Transcript Reading Weather Maps

Symbol
Meaning
2. What is barometric
pressure? What does it
mean?
3. What are isobars?
What do they show?
4. Why is forecasting
important?

Forecasting:
predicting weather
before it happens
◦ Sometimes it works,
sometimes it doesn’t
◦ It’s not an exact
science!
•
Before the 1950s, all
forecasting was based
on observations
–People would look for
changes in:
•
•
•
•
Air pressure
Wind speed/direction
Temperatures
Current weather conditions
•
Forecast models from
computers are now used to
predict the weather. Each
forecast model will create
several different outcomes
– Humans are still required
to pick the best possible
forecast model
– The atmosphere is
unstable and constantly
changing, which means
that forecasts become less
accurate the farther out
they are made
How to draw/read
weather symbols
• First, look at the
symbol in the middle
and match up weather
conditions
• Look to see if there’s
a line that means wind
– Figure out the speed of
the wind and the
direction it’s blowing
Symbol
Meaning
Symbol
Meaning
Clear skies
T
Thunderstorms
Cloud cover 25%
S
Snow
Cloud cover 50%
(partly cloudy)
f
Fog
Cloud Cover 75%
F
Freezing rain
Cloudy
Wind (will be
different speeds
and directions)
R
Rain
Hurricane
•
Barometric pressure
(air pressure): The
amount of force the
air is pressing down
on the Earth.
– About 15lbs per
square inch at sea
level
– Measured in inches of
mercury (mb)

Barometer- The
instrument that
measures air
pressure

Used for forecasting
 High air pressure, or air
pressure increasing—
nice, calm weather
 Low air pressure, or air
pressure decreasing—
weather changing,
storms more likely

Isobars—Lines on a
weather map that
connect areas of equal
pressure
◦ Areas that are connected
on the line have similar
weather
*Weather in the USA moves
from west to east

Isobars show differences in air
pressure. Air flows from areas of high
pressure to areas of low pressure.
◦ The more tightly packed the isobars the
faster the air moves across them.
 Just as water flows faster downhill
where there is a steep slope, air moves
faster "downhill" (down pressure)
faster where there is a steep slope.
Closely spaced isobars means a steep
slope.
 Therefore, tightly packed isobars
would mean strong winds and widely
spaced ones would mean slow winds
How to draw isobaric maps
1. Find all the areas on a map that have similar
numbers
–
2.
3.
4.
Generally go by increases of 4 mb
Draw a line around the similar numbers
Lines cannot cross or touch
Find areas of high and low pressure
H=high pressure (biggest numbers)
L=low pressure (lowest numbers)
(10)00
97, 98, 99, 00
(10)04
01, 02, 03, 04
(10)08
05, 06, 07, 08,
(10)12
09, 10, 11, 12
(10)16
13, 14, 15, 16
(10)20
17, 18, 19, 20
(10)24
21, 22, 23, 24

Forecasting is important
for multiple reasons.
◦ Weather warnings are used
to protect life and
property.
◦ Forecasts based on
temperature and
precipitation are important
to farmers, and may cause
food prices to
increase/decrease

Forecasting is important for
multiple reasons.
◦ Temperature forecasts are used by
electric companies to estimate
demand over coming days.
◦ People use weather forecasts to
determine what to wear on a given
day.
◦ Forecasts can be used to plan
activities around outdoor events
How to Forecast
• Weather in the USA generally
moves from west to east
– Take a look at the different fronts
and how they’ve been interacting in
the past few days
– Pay attention to the barometer
• Falling pressure: bad weather
• Rising pressure: good weather
– Make predictions based on the
above information, and the time of
year