Hodographs

Objectives

Upon completion of this web page you will be able to:

1. Define what a hodograph is.
2. Explain how to construct a hodograph.
3. Identify backing and veering on a hodograph.
4. Identify hodographs typically associated with single cell, multicell, and supercell thunderstorms.

What is a Hodograph?

A hodograph is a display representing the vertical distribution of the horizontal wind. It is a different from the vertical wind profile typically found on the right side of a skew t-log p diagram.

Constructing a Hodograph

The hodograph is a polar coordinate plot of wind data. Wind can be represented as a vector where the wind speed is the vector magnitude and the wind direction orients the vector. The diagram below shows four wind vectors extending from the origin of the diagram. Note that a direction of 180 degrees is at the top of the diagram. This axis rotation is used to make it easier to plot a hodograph from upper air data. Recall the a 180 degree wind comes from the south but its vector representation points to the north. North is usually placed at the top of the page.

Constructing a Hodograph

To construct a hodograph:

1. Plot the wind vectors.
2. Connect the tips of the wind vectors, starting from the surface and working upward.
3. The original wind vectors are deleted. This leaves only the line connecting the tips of the wind vectors.
4. Label the tips of each wind vector with the appropriate height or pressure level.

The line connecting the tips of the wind vectors is the hodograph. This line also represents the shear vectors between any two adjacent wind levels. Thus the hodograph is the series of wind shear vectors connected head to tail, starting with the surface wind. Mathematically it represents the vector addition of the wind shear vectors. The diagrams below show typical hodographs (without labels).

Interpreting a Hodograph

From the study of thermal wind you know that warm air advection is associated with a wind profile that turns clockwise or veers with height. This pattern is easily recognized on a hodograph and is shown in the figure below.

Hodograph with a Veering Wind

Cold air advection is associated with a wind profile that turns counter-clockwise or backs with height. This pattern is easily recognized on a hodograph and is shown in the figure below.

Hodograph with a Backing Wind

Not all hodographs are as easy to interpret as the two above. In some cases the wind veer in one layer, back in another, or not turn at all through some layer. But if you are familiar with the basic patterns of veering and backing, you should be able to identify these layers and imply the presence of warm or cold advection.

Although not covered in this lesson, hodographs can also be used to evaluate the magnitude of storm-relative helicity for various storm motions in severe weather situations.

Storm Type from Hodographs

Studies of severe thunderstorms have correlated specific hodograph patterns with specific thunderstorm types. These patterns are illustrated below.

Hodograph for a Single Cell Thunderstorm

When the wind shear is weak and relatively random, thunderstorm updrafts are vertical and produce single cell or ordinary "garden variety" thunderstorms. The hodograph above shows these weak vertical wind shears and their random nature.

Hodograph for a Multicell Thunderstorm

As the wind shear increases, the storms become multicell, the hodograph tends to be linear. In this case the winds veer with height and increase in speed.

Hodograph for a Supercell Thunderstorm

If the hodograph veers and shows curvature, supercell thunderstorms are typically the result.

Optional Exercise

Plot the following wind data on polar coordinate graph paper.

What type of thunderstorm would you expect to occur with this type of environmental wind shear?

Review Questions

Instructions: Place the cursor over the answer of your choice. If you are correct, it will be highlighted in green; if you are incorrect, it will be highlighted in red.

A hodograph is best described as:

1.  a series of wind vectors plotted head-to-tail
   
   
2.  a series of wind shear vectors plotted head-to-tail
   
   
3.  a Cartesian plot of wind data using wind barbs
   
   
4.  a strange way to display wind information

When you plot a hodograph:

1.  calculate wind shear values before plotting
   
   
2.  adjust the wind vector to magnetic north
   
   
3.  delete the wind vectors after drawing the wind shear vectors
   
   
4.  use tail feathers to represent wind speed

A hodograph that turns clockwise with height indicates:

1.  veering and warm air advection
   
   
2.  veering and cold air advection
   
   
3.  backing and cold air advection
   
   
4.  backing and warm air advection

A hodograph with a linear profile is usually associated with:

1.  garden variety thunderstorms
   
   
2.  supercell thunderstorms
   
   
3.  single cell thunderstorms
   
   
4.  multicell thunderstorms