Skew-T Diagrams and
Hodographs, although extraordinarily confusing at first glance, are
indispensable resources in analyzing the overall temperature and wind profile of an air column. In
addition to a display of the temperature, dew point and wind at all levels of
the atmosphere, a skewT diagram can be used to determine a number of
convective indices used by forecasters to assess the potential for severe
At least twice each day (additional
observations are sometimes conducted during severe weather outbreaks) at
various locations across the United States, an instrument package, called a radiosonde, is
attached to a large balloon and released into the atmosphere. A small
radio transmitter sends back information regarding temperature,
pressure, humidity, wind speed and wind direction as the unit is carried
This data is then plotted on a Skew-T
Log-P diagram such as the example above from 12Z on November 4, 1998:
Before a forecaster can use a Skew-T to
determine the value of various convective indices, the pressure level of
the Convective Condensation Level (CCL), the Lifting Condensation Level
(LCL), the Level of Free Convection (LFC) and the Equilibrium Level (EL)
must be identified. The National Center for Atmospheric Research (NCAR) offers an annotated diagram that provides this information at the
calculated height along the left margin. See, for example, the NCAR
skew-T from 12Z on February 16, 2006 (below).
- Isobars appear in gold and
represent lines of equal pressure. The pressure level appears along the left side of the diagram and is represented in millibars.
- Isotherms, shown in orange,
represent lines of equal temperature and run diagonally from the
lower left to upper right. The scale, in degrees Celsius, runs
along the bottom of the chart and appears in green.
- The Dry Adiabatic Lapse Rate is
shown in green and represents the temperature a dry air parcel would
reach as it ascends. The dry adiabatic
lapse rate is a decrease of approximately 10° C per 1,000 meters
- The Moist Adiabatic Lapse Rate shows the temperature a saturated air parcel would assume as it
ascends, and is shown in purple. The moist adiabatic lapse rate is not constant, but is an average
decrease of 6° C per 1,000 meters of increased height.
- The Saturation Mixing Ratio,
represented by blue dashed lines, is the ratio of the mass of water
vapor to the mass of dry air in grams per kilogram. The values for
this variable appear in yellow at the bottom of the diagram.
- The Dew point Plot, in solid
blue, is the vertical plot of the dew point temperature.
- The Environmental Sounding appears in red and represents the temperature at the specified level
of the air column.
- Wind direction and speed are
provided in the form of standard wind barbs along the right side of the diagram.
- Various Sounding Parameters such as convective indices are sometimes provided along the right
side of the diagram.
from 12Z on February 16, 2006 from NCAR.
In the event that these
parameters are not provided on the skew-T, it is a relatively
straightforward task to determine them manually.
A basic comprehension
of atmospheric dynamics and a Skew-T diagram, enable meteorologists to determine the
level of instability in an air column and assess the potential for an
outbreak of severe weather.
Level (LCL): the pressure level at which a lifted parcel reaches
saturation. The LCL is identified by using the temperature and dew
point at a specific pressure level. The intersection of a line drawn
parallel to the mixing ratio line from the dew point, and parallel to
the dry adiabat from the temperature, is the LCL.
Condensation Level (CCL): the pressure level at which condensation
occurs in an air parcel if the only mechanism for convection is
heating. The CCL is found by drawing a line upward from the surface
dew point, along or parallel to a mixing ratio line until it
intersects the temperature line.
Level of Free
Convection (LFC): the pressure level at which the lifted parcel
becomes warmer than its environment, and therefore positively
buoyant. The LFC is found by following the moist adiabat from the
LCL until it intersects the temperature line. In stable air masses,
there may not be LFC.
(EL): the pressure level at which a rising parcel becomes colder
than the environment. The EL is identified by following the moist adiabat from the LFC until it crosses the temperature line.
Hodographs display the change in wind speed and direction
(wind shear) at prescribed
levels of the atmosphere on a simple polar chart (see
below). In contrast to an actual hodograph, this instructional sample
includes wind barbs of the observed wind at 1 kilometer intervals along
the right side of the diagram.
For simplicity, the wind speed is equal at all observed levels, while the
direction changes from easterly at the surface to westerly at a height
of 6 kilometers. The wind speed and direction are plotted as arrows,
beginning at the center pointing in the direction that the wind is
blowing and extending outward to the appropriate circle for the observed
speed. Only the end points are plotted to
prevent the hodograph from becoming unnecessarily cluttered. Once each of the observed
levels are plotted, the hodograph is completed by connecting the end
from The COMET Program.
The finished hodograph
provides a quick representation of the wind shear above the observation
site. In the example above, the wind veers (shifts in a clockwise
direction) with increasing
heights. However, the existence of constant
wind speeds with increasing heights is highly unlikely. The two
hodographs provided below are far more representational of
actual vertical wind patterns.
Hodographs from The COMET Program.
When severe weather
threatens, experienced meteorologists analyze hodograph patterns to
assess wind shear, a necessary ingredient for the for the development of supercell thunderstorms and