Good question about rainfall and raingauges

[calvinbiss]

I was just having an "discussion" with coworkers about rainguages. If the rain is falling at a steep angle due to wind, say 45 degrees, will the raingauge not read acturatly? Will the be different readings in the gauge if half an inch fell straight down, as opposed to half an inch falling at 45 degrees?

 

[notfred]

No, It wont.

 

[dullard]

So you are saying the higher the wind, the less rain strikes the ground? Using your logic, if I dump a bucket of water in still air, one bucket hits the ground, but if I dump one bucket in moving air, then less than a bucket hits the ground. Rethink that and post again.

 

[AgentBehemoth]

dullard I was gonna put it a little different but that'll do.....Pig.

 

[notfred]

Originally posted by: dullard
So you are saying the higher the wind, the less rain strikes the ground? Using your logic, if I dump a bucket of water in still air, one bucket hits the ground, but if I dump one bucket in moving air, then less than a bucket hits the ground. Rethink that and post again.

No, I'm saying if you put a bucket on the ground and spray the side of it with a hose, less water gets in it than if you spray it from above with a hose. Do you even know what a rain guage is? It's not in any way analagous to the entire surface of the earth.

 

[dullard]

Originally posted by: notfred
No, I'm saying if you put a bucket on the ground and spray the side of it with a hose, less water gets in it than if you spray it from above with a hose. Do you even know what a rain guage is? It's not in any way analagous to the entire surface of the earth.

That is true since you use a tiny stream of water - meaning you have edge effects. Some misses the bucket and since you only have one tiny stream, nothing replaces the water that missed.

As long as we aren't talking about the exact edge of the storm, the rain guage is identical regardless of the angle. Yes some moves over from the wind, but more moves in from the wind as well (since rain isn't a point source).

 

[Mr.IncrediblyBored]

Originally posted by: notfred

Originally posted by: dullard
So you are saying the higher the wind, the less rain strikes the ground? Using your logic, if I dump a bucket of water in still air, one bucket hits the ground, but if I dump one bucket in moving air, then less than a bucket hits the ground. Rethink that and post again.

No, I'm saying if you put a bucket on the ground and spray the side of it with a hose, less water gets in it than if you spray it from above with a hose. Do you even know what a rain guage is? It's not in any way analagous to the entire surface of the earth.

Which still isn't correct.

Picture in your mind a flat surface with 10 sticks protroding perpendicular from an area of 10 square inches. Angle these 10 sticks at 45 degrees from the flat surface. Notice how the ends of the sticks still touch the ground on the same 10 square inch surface area?

These sticks signify the flight path of a rain drop in the wind. The rain gauge will still read correctly.

 

[notfred]

You guys are right. I am wrong. I see now.

 

[FoBoT]

i saw something related to this on discovery or national geographic or one of those channels last night

in the old days, raingages were relied on pretty heavily to measure precipitation and since they are prone to the type of problems you point out, and also, since they are spread out so much (only one every dozen/tens/hundreds of miles) , localized rainfall was never measured

apparently these days, according to this tv show, instead of measuring precipitation primarily with raingages, they use Radar to measure precipitation. radar isn't susceptable to the problems you mention and it can measure large areas from a single radar installation. so a major weather station can measure rainfall over several hundred square miles and many times more accurately than raingages

 

[dullard]

Originally posted by: notfred
You guys are right. I am wrong. I see now.

It can read improperly for other reasons though.
1) Poor installation so that it is blocked by an object (say a nearby tree). If the wind changes, the object may or may not block the rain guage. Thus on a windy day the tree may block some rain when the tree might not block it on a calm day.
2) Splashing out of the guage.
3) A guage that isn't pointing straight up (then the angle really affects the amount collected). This is the major effect that home users will see - and thus you are partially correct Notfred. But in the ideal straight up case, you are not.