CONTOUR ANALYSIS

Interpreting a group of meteorological observations is a great aid to understanding the information from many data points. Meteorologists use a technique called contour analysis to get a better understanding of what information the data is providing. Correct interpretation of the data is crucial in determining atmosphere structure and forecasting its evolution.

Contouring, sometimes called isoplething, is the process of connecting lines of equal valued data. Meteorologists use contouring to create weather maps that connect data taken from weather stations and weather balloons (radiosondes). These measurement sites can be relatively close together (a few kilometers) to great distances apart (hundred of kilometers). The size of the region one analyzes and the time between measurements determines the scale of the analysis (Table 1)

Table 1:  Basic Scales of Atmospheric Motion                               
                                                                           

Term                Spatial size      Temporal size    Example                

Global              1,000-10,000 km   days - weeks     global analysis of     
                                                       the upper atmosphere   

Regional or         100-1,000 km      hours to days    Entire USA to          
Synoptic                                               regional weather       
                                                       charts                 

Meso                1-100 km          minutes - hours  Wisconsin to Madison   

Micro               .001-1 km         seconds -        your backyard          
                                      minutes                                 

The most important measurements to weather forecasters are pressure, temperature, moisture, wind, and clouds. All these fields are vital to understanding the current and predicting the future atmospheric state. If a forecaster looks at a map of temperature readings from the entire country, it would take considerable time to get a feeling for which parts of the country are experiencing warm conditions and which are experiencing colder conditions. However, by drawing lines of equal temperature, the meteorologist can quickly see patterns emerge. Contouring is vital in finding the location of fronts, the separation of warm and cold (moist / dry) air masses. Contouring winds can show where the warm and cold air is being advected, the horizontal transport of air, and thus where it might get colder or warmer in the future.

Data is collected at weather stations all around the world (often at airports). The data important to meteorologists includes measurements taken at the surface (surface data) and within the atmosphere (upper air data). Contouring both surface and upper air data is vital to the meteorologist to understand the 'whole story' of how air masses and storms are moving.

Storm systems are usually located where the atmospheric pressure is the lowest, commonly called a Low. Where atmospheric pressure is highest (a High), the weather is usually pleasant. The technique of contouring will make these features readily apparent and of great use not just to the forecaster but also those who plan on this information.

Contours and contour analysis are found everywhere; you see them when you look at the pretty colored temperatures in the weather section of the newspaper, and also when you look to see how steep the mountain is you are about to climb. Every time you look at a map of the Earth you see contours which are the lines of equal latitude and longitude. Nature also creates some of its own contours. When you look at tree stump it is easy to distinguish the tree rings. The age of a tree is easy to identify as each year of its life is represented by a contoured ring of its annual growth.

The importance of contouring has been established. Now it's time to learn and practice the rules in becoming an expert in contour analysis.

OBJECTIVE: An analysis is performed to separate higher values from lower values. This is done by drawing lines connecting equal values, or Isopleths. You must choose the interval between isopleth values to best interpret the data (e.g. contouring temperatures at 75, 80, 85, and 90).

GENERAL RULES:

1) Preparation

• A. Search for continuity (time and space) on the weather map

• B. Eliminate faulty (incorrect) data

• C. Review data to determine isopleth spacing

2) Drawing

• A. Use a pencil !!

• B. Make smooth lines

• C. Interpolate between values to correctly place the isopleth

• D. Isolines cannot touch or cross

• E. Be neat and accurate

• F. Label isopleths

3) Review

• A. Do a gross check of isopleths for accuracy

• B. If necessary, trace over pencil with final marker (be neat)

• C. Check labels

• D. The map is now ready for meteorological interpretation

  (note, you probably understand the field very well already, which is one of the benefits of doing contour analysis.)

Name of common meteorological isopleths:

• Isobar: pressure
• Isotherm: temperature
• Isodrosotherm: dewpoint
• Isotach: wind speed
• Isohyet: rainfall

Contouring increments of atmospheric variables:

• Temperature: 5°F surface/2°C for upper-air
• Dewpoint Temperature: 5°F surface/2°upper-air
• Surface Pressure: 2mb starting at 1000mb (most weather maps are every 4mb)
• Isotachs (500mb and 250mb): Every 10 knots.
• Geopotential Height: (500mb) 60m starting at 5700m :(250mb) 120m starting at 10800m

Maps of uncontoured dewpoint temperatures are provided. Contour the dewpoint temperatures in increments of 5°F. Click on the contoured version to check the results.

Interpretation of winds will be critical in analyzing the Oakfield tornado. Winds can either be streamlined or contoured. Contouring is generally used at upper levels to help indicate locations of jet streams and streaks. Streamlining is important at the surface and in the lower levels of the atmosphere as it shows the general flow of the wind. A map of unstreamlined winds are provided. Streamline the winds in the center of the country by drawing lines parallel to the wind barbs and connecting closely related regional flow. You can check your results with the streamlined version provided.