Why Weather Forecasts Are So Difficult

[quasigeostrophy]

There was some kvetching on the radio this morning about the missed forecasts we've had this month, especially with snow coming today, and it just annoyed me.

This is one of my pet peeves, and it was so even before I headed down this career path. It's also one reason (of many) I don't want to go into forecasting.

Try this at home. I dare ya:

- Take a huge not quite spherical ball.
- Cover it with a very irregular surface, about 3/4 of which is water.
- Cover that with a thin mixture of gases that gets exponentially thinner as you go up from the surface.
- Tilt it 23.5 degrees, with a bit of wobble.
- Spin it.
- Heat it irregularly via a huge ball of fusing gas about 93 million miles away.
- Send it around that ball of gas in an elliptical orbit so its distance to the heat source changes.
- Accurately tell me what all the gases covering the first ball are going to do at any given time and place in the future.

Go on.

The other problems?

1. We know the set of mathematical equations that describe how that mix of gases flows around the big ball, with and without varying amounts of water vapor. Unfortunately, that set of equations cannot be solved exactly by any known means.

So the forecasters build models that make approximations. Approximations introduce error. What a concept.

2. We don't have accurate observations in enough places or often enough. I've heard via a third party someone (on faculty here at Purdue, supposedly, but I don't know whom) theorized that if we had observations about every square kilometer and every 300 meters in altitude through the troposphere (the bottom 11 km or so), we might have enough observations to calculate the models accurately.

Forecasters make mistakes. Now you know why.

Comments

[anmorata.livejournal.com]

It seems everyone complains about it. I crack up when people (news forecasters included) predict a foot of snow, and we get 3 inches. At least, we get 3 inches -at my house-, which is less than a mile from the lake, so the weather conditions are different given the temperature of the big huge body of water that I'm next to, and the topography of the land around us. It just makes me laugh a little more when it's a family member, because I think that certain family members like to play up the ZOMG!! factor in such things. I am notorious for low-balling my estimates, and very much a believer in "I'll believe it when I see it happen!" where it comes to snow accumulation.

Case in point, the prediction for my area for today is 5-8 inches. There's already about 2 inches on the ground. I'm going to guess that we get.. eh, 4, *maybe* 5 inches. I'm not using any calculations at all, just my rough estimate based on the rate of snowfall (by observation) and gut feeling. When I went out earlier, it was already a few inches deep but very fluffy, not nearly as dense as I'd expect. Over 200 schools in the greater Cleveland area closed today, but not the local school district here. The roads are fine.

I do the same things for fuel. Every year, I hear "OMG They say that gas is going to be over $X.XX by Memorial Day!!!" .. and it never is. Ever. It might peak, sure, but there's a median price for most geographical areas within a 20-to-30 cent price range, and the last few years, minus a few spikes, it's stayed in that range.

Wow I'm rambly this morning. Sorry about that. ;) Have a good day. :)

[quasigeostrophy.livejournal.com]

ust my rough estimate based on the rate of snowfall (by observation) and gut feeling

Sometimes those gut feelings are worth more than a dozen supercomputer-based forecast model runs. ;-) The human brain can assimilate a lot based on experience. It's just impossible to quantify "gut feelings".

Ramble away. :-)