METEOROLOGIST JEFF HABY
The following is a list of all the important isopleths encountered in operational meteorology. An isopleth
is a line or curve of equal values.
Isobar- A line of constant pressure. Isobars are found ONLY on surface charts. They most commonly connect
lines of equal pressure in the units of millibars. High pressure isobars generally occurs with isobars above
1010 mb while low pressure isobars occur with lower than 1010 millibars. Isobars "kink" along fronts and
otherwise have a smooth curved trajectory. Isobars represent the pressure at zero geopotential meters. This
is done to compensate for elevation changes. Isobars of close proximity represent higher wind speeds than
isobars of wide spacing. When isobars are "pack together", this represents an increase in the pressure gradient
force and thus stronger winds. The pressure on an analysis chart in Colorado may be near 1028 mb on the chart
when in reality the surface pressure is closer to 850 mb.
This surface chart has isobars (solid lines).
Isohypse (aka height contour)- A line of equal geopotential height. Geopotential assumes the earth is perfectly
flat and a perfect sphere. The geopotential height is the distance above the Earth's surface if it was a
perfect and flat sphere. Isohypse are shown on a constant pressure surface. For example, when looking at a
850 mb chart, all isohypses no matter their value are located at 850 mb. Regions of low isohypse values are
correlated with low pressure (trough) while high isohypse values are correlated with high pressure (ridge).
This 850 chart has height contours. The
lines are the isohypses. Above 850 mb, the
wind flows close to parallel to the height contours. A curving down of the height contours represents a
trough while a curving up a ridge.
Thickness lines- Same as an isohypse except they represent the distance from one pressure level to a selected
pressure level (usually 1000 to 500 mb thickness). They are used to forecast snow, cold air advection, and warm
air advection. This image
is an example of thickness lines (dashed lines). Colder air (since it is denser)
will have a smaller thickness than warm air.
Constant Pressure Surface- Most analysis and model images are shown using a pressure surface. The most common
are the 1000 mb, 850 mb, 700 mb, 500 mb, and 300 mb surfaces. Every location on the image has the same pressure,
however, heights will vary (thus the contouring of height contours). Below is a listing of pressure surfaces
and their approximate height above zero geopotential meters.
1000 mb-- near surface
850 mb-- 1,500 meters (5,000 feet)
700 mb-- 3,100 meters (10,000 feet)
500 mb-- 5,500 meters (18,000 feet)
300 mb-- 9,300 meters (30,000 feet)
Isotherm- A line of equal temperature. Each of the analysis charts will show
isotherms in either a 2,4,5 or
10 degree increment. They are most commonly used at pressure surfaces below 500 millibars and on surface charts.
Isotherms are used to find regions with warm air advection and cold air advection as well as short waves, fronts,
temperature gradient boundaries, and instability zones.
This 850 mb model panel shows isotherms.
Isallobar / Height change contours- A line of equal pressure change. They are used to forecast the propagation
of low and high pressure
systems. Low pressure tends to develop toward regions of the greatest pressure falls (height falls). Heights
and pressures fall due to the evacuation of mass in the upper levels of the atmosphere and the chilling of air
within a vertical atmospheric column.
Isotachs- These are lines of equal wind speed. They are most often contoured in
the upper levels of the atmosphere,
especially at the jet stream level. They are important for locating the jet stream and jet streaks within a jet
This 300 mb image is an example of isotachs.
Streamlines- Lines of equal wind direction. They are not a pure isopleth by definition. Streamlines are
used primarily in tropical regions since the pressure gradient is weak. They show areas of convergence,
divergence and pressure circulation.
Isodrosotherms- A line of equal dewpoint. They are contoured most often
in the low levels of the atmosphere.
Isodrosotherms can be used to locate frontal boundaries, regions of moist air or dry air advection, and mesoscale
The following image is an example of isodrosotherms (the colored lines). The highest dewpoints are
often found bordering the Gulf of Mexico.
Isodop- Contour of constant doppler velocity values.
Isohyet- Contour of constant rainfall. Used to assess soil moisture, flooding potential, mesoscale wet/dry
boundaries, and rainfall coverage as well as intensity.
This image shows an example of isohyets.
1. What is meteorologically significant about isobars or isohypses being closely spaced?
Answer: A forecaster can determine the amount of thermal advection (temperature advection). Closely spaced
contours indicate stronger wind.
2. How can height contours be used to tell if temperatures are above or below normal? The wind
direction and wind speed? Surface pressure? Precipitation is likely or not?
Answer: High heights indicate the air under that pressure level is warm. Low heights indicate the air under
that pressure level is cool. Above the PBL, the air flows nearly parallel to the height contours in the mid-latitudes
(quasi-geostrophic flow). Closer height contours indicate stronger wind. Precipitation is most likely to the right
of a trough axis. Height contours can be used to locate the trough axis.
3. Negative thickness advection is lower values of thickness moving toward a fixed point. How can this be
determined by examining a thickness chart?
Answer: If height contours or isobars are overlaid on the thickness field, a forecaster can determine
the speed and direction cold air will have. If the lower thicknesses are advected by the height contours
or isobars toward a fixed point, that fixed point will experience a cooling of weather.
4. In feet and meters, what is the average height of the 850, 700, 500 and 300 mb surfaces?
850 mb (1,500 meters, 5,000 feet);
700 mb (3,100 meters, 10,000 feet)
500 mb (5,500 meters, 18,000 feet);
300 mb (9,300 meters. 30,000 feet)
5. What meteorological information can isotherms give an analyzer?
Answer: The temperature gradient, location of fronts, strength of warm air and
cold air advection
6. How are isallobars significant to weather forecasting?
Answer: They give the indication of pressure change. Lows tend to move toward the region with the greatest
height falls. Values tell a forecaster if a low or high is increase or decreasing in intensity.
7. Where in the atmosphere is the jet stream found? What causes the winds within the jet stream to increase?
Answer: Near the 300mb level. Wind speeds increase when the low level temperature gradient is large. If
a polar air mass moves next to a tropical air mass, the upper level winds will increase (formation of jet
stream or a jet streak).
8. What processes cause dewpoint to increase? Decrease?
Answer: INCREASE: Advection of maritime tropical air into forecast region; evaporation or sublimation
of water vapor into the air; movement of air from a moisture source
DECREASE: Advection of drier air into forecast region is primary reason