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WHAT DOES THE WEATHER FORECAST MEAN
WHEN THE GUY SAYS "COLD FRONT"?

We all complain about the weather, and we all tease the
people who predict the weather. Nobody has an inside track
on what is REALLY going to happen in any arena, including the
weather. However, over time, people have observed that there
seem to be patterns in which things happen.

We have observed that people, under certain types of circum-
stances, display the same or similar kinds of behavior for
those circumstances. We can avoid causing people to behave
poorly in a social setting by seeking to allay hostility. By
observing animal behaviors, we have devised methods of
training animals to behave in specific ways. We can train
dogs to lead blind people, to herd cattle or to find people
who are hiding. We have learned many valuable lessons merely
by careful observation.

We cannot DO anything about the weather, but we can avoid
being severely harmed by naturally-occurring events. As
recently as last week, the news media were instructing people
to take precautions against serious winds and flooding. They
told us to pack emergency provisions in case we have to
evacuate our homes. When one inch of rain falls instead of
the predicted one foot of rain, we laugh at the weather guys
and suggest that they are stupid. However, we must remember
that they can only read the signs that are present and from
those signs, make educated guesses as to what the signs mean.
We can assume, when the news media issues storm warnings,
that something serious is probably going to happen, and set
about to ensure that harsh results are minimized.

Those who earn their livings by growing and harvesting our
food can make tentative plans for planting, spraying, etc.,
by paying attention to weather signs and predictions of
meteorologists. Some information is better than none.

Let's create some impending weather patterns and see how
natural events develop:

An aquarium can be structured to show what generally takes
place when differently heated air masses come in contact.
To do this, use plastic model cement or contact cement to
glue plastic rib binders, such as the type used by students
to hold report papers together, along the bottom and sides
of the aquarium. Be sure the ribs are glued securely, to
form a watertight guide for a glass partition. Now make a
partition from a sheet of glass.

Cut the partition so that it will just slide in the plastic
guides and is higher that the top edge of the aquarium.
Several different demonstrations pertaining to the contact of
warm and cold air masses (weather fronts) can be observed:

a. Warm and cold fronts.

Place warm water in one side of the aquarium and cool water
in the other. Use food coloring to color the warm water red
and the cool water blue. Add half a spoonful of salt to the
cool water. The red water represents a warm air mass and the
blue represents a cool air mass. Such air masses are like
huge invisible flat bubbles -- they are often more than 10
miles high and hundreds of miles across.

Remove the partition, and describe what you see. The
dividing line between the masses is called a front. When a
cold mass (blue water) moves against a warm mass (red water),
there is a cold front. You will see that the warm mass is
forced upward over top of the cold mass. Predict what warm
moist air would do under such circumstances.

Generally, a sequence of cloud types progress along the front
bringing a sequence of weather events:

(1) The chilling of the warmer moist air near the
ground causes a general haziness, light flow or
fairly low clouds to appear.

(2) The rapid rising and cooling of the warm moist
air over the cold mass front causes the somewhat
higher thunderclouds (cumulonimbus) to appear.
Heavy rains fall, and, on the ground, strong winds
blow.

(3) Diminishing numbers of cumulus clouds can be seen
high in the sky. On the ground the wind is calmer,
and the air is cooler or even cold.

There may be some variations in this sequence, depending upon
the degree of difference in temperatures between the cold
and warm masses. If the difference is slight, the stormy
portion is less violent -- possibly just changing from mild
to light rains and to slightly cooler temperatures. In the
winter, the cold front may bring snow or sleet instead of
rain.

When a warm mass (red water) moves against a cold mass, there
is a warm front.

In the same demonstration, you can imagine how the warm mass
can advance over the cold mass and bring a sequence of
weather events:

(1) Feathery bands or rows of cirrus clouds appear about
8 miles up in the sky where the air is so cold that
water vapor freezes. The clouds are made up of ice
crystals caused by the warm mass being chilled by
the cold mass.

(2) At a lower altitude, thin sheets of altostratus
clouds appear.

(3) Nimbostratus clouds appear, and steady rains fall.
The clouds seem to come lower and lower as the front
progresses and may touch the ground as fog.

(4) The sky clears, and the air is warmer and damp.

Many variables can affect this general sequence. Note that
when a cold front advances, the progression of clouds
increases in altitude. They decrease in altitude when a warm
front progresses.

By knowing what type of air mass is approaching and how fast,
the meteorologist can generally predict the type of weather
and when it will arrive.

b. Occluded fronts.

Insert the glass partition in the aquarium. Place warm water
in one side of the aquarium and cool water in the other. Use
food coloring to color the warm water red and the cool water
blue. Add half a spoonful of salt to the cool water. Remove
the partition, and allow the water to intermix.

Reinsert the partition; stir the water on one side of it,
then remove the partition again. The three colors of water
now visible represent three different air masses -- warm,
cold, and colder.

You will see the air mass of intermediate temperature forcing
its way between the warmer and colder layers to form three
distinct layers. This relationship forms an occluded front.

In a sequence of events, this triple-mass system passes high
above the ground. The clouds move downward (e.g., cumulus,
stratus, nimbostratus), then back up again (e.g., nimbo-
stratus, altostratus, cirrus).

The lowest clouds are almost always some distance above the
ground. The weather changes from clear and cool to stormy,
then to clear and cool again.

Use the aquarium to see what would happen if no salt was used
in the beginning, or if the two compartments contained water
of the same temperature. You might find the temperature
differences among the three layers in an occluded front.

Other liquids, such as white syrup or glycerine, might be
tried to see if they slow down the changes that take place.


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