I never appreciated the benefits of weather radar until I got a weather app on my phone.
It’s amazing to see rain on the left side of the map (west) and watch it approach my city. Or, a big rain storm is dumping rain on me and I’ll open the weather map. I can see the rain over my area and have an idea if it’s localized or a big front that will stick around.
I’ve learned to trust these radar maps. If I see a big rain pattern on my states western border, then I know it’s time to run any errands. Because that rain will be on me in an hour or so.
I’ve noticed the rain ALWAYS comes straight across left to right (West to East) or it’ll come down from the top and drift to the right. (down from the North and then towards the East).
I’ve never seen it go East to West. Or come up from the South.
Is that just how it works? What causes it to do that?
The prevailing is west to east, but there are some exceptions. When storms come from the other directions, it usually ain’t good. Ancient weather lore recognized this.
When the wind is from the North, the skillful fisherman goes not forth
When the wind is from the South, it blows the flies in the fishes mouth
When the wind is from the East, 'tis good for neither man nor beast
When the wind is from the West, then that is the very best.
Physics. Precisely what physics again depends on your location.
You probably don’t have wind coming absolutely always from one direction, but from one direction it comes over land and/or sea that allows it to pick up moisture, and from others it comes over mountains and has been forced to release moisture. (Just a rough example out of many possible.)
The Westerlies lie between around 30º and 70º latitude (both hemispheres), prevailing winds are from the west. Between 30º latitude and the equator, we have the Trade Winds, or less commonly called the Easterlies, prevailing winds are from the east. Above 70º, the prevailing winds blow toward the equator with a component from the east, and I don’t know if they have a special name except perhaps polar winds. These winds blow these directions generally, any storms or cyclones in the local area will cause the winds to blow in all the other directions, as illustrated by Aspidistra’s link.
The OP asks what causes this, and it’s complicated … let’s keep in mind we’re speaking to surface winds … and we can save some words with a pretty picture of the large scale atmospheric circulation patterns.
Note to the right, we have the vertical structure of this circulation … the air is heated the most on the surface at the equator … this air rises and once at the “top-of-atmosphere” (tropopause) the air flow splits toward the poles … and it cools itself off by radiating into space … becomes more dense and falling back to the surface where it returns to the equator …
Ah, that’s the first key … the air at the surface is moving toward the equator in the tropical zones … towards the poles in the temperate zones … and finally toward the equator in the polar zones … these circulation cells are like a gear train, each adjacent gear spins the opposite direction …
Now we just add heaping doses of Newton’s Second Law of Motion, as applied to a spinning spherical shell structure of fluid and this existing north-south motion takes on the east-west motion … easy peasy …
Yeah, right … but it’s 2am here right now and I’m going back to bed for few more hours … no way am I going to try and explain THAT without a belly full of coffee … check back later for more details that I hope won’t be too terribly weird … but yeah, the pressure force involved is actually points opposite these prevailing winds …
The explanation for the OPs question may be this, but the question asked is why rain always moves like that. Which could also have different answers, although possibly people know enough of the OPs location to render them unlikely.
I’ll just add that a large weather system could be rotating counter-clockwise while moving from west to east, so when the leading edge passes over your area the wind and rain would be coming out of the south. Then as the trailing edge passes over, the wind and rain would be coming out of the north.
Smartphones are giving us such great tools to observe weather. But they often raise questions too. I had never considered the movement of storms until I could observe it on weather maps.
Knowing the OP is someplace in the US, but not where he is …
As a general matter some parts of the country have fairly consistent large scale weather evolution. Other parts are more chaotic, where weather can come from any direction depending on the mood du jour.
The weather pattern may usually follow certain rules but there are always exceptions. As noted above, some areas may be especially chaotic, while others are more regular. No area is absolute, which is why even now we model model weather and employ meteorologists. The weather is a vast and complicated (global even!) system.
Coffeeeeee … it’s why they invented chocolate creamer …
I need to learn to not post a 2am … what the hell was I thinking … but I guess I better finish …
I’ll use the temperate circulation cell for my example, as this is what most of us experience in our day-to-day lives … from the diagram above we note that at the surface we have air moving up in latitude … so, at 30º, the solid Earth and gaseous atmosphere are moving at around 900 mph (1040 mph x cos 30º) towards the east, the anemometer bolted to the ground will read 0 mph, dead calm … we move an air parcel up to 45º, and this is where the 2nd Law of Motion comes in … this air parcel remains moving at 900 mph … however, the anemometer bolted to the ground is only moving at 735 mph … it will read 165 mph towards the east … Chicago’s windy, but it ain’t that windy … a force has acted on this air parcel, towards the west, to slow it down … but not completely, let’s say down to 745 mph so the anemometer reads 10 mph and still towards the east … thus our general weather pattern in the temperature zones is from west to east since our westward forces are a little behind the poleward forces …
Magical forces do not exist … this westward force requires explanation … as the air parcel is moving up in latitude, it begins to move toward the east faster than the air it’s moving through … and the air begins to pile up in front (on the east side) and evacuate behind (on the west side) … this causes air pressure to rise in front, and fall in behind … the pressure force then kicks in moving air from a high pressure to a low pressure … and in our example this pressure force is pointed to the west …
The word “Westerlies” describes from what direction the wind is coming from, out of the west … so remember that the actual velocity vector is pointed east …
My posts are in generalities … here’s a map of Hurricane Sandy’s track … totally wack-o … she got drove into the New Jersey shore from the east … I believe that was unprecedented for a tropical cyclone to turn retrograde like that … so storms can move in any direction, but for the most part they move west to east in the temperate zones …
What’s amazing is that our numerical models we run on them super-computers were actually able to predict this track a full 96 hours ahead of time … simply amazing …
It’s almost as if they are following a set pattern, if only we could find a word that describes this sort of behavior. Something like weather consistency or weather predictability. Hmmm, it’s right on the tip of my tongue, a weather . . . ???
Right, but there are also remarkable consistencies over suitable time or geographic scales, and I’m surprised you didn’t mention the fact that we get clues about this from aviation. Over most of North America and Europe and the pond in between, for example, westbound flights take substantially longer than eastbound ones. NY to LA, for instance, averages around 6h 15m while LA to NY is about 5h 10m. Granted, this is jet stream behavior, but it also reflects the overall trend of west-to-east weather system movement in this broad geography.
One sees a similar aviation phenomenon at a local level. The major international airport at Toronto is located west of the city, and there’s a good reason for that. Due to prevailing westerlies, most of the time flights take off to the west, where there is a big buffer of industrial areas, and city overflights are usually planes making their landing approach. When planes take off toward the east, it means there’s an unusual weather pattern and a fairly reliable indication that some type of weather disturbance is coming.
Low pressure areas (which have a counterclockwise spin - as opposed to high pressure areas which have a clockwise spin around the center) move with the prevailing westerleys if they are frontal lows, which is the normal situation in the states. However, tropical systems are not associated with fronts. Their directions are determined by numerous factors: blocking highs, shearing winds, blocking lows too. Those that come off Africa are caught in the horse latitudes and move east to west until other steering factors come into play. A high off our coast can block them and steer them north into the mid-Atlantic. Tropical storms are adiabatic and have a hot core. Frontal storms are caused by differences in barometric pressures and have a cold core.
Some tropical storms develop in the Gulf of Mexico and can move either east or west depending upon steering currents. If there are no strong steering currents, they can be practically stationary and can maintain their strength as long as the sea surface temps remain above 79 degs F. However, eventually they will bring up deeper, colder waters and their energy source ceases. Usually these storms if off our coast in the Atlantic will be picked up by a front and move with the front, becoming a frontal storm and extra-tropical. Sandy was not considered a hurricane even though it had hurricane force winds because it lost the heat source for its energy. Many hurricanes are picked up by fronts in the north Atlantic and head for England. They are no longer tropical cyclones but still may have hurricane force winds.
