Hodographs
Posted on 29. Apr, 2010 by Ryan in Education
You may have heard forecast write-ups referring to hodographs in their forecasts and wondered either: how do they work? or how do they improve my forecasting? This article will first walk you through what a hodograph is, what to look for and how it can help in severe weather forecasts.
Meteorologists use a hodograph to plot the winds through the atmosphere in one given location. They are typically attached with forecast soundings. Above is an example of a sounding with an attached hodograph. If you have ever look at a forecast sounding and seen the wind barbs on the side of the graphic, a hodograph is simply another way to show it.
So what are all those lines and circles on a hodograph?
The 2 lines straight lines that all cross in the middle do two things: they separate the 4 cardinal wind directions and create a legend for the wind speed. The circles on the map are simply an extension of the wind speeds to make it easier to plot more accurately.
To create a hodograph (often done by computers, but for this example we will make our own) we will pick out a sounding. And in this case, significant severe weather appears likely in the area where we picked the sounding. But does the hodograph show favorable winds?
12/01 JAN sounding
Yes, the hodograph is already plotted on this sounding. We will use that to check our work Lets start by writing down the various wind speeds and directions from this sounding.
Height(mb) Speed(kt) Direction
1000 10 180
850 40 200
700 40 225
500 45 230
300 90 265
At each height, place a dot with the corresponding wind direction and speeds. It should look like this:
All you have to do now is connect the dots in order of their height, away from the surface.
Now you understand how to make a hodograph, you can better understand one when you see it. But what do the winds really mean? Well for a look how the different level winds affect storm growth and development, I will refer you to the ‘Upper Air’ section HERE. But looks for a big ‘kink’ near the 850mb level has shown links with tornadoes, along with strong low level directional shear.
Resources:
You can print off your own hodograph to use in the field based on changed you monitor from VWP’s and soundings: http://www.weathergraphics.com/edu/hodograph_blank_s.gif
Jon Davies has an excellent, in depth write up from stormtrack. It can be viewed here : http://www.stormtrack.org/library/chasing/hodo.htm
