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

- Define what the term
*thickness*means. - Describe five uses for thickness.

Thickness is a very simple concept. By definition, thickness (h) is the difference in height (z) between two pressure levels. In equation form:

where the lower pressure level is denoted by 0 and the upper pressure level by 1.

Thickness can be derived theoretically from the hydrostatic equation, assuming a mean temperature for the layer. The result is the hypsometric equation:

where:

R and g are constants. For any given layer, the thickness of the layer is proportional to the mean temperature of the layer. As a result, it is common to use thickness and mean temperature interchangeably when discussing thickness fields. For example, you may hear a meteorologist talk about cold thickness values instead of lower thickness values.

As a layer warms and cools, the thickness of that layer changes in response. If a layer warms, its thickness increases. If a layer cools, its thickness decreases. Keep these basic relationships in mind for future lessons.

Thickness charts can be developed in two ways:

- Calculate thickness values for a specific layer from rawinsonde data and then analyze these data to produce a thickness field.
- Subtract the height fields for two layers. In the old days this was done graphically. With gridded data sets, it is a simple subtraction of height values for vertically stacked grid points.

The thickness that has been traditionally used by meteorologists is the 1000-500 mb thickness. Contours of 1000-500 mb thickness are usually found on surface charts (analysis and prognoses) along with sea level isobars, and perhaps surface wind barbs. With the general availability of gridded data sets, thickness calculations are no longer limited to this one layer. Specific uses of thickness values for various layers is outlined below and in later lessons.

Thickness values can be used in several ways.

One of the more important uses of thickness is in defining the location of fronts. A front is a three-dimensional structure that represents a contrast in temperature over a relatively short distance. As a result, thickness is an excellent way to identify a "true front" as compared to a trough or wind shift area. The 1000-500 mb thickness mentioned above can be used for this purpose, but the 1000-700 mb thickness is a better measure because typically the strongest frontal temperature gradients are in the lowest 10,000 feet of the atmosphere. If you are looking for fronts that do not reach the surface, a different layer may be appropriate. For example, for idientifying a cold front aloft, the 850-500 mb thickness may be useful.

If you have to prepare a surface prognosis based on numerical output, a combination of a sea level isobar pattern, surface wind barbs, and 1000-700 mb thickness are very useful is defining frontal locations.

Over the years, statistical relationships have been developed between thickness values and precipitation type. Critical values for identifying the rain-snow line or areas of freezing precipitation (ice pellets and freezing rain) had been identified and are covered in detail in the web page on **Thickness and Precipitation Type**.

The movement or propagation of mesoscale convective systems (MCS) as seen in satellite imagerg has been correlated with the 850-300 mb thickness pattern. This forecasting technique is covered in detail in the web page on **MCS Propagation**.

The basic data observed by a radiosonde are temperature, relative humidity, and pressure. The height of specific pressure surfaces is part of the radiosonde observation message, but is not directly observed. This height is calculated using the hypsometric equation, i.e., thickness.

A plot of temperature versus pressure is constructed from the base radiosonde data. The thickness of a series of relatively small layers from the Earth's surface to the top of the sounding is calculated. Then, starting with the elevation of the launch point for the radiosonde, these thickness values are used to produce a height versus pressure curve. This curve provides the height of the 925 mb, 850 mb, 700 mb, etc., levels that is used to construct standard level charts.

Techniques exist that correlate thickness values with surface temperatures. These relationships will not be discussed in this course.

You can easily see that thickness, although a relatively simple concept, has many practical uses. With the availability of gridded data sets and modern computer displays, it is easy to examine a variety of thickness fields and relate them to your forecast requirements. Thickness is also related to something called ** thermal wind** which is covered in the next lesson.

*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.

*Thickness is defined as:*

- height difference between the surface and 18,000 feet

- height difference between any two pressure levels

- a measure of frontal layer width

- pressure difference between two isentropic surfaces

*Thickness values can be used for:*

- rain-snow delineation

- identifying the location of a front

- deciding where a MCS will move to

- all of the above

*The only useful thickness value is that for the 1000-500 mb layer.*

- True

- False ... many layers are useful; for example, the 1000-700 mb thickness is good for identifying fronts