Expert: Tom Whiting - 11/22/2009

Question
QUESTION: Hi TOm, I got a question. I am stationed in Liberia (africa) and there are two seasons which is the wet from May to October and the dry from November to April. It seldom rains during the dry season, though there are dry period during the rainy season, including a dry spell in July or August lasting about two weeks. What would be the isotherms?

ANSWER: Hi Cynthia,
Isotherms, by definition, are lines of constant temperature on a weather map over a large area.  They have nothing to do with wet/dry air, rainfall amounts, cloud cover, etc. Simply (and only) temperature related, isotherms can change slowly on a synoptic weather map during the 24 hour day. So...for this reason, I don't understand the question or relationship to your information given above, to the question about isotherms. (iso means constant).
Clear Skies,
Tom
PS...other weather iso- related terms are:
isobars - lines of constant pressure on a synoptic weather map.
isotachs - lines of constant wind speed on a synoptic weather map.


---------- FOLLOW-UP ----------

QUESTION: Sorry Tom,

What i meant to say is that if the geographic coordinates for the country i am in now is 6 30 N, 9 30 W. What is the isotherms throughout January and July.

ANSWER: Location is not germane to what or where an isotherm line is on a weather map.
As stated earlier, it's a line of constant temperature over a large area.
In the morning hours you might have the 60 degree isotherm line pass through your
area, parallel the Atlantic and Mediterranean coast, thru Cairo and onward into the Red Sea
region.
Then by noon, the 80 degree isotherm line might replace that same line, and so on into the
day.  Isotherm lines change from hour to hour as the daily temperature goes up, and
as it goes down during the night.  I would presume that along the coastal regions, the
isotherm lines of 60, 70, 80, 90 degrees F., would all run parallel to the shore line,
and of course to each other. (Isotherm lines, like isobar lines, can never cross over
each other...they can get very close at times, but never cross over...as they represent
lines of a constant temperature on Earth's surface.)

Perhaps what you really want to know is the average daily temperatures, the highs and lows,
for those seasons of January and July in Liberia... if that's the case, sorry, but I have
no idea...I would have to just check on Google to see if that information is even available.
(and BTW, isn't this more of a meteorlogical question rather than an astronomy question?)
Clear Skies,
Tom

---------- FOLLOW-UP ----------

QUESTION: Yes it is more of a meteorology, I saw on your profile that you also taught meteorlogy. WHy is Isotherm important?

Answer
Isotherm lines on a synoptic weather map are important in a historical sense.
One can go back say 50 years in the climatological record and see where say the
90 degree isotherm line was in say, the sub-Sahara region in 1959 at a certain date
and time. If that line has moved southward today on the same calendar date and time,
(and maybe use a couple of other years of data in combination with it)....
then we can infer that the Sahara Desert is now warmer and expanding southward in size.
(Then tell the people living in that region that they'd better move and get out of there
because the heat and desert region is expanding and getting worse over the years).

For one that likes to hike in, say a coastal region, but I don't like temperatures over,
say 80 degrees, I can check an isotherm map of the region and avoid those hiking paths
that take you into very warm regions by just following the 60 or 70 degree isotherm lines.

But most of the time, they are used more just for informational purposes. For instance, out
over the Atlantic Ocean and Gulf of Mexico, meteorologists like to follow the 80 degree oceanic
isotherm so they know what part of the sea is 80 degrees and warmer...because hurricanes
thrive and grow stronger at, and above 80 degree temperatures, whereas they shrink and decay below 80 degrees. So the 80 degree isotherm (of the water) is very important to know.

Isotherm lines are not as important as the other two, isobars (pressure) and isotachs.
Pilots like to know the winds aloft at the 10, 20, and 30 thousand foot level, so they can
plan their commerical flight and take advantage of the winds aloft forecast.
So they refer before flight to the winds aloft forecast which is presented in isotachs (wind speed and direction) at different altitudes.

Isobars are important in determining where the location of High pressure and Low pressure
regions are on the synoptic weather chart...and the isobars also show where the frontal systems connected to the Low pressure systems, are located. Also, closely packed isobars show where
the highest surface winds are located so they can make accurate predictions of wind speed
and direction over an affected region.
Hope this helps,
Clear Skies,
Tom Whiting...retired Air Force pilot.