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Weather Fronts
Fronts Signal the Arrival of Warm Air, Cold Air, and
Precipitation
by Tiffany Means
Known as the colorful lines
that move across weather maps, weather fronts are boundaries that separate air
masses of different air temperatures and moisture content (humidity).
A
front takes its name from two places: it is the literal front, or leading edge,
of air that's moving into a region; it is also analogous to a war battlefront,
where the two air masses represent the two clashing sides.
Because fronts are zones
where temperature opposites meet, weather changes are usually found along their
edge.
Fronts
are classified depending on what kind of air (warm, cold, neither) is
advancing onto the air in its path.
Read on for a more in-depth
look at the main types of fronts.
Warm Fronts
If warm air moves in such a
way that it advances onto and replaces the cooler air in its path, the leading
edge of the warm air mass found at the earth's surface (the ground) is known as
a warm front.
When
a warm front passes through, the weather becomes noticeably warmer and more
humid than it was before.
The
weather map symbol for a warm front is a red curved line with
red semi-circles. The semi-circles point in the direction the warm air is
moving.
Cold Fronts
If a cold air mass spills
onto and overtakes a neighboring warm air mass, the leading edge of this cold
air will be a cold front.
When
a cold front passes through, the weather becomes significantly colder and
drier. (It isn't uncommon for air temperatures to drop 10 degrees Fahrenheit or
more within an hour of a cold frontal passage.)
The
weather map symbol for a cold front is a blue curved line with blue triangles.
The triangles point in the direction the cold air is moving.
Stationary Fronts
If a warm and a cold air mass
are next to one another, but neither is moving strongly enough to overtake the
other, a "stalemate" occurs and the front remains in one place, or stationary. (This can happen when winds blow across
the air masses rather than toward one or the other.)
Since
stationary fronts move very slowly, or not at all, any precipitation that
occurs with them can stall out over a region for days on end and cause a
significant flood risk along the stationary front boundary.
As
soon as one of the air masses pushes ahead and advances onto the other air
mass, the stationary front will begin to move.
At this point, it'll become
either a warm front or a cold front, depending on which air mass (warm or cold)
is the aggressor.
Stationary
fronts appear on weather maps as alternating red and blue lines, with blue
triangles pointing towards the side of the front occupied by warmer air, and
red semi-circles pointing towards the cold air side.
Occluded Fronts
Sometimes a cold front will
"catch up" to a warm front and overtake both it and the cooler air
out ahead of it. If this happens, an occluded front is born.
Occluded fronts get
their name from the fact that when the cold air pushes underneath the warm air,
it lifts the warm air up from the ground, which makes it hidden, or
"occluded."
Occluded
fronts usually form with mature low-pressure areas. They act like both warm and cold
fronts.
The
symbol for an occluded front is a purple line with alternating triangles
and semi-circles (also purple) pointing in the direction the front is
moving.
Drylines
Up until now, we've talked
about fronts that form between air masses having contrasting temperatures. But
what about boundaries between air masses of different humidity?
Known
as drylines, or dew point fronts, these weather fronts separate warm, moist air
masses found ahead of the dryline from hot, dry air masses found behind it.
In the US, they're most often
seen east of the Rocky Mountains across the states of Texas, Oklahoma, Kansas,
and Nebraska during spring and summer.
Thunderstorms and
supercells often form along drylines, since the drier air behind them lifts up
the moist air ahead, triggering strong convection.
On
surface maps, the symbol for a dryline is an orange line with semi-circles
(also orange) that face toward humid air.
Tiffany Means
· Studied
atmospheric sciences and meteorology at the University of North Carolina
· Former
administrative assistant for the National Oceanic and Atmospheric
Administration
· Member
of the American Meteorological Society
Experience
Tiffany
Means is a former writer for ThoughtCo who contributed articles about weather
for five years. She has interned with the domestic and international
weather departments at CNN, written monthly climate reports for NOAA’s National
Centers for Environmental Prediction, and participated in a number
of science outreach events, including the Science Olympiad Competition.
Means has personally experienced such weather greats as the Blizzard of 1993 and the floods of
Hurricane Francis (2004) and Ivan (2004).
Education
· Bachelor's
degree in atmospheric sciences and meteorology from the University of North
Carolina at Asheville
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