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1. Skew-T Description

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Introduction

Photo of 2 men launching a weather balloon

The skew-T/log-P thermodynamic diagram used to plot vertical profiles of atmospheric temperature, moisture, and wind. (See an example blank skew-T, which opens in a separate window.)  It has been used for many decades to assess a variety of meteorological conditions, most notably atmospheric stability.

Data for the vertical atmospheric profiles, also known as sounding plots, come from numerous sources such as radiosondes, dropsondes, pibals, aircraft, NWP model output, and satellite sounders.  To decode data messages generated from a particular data source, one should refer to the appropriate reference manual. While most soundings today are plotted electronically, the instructions in this module are also relevant for manually plotting data on a skew-T diagram.

Since pressure decreases logarithmically with increasing height in the atmosphere, the skew-T diagram has lines of constant pressure (isobars) spaced logarithmically. This leads to the skewed appearance of lines of constant temperature (isotherms). Thus the diagram is called a skew-T/log-P diagram and is frequently referenced (including hereafter in this module) as a skew-T diagram.

This section describes the various lines on the skew-T/log-P diagram and how data are plotted on it.

Lines on the skew-T

Lines on the skew-T Isobars

Isobars on a skew-T

The nearly horizontal lines are isobars, spaced logarithmically from about 1050 hPa at the bottom to 100 hPa at the top.  They are plotted every 50 hPa.

On paper versions of the skew-T/log-P diagram, the ICAO standard atmosphere heights are printed under selected isobars on the right hand side of the diagram in feet (in parentheses) and meters [in brackets]. There is also a height scale to the right of the diagram.

The link below (which opens in a new window) goes to a table that provides a mapping from standard pressure values to height assuming an ICAO standard atmosphere:

Table of pressure vs. height in a standard atmosphere

Lines on the skew-T Isotherms

Isotherms on a skew-T

Isotherms are lines of constant temperature.  On the skew-T diagram, isotherms are straight, solid lines, sloping from the lower left to upper right.  In this example, the isotherm interval is 10°C and those from -40°C to +40°C are labeled at the bottom of the chart.

Lines on the skew-T Dry Adiabats

Dry adiabats on a skew-T

Dry adiabats represent lines of constant potential temperature. On the skew-T diagram, dry adiabats are the slightly-curved, solid lines sloping from the lower right to upper left. They indicate the rate of temperature change in a parcel of dry air rising or descending adiabatically, i.e., with no loss or gain of heat by the parcel. The dry adiabat for each multiple of 10°C shares a label with the isotherms, with the Celsius temperature value of its point of intersection with the 1000-hPa isobar.

Lines on the skew-T Saturation Adiabats

Saturation adiabats on a skew-T

Saturation adiabats, also known as moist adiabats or saturation pseudo-adiabats, represent lines of constant equivalent potential temperature.  On the skew-T diagram, saturation adiabats are the slightly curved lines. Note that the slope and spacing of the lines varies significantly with height and temperature, particularly at lower levels. Saturation adiabats represent the rate of temperature change in a rising parcel of saturated air (assuming that all the condensed water vapor is liquid and falls out immediately as the parcel rises—the pseudo-adiabatic assumption).  Note that the saturation adiabats become parallel to the dry adiabats at low values of moisture, temperature, and pressure.

Each saturation adiabat is labeled with the Celsius temperature value of its point of intersection with the 1000 hPa isobar, and shares its label with the isotherms and the dry adiabats. Note that the saturation adiabats tend to become parallel to the dry adiabats at low values of moisture, temperature, and pressure.

Click on the graphic to compare the saturation adiabats to dry adiabats.

Lines on the skew-T Saturation Mixing Ratio Lines

Saturation mixing ratio lines on a skew-T

Saturation mixing ratio lines, also known as humidity mixing ratio lines, represent constant values of water vapor capacity—specifically, the number of grams of water required to saturate one kilogram of dry air at a particular temperature and pressure.  On the skew-T diagram, the saturation mixing ratio (ws lines are the slightly-curved, dashed lines sloping from the lower left to upper right. They are labeled at the bottom of the diagram for a range of 0.1 to 40.0 grams per kilogram; i.e., in parts of water vapor per 1000 parts of dry air. Note that since the vapor capacity of air varies non-linearly with temperature, the labeling interval for ws lines is not uniform.

Lines on the skew-T Putting It All Together

Combining the isobars, isotherms, dry adiabats, saturation adiabats, and saturation mixing ratio lines onto a single diagram creates the skew-T:

Blank skew-T showing all of its lines; no wind staff

Lines on the skew-T Question

Skew-T with certain lines highlighted and labeled

On this blank skew-T diagram, certain lines have been highlighted and labeled. Using the dropdown boxes, identify each of the labeled lines.

A  

The correct answer is: Isobar

B  

The correct answer is: Dry Adiabat

C  

The correct answer is: Saturation Adiabat

D  

The correct answer is: Isotherm

E  

The correct answer is: Saturation Mixing Ratio

Sounding Data

Sounding Data Temperature/Dewpoint

Sounding Data » Temperature/Dewpoint Description

Sounding plotted on skew-T with dewpoint, temperature, and wind profiles labeled

To create temperature and dewpoint curves on a skew-T, first plot the temperature and dewpoint at each individual pressure level in the sounding.  All mandatory and significant level sounding data should be plotted.  Then, connect the temperature points with straight lines between successive pressure levels, often using red.  Similarly, connect the dewpoints with straight lines, often using green or blue.

The dewpoint is the temperature to which a parcel of air at a constant pressure becomes saturated without any addition of water vapor.  For instance, at ground level this is the temperature at which dew would be expected to form.

Sounding Data » Temperature/Dewpoint Question

Skew-T with single temperature and dewpoint plotted

On this skew-T diagram, determine the pressure (P), temperature (T), and dewpoint (Td) for the labeled points. Enter your answers in the text boxes below, then click Done.

P = hPa   


T = °C   


Td = °C   

Sounding Data Wind Staff

Sounding Data » Wind Staff Description

Wind profile plotted on a skew-T

Wind is plotted using the conventional wind plot model.

Wind speed is specified in knots using barbs and flags on the staff: each flag counts 50 knots, each full barb counts 10 knots, and each half barb counts 5 knots. For example, three full barbs and one half barb represents 35 knots, while one flag, one full barb, and one half barb represents 65 knots.

The end of the staff where the barbs or flags are located marks the direction from which the wind is coming. For example, a horizontal staff with flags and barbs on its left side pointing up (as is the case for most of the wind staffs plotted on this example) signifies a wind coming from the west.

Winds are plotted in black at individual levels along a separate axis, the wind staff, on the right side of the skew-T. The wind staff shows the particular height or pressure level at which the wind is reported.

Sounding Data » Wind Staff Question

What is the wind speed and direction for the following wind barbs?
Enter your answers in the text boxes below, then click Done.


Wind barb showing a southwesterly wind of 55 knots for wind barb question 1

Speed = kt   


Direction = °   


Wind barb showing a southeasterly wind of 35 knots for wind barb question 2

Speed = kt   


Direction = °   


Sounding Data Hodograph

Sounding Data » Hodograph Introduction

The Hodograph

Sounding including hodograph taken at Denver, CO, 0000 UTC, 27 Jan 2013

The hodograph is a graphical tool that helps forecasters evaluate vertical wind shear. It can be helpful at all times of the year, particularly for aviation concerns. In a convective environment, an understanding of the vertical wind shear is tremendously important for anticipating convective storm type, the likelihood of supercell storms, where new storms may form, and even storm and storm system motion.

Hodographs are frequently included with soundings on skew-T diagrams, usually in one of the upper corners of the diagram. This figure shows an AFWA MM5 model sounding, along with the associated hodograph in the upper-right corner, downloaded from the Joint Army-Air Force Weather Information Network (JAAWIN) Website. Similar sounding/hodograph plots are available from many sources.

Sounding Data » Hodograph Wind Barbs vs. Wind Vectors

Comparison of wind barbs and wind vectors in a vertical wind profile

Meteorologists are all familiar with the traditional vertical wind profile from a radiosonde that uses barbed lines to indicate wind direction and speed at various levels. The hodograph communicates the same information. However, since its primary purpose is to reveal vertical wind shear, the hodograph is based on wind vectors. Unlike the wind barb, a vector indicates speed by its length rather than a combination of barbs.

Sounding Data » Hodograph The Polar Coordinate Chart

Animated hodograph of wind vectors on a polar coordinate chart

For a hodograph, wind vectors are plotted on a polar coordinate chart. The axes of the chart represent the four compass directions. All the wind vectors extend from the origin and point in the direction of the wind's movement. Since the vector length indicates speed, concentric circles drawn around the origin represent constant wind speeds. For example, this hodograph shows that both the 4- and 5-km winds are 25 m/s, although their wind directions are from the west and west-northwest, respectively.

Sounding Data » Hodograph Creating the Hodograph

Animated image depicting creation of a hodograph

Typically, the actual wind vectors are not drawn on the hodograph, but are indicated only by their endpoints on the polar coordinate chart. The hodograph is plotted by connecting the endpoints of each of the wind vectors.

More information on hodographs and their application to convective storm environments can be found in the COMET module: Using Hodographs.

Sounding Data Legend, etc.

Example of multiple soundings plotted on a skew-T with legend, date field, and calculated quantities highlighted

A legend on the skew-T diagram provides essential identifying information about the sounding.  The legend must include the sounding location (WMO station number, three- or four-letter station identifier, or station name), as well as the UTC time and date.

It is sometimes instructive to plot two soundings on the same skew-T diagram, for instance, to compare atmospheric changes over time at a particular location or to compare soundings from two different NWP model runs.  While there are several conventions for plotting multiple soundings, it is most important to plot them such that you can distinguish them (e.g., solid/dashed lines, different colors, etc.).  Winds for different soundings can be plotted on different wind staffs (appropriately labeled) or on the same wind shaft using different colors.

For electronically plotted soundings, various calculated quantities, levels, and indices are often included on the right-hand side of the diagram.