubble boundary is moving. Air that is in the high pressure bubble can hold the moisture, air that is in the low pressure bubble dropped its moisture long ago. They have both already equalized, only along the boundary is the moisture in the air changing to the degree that water needs to be shed in the form of rain.
All weather is ultimately created by the interactions these bubbles have. Without getting too far in the weeds high pressure bubbles rotate one way and low pressure systems rotate another. In very flat places like tornado alley, where there aren’t a lot of hills and mountains to disrupt the air flow, these rotations get pretty powered up and when the two bubbles collide, one going clock-wise and the other counter-clockwise, the air on the boundaries start spinning into each pretty aggressively, creating tornadoes. Likewise hurricanes are created by ocean water heating up in the summer and creating big air bubbles of hot water filled air that gets a ton of spinning energy because the ocean is way flatter than land, that then move over land, where they cool and start dropping rain and then spinning does a bunch of wind damage.
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