Background on Contouring
Last week we learned how to reads station models and we can now read the observations reported at an individual station. We also looked at the lastest observations across Wisconsin. Sure we observed that it is warmer to the south and colder to the north, but what does the actual temperature pattern look like? What does the pressure pattern look like? An accurate representation of this data is key in it's interpretation. Meteorologist use a technique called contour analysis, it allows us to see numerical data in a picture-like way.
A contour line is a line of a constant value or an isopleth. Where iso means same and pleth means value. A contour analysis involves drawing multiple isopleths. Depending on what you are contouring, the lines have different names:
- Isobar - Line of constant Pressure
- Isotherm - Line of constant Temperature
- Isotach - Line of constant Wind Speed
- Isogon - Line of constant Wind Direction
- Isoshear - Line of constant Wind Shear
- Isodrosotherm - Line of constant Dew point
- Isohyet - Line of constant Precipitation Accumulation
- Isohaline - Line of constant Salinity (saltiness in the ocean)
- Isopycnic - Line of constant Density
- Isohume - Line of constant Humidity
- Isoneph - Line of constant Cloudiness
- Isohel - Line of constant Sunshine
The contour analysis is the first step in the analysis of data. Contour analysis are vital in:
- Finding the location of atmospheric and oceanic fronts.
- Locating regions with the potential of severe weather
- Tracking storms, such as hurricanes and blizzards
- Tracking the movement of pollutants
Examples of Contour Maps
So how do you contour?
Contouring data can be difficult because observations are not made everywhere as we saw on the observation map of WI. Gaps in the observations exist and we must interpolate between the existing weather observations. For example, assume the image below is temerature observations. The numbers are the observed temperatures, in degrees Fahrenheit. The city of Madison, WI has a temperature of 39F while Janesville, WI has a temperature of 43F, the blue numbers. If you where to drive from Madison to Janesville on I90 (the blue line) with a thermometer, along the way you would measure temperatures of 40F, 41F and 42F. If we were to draw an isotherm of 40F, this isotherm would have to pass between Madison and Janesville, and through the city of Platteville, WI where the observed temperature is 40F.
Above the red 40F isotherm, the temperatures are colder than 40F. Below the 40F isotherm temperatures are warmer than 40F. Along the isotherm the temperatures are exactly 40F.
Contouring Rules
There are a few general rules you should always follow when you contour a map:
Now we can use these rules to draw another isotherm on the map of temperatures. Lets try the 39F isotherm. It can not cross the 40F isotherm and must stay on the cold side of 40F. Even though we see several 39F observations, we generally should try to have one line connecting all of them.
Above the red 39F isotherm, the temperatures are colder than 39F, the blue area. Below the 40F
isotherm temperatures are warmer than 40F, the pink area. Between the 39F isotherm and the 40F
isotherm the temperatures are warmer than 39F but colder than 40F, we would find 39.5F here in
the yellow area.
Practice contouring with
some online help
What do these contour maps tell us?
We look for gradients for answers...
Isotherm maps: We use these maps to find the location of fronts. (We will talk about
fronts, in detail, later this semester.) Generally, the regions where the isotherms are close
together are the regions where we find fronts, the green circled areas on the map below. These are
the regions with strong temperature gradients.
Isobar maps: We use these maps to identify regions of strong winds and estimate their
general direction. The wind flows counterclockwise, cyclonic, around a Low and clockwise,
anticyclonic, around a High. Regions where the isobars are close together (where there is a
large pressure gradient) are the areas where the winds are the strongest, the green circled area on
the map below.
Current Wisconsin Observation Source: NWS
Click on the image for a better quality view and the pressure contours:
