Eli5: How come it doesn’t rain all year round?

Submitted by TheRunningMD t3_zye2lk in explainlikeimfive

I know that there are places that have rain all year long, but it isn’t for most places. In most places it rains mostly in winter, a little bit in fall and spring and no rain in summer.

Basically as a kid we learned: Evaporation-> Condensation-> Precipitation

Evaporation happens all year long. Probably more at summer because of the heat.

All that evaporated water must condense at some altitude because it is basically freezing that high up, even in summer.

So where is the precipitation part?

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chemist612 t1_j25anpg wrote

Evaporation happens where it's hot (summer) and the vapor moves to where it is cold (winter). It may be cold way up high, but before enough moisture builds there, the winds will generally move it somewhere colder close to the surface.

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TheRunningMD OP t1_j25azj4 wrote

But wouldn’t that mean that the water droplets would have to travel thousands of miles “close to the surface” because usually those places are far away, and not just go up like 2K up?

Up cold is a lot closer than low cold.

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chemist612 t1_j25b7kk wrote

Yes, but the jet stream is powerful and mixes the air pretty fast. Summer storms have a lot of energy, which is why they tend to be so destructive

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TheRunningMD OP t1_j25bdk5 wrote

Sweet. Is there a physical reason why wind moves to colder places?

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chemist612 t1_j25bqdi wrote

Wind is just hot air and cold air moving past each other. Areas of high pressure are where it is hotest and areas of low pressure are colder, so evaporation happens somewhere warm and high pressure then moves via wind (advection) to somewhere colder with low pressure.

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TheRunningMD OP t1_j25bygl wrote

My chemist friend, you have enriched my life. Thank you and have an amazing day!

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chemist612 t1_j25cl7d wrote

Happy to help. Go forth and question everything!

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keeltheone t1_j25cd2q wrote

>In most places it rains mostly in winter, a little bit in fall and spring and no rain in summer.

Is this actually true?

I've lived in Michigan, US most of my life and we definitely have rain in summer, spring, fall and rain, sleet, hail and snow during winter.

My mind is blown that this might be the exception rather than the rule for "most places" (I do understand that some places rarely experience snow, and some are deserts, rain forests, etc.)

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attorneyatslaw t1_j25l8c3 wrote

Its true on the west coast, mostly, and not really anywhere else in the US.

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TheRunningMD OP t1_j25cv92 wrote

I think you are the exception. Some places might have very light rainfall during summer, some places not a drop, but usually most rain is in the colder months.

I honestly haven’t looked at statistics, but I’ve traveled a lot and from my experience unless you are close to the equator where it rains year round (like Hawaii), cold months have way more precipitation.

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LochFarquar t1_j25bpkz wrote

Most precipitation comes as part of large weather patterns that move over thousands of miles (although this is less true in hot, humid places that create enough of their own moisture for localized storms). Generally storms start as big masses of warm, wet air over an ocean and then blow across land. Seasonal weather patterns will affect which times of year a particular location gets more of these warm, wet air masses bringing large amounts of rain.

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