Monday, February 9, 2015

Weather Monday 2/9/15 (Weather Forecasting Part 2: Forecasting Tools)

 smallercard Winter postcard made for an exchange.

As I write this week’s weather blog my daughter in Boston is getting another foot of snow. We are having freezing rain in central Jersey. Will this winter ever end?

Forecasting tools

A weather forecaster has a lot of tools to help forecast the weather. I will discuss those tools.


Computer models that do a lot of the work these days. Up to the 1950’s all the charts were hand drawn. Now computers plot the data, analyze the data by plotting fronts, pressure lines, etc., and forecast the weather. If fact one of the first computers was built to do weather forecasting. What exactly is a computer model? In one word …MATH! They are mathematical formulas to describe the complicated behavior of the atmosphere. We understand the physics of motion, heat, forces and we have many mathematical equations to describe these things. So the computer models are a bunch of mathematical equations that need to be solved.

Even though we have a fair understanding of how the atmosphere works many of these equations are approximations of its behavior. The atmosphere is a fluid and fluids are very complicated. Wikipedia has a good article on forecasting. There are several different computer models that are used. The two you hear a lot about these days are the GFS and the European models. The different models use slightly different science and mathematics to describe the atmosphere. Some of the models even take into consideration some of the geography of the earth such as mountains and large lakes. If the models were perfect we would only need one.

Even though we have several models they all work pretty much the same way.

  • Current observations of the weather are taken at thousands of locations and put into the equations in the models.
  • The model then makes a 5 minute forecast. Yes I did say 5 minutes. The model says what it will be like in 5 minutes at all of those locations.
  • The conditions 5 minutes out are then fed back into the equations and another  5 minute forecast is made.
  • Then another 5 minute forecast, etc until we have a 12, 24, 36, 48 hour forecast. So these forecasts are created 5 minutes at a time. A 24 hour forecast is made 5 minutes at a time. So the computer has to do the calculations for all of these equations for all of these locations almost 300 times to get a 24 hour forecast. You can see why we rely on computers to do it. It would take a person a lot longer to do the same thing.

If you know anything about measurements you know that they always have errors. For example you could be off a degree when measuring temperature. When you put something with error into a calculation the error compounds or gets worse. So the more calculations you put it in the worse the error gets. So that is why a 12 hour forecast is more accurate than a 24 hour forecast. And why a 24 hour forecast is better than a 48 hour forecast. And why there is so much error in 5 or 10 day forecasts. It is like a snowball rolling down a hill…it gets bigger and bigger the more it rolls. Or if you are a quilter you know that if you are off a little bit with each seam of a block that has a lot of pieces you will be off a lot by the time the block is complete.

Weather Satellites that orbit the earth and take images of the clouds. Do you see the comma cloud in this satellite image from 2008? See how big this storm is? Just checking….


Here is a satellite image from Thompson Learning. It shows that you can see different types of storms from space. Click on the image and you will see how thunderstorms, hurricanes and middle latitude storms (comma clouds) look.


Soundings which show the conditions in the upper atmosphere. These are created by launching weather balloons with and instrument package attached called a radiosonde. The radiosonde is about the size of a soda can. It measures temperature, humidity, etc as it is carried aloft by the weather balloon. The information it collects is radioed back to someone on the ground. The information is then used to plot upper level maps and soundings. Here is what a sounding looks like. Unless you are a meteorologist you probably won’t be able to understand this graph. It has a lot of information on it. The only bad thing is that it is only done twice a day at midnight and noon GMT.

Meteograms are plots of the surface conditions over a 24 hour period. The plots show how temperature, dew point, pressure, winds, precip and clouds have changed during this time period at a specific location such as the Philadelphia airport. It is done for many locations. Here is the current meteogram for Boston. The little XX’s in yellow are snow. Boston was having snow almost the entire time period represented by this meteogram. If you want to understand how to read a meteogram go to this link.


Radar shows the precipitation. It shows where the precip is, how heavy it is, and what kind it is. Here is the radar of the NE for today’s storm. I love the Weather Channel’s radar images. You can clearly see who is getting snow, freezing rain, and rain.


Thanks for reading. Next Monday I will discuss other methods of forecasting and see how they stack up against each other.



  1. Terrific little postcard. As for the weather - can't believe what's happening in Boston, but am surely glad it is not happening to me. I know how grueling that day after day of snowfall is, even if it's not in the numbers Boston has been getting.

  2. Very interesting info. The postcard is darling. I'm am unhappily awaiting a cold front coming through which is supposed to challenge a 30 year low record on Sunday night. I don't miss living in PA at all! We've had a pretty mild winter... where's the wood when I need it?


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