Download “Making the Invisible Visible: Monitoring Weather

Understanding Weather
What Is Weather?
“Weather” refers to the conditions of the
atmosphere at a certain place and time.
Weather differs from “Climate,” which
deals with long-term conditions in the
atmosphere—patterns, averages, and
extremes (most/maximum and least/
minimum).
• “Climate is what you expect, weather is
what you get!” – Robert A. Heinlein
• Ed Lorenz (MIT): The Father of Chaos
What does weather include?
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Air temperature
Cloud cover
Amount of sunlight
Relative humidity
Precipitation
Wind speed
Wind direction
We Can’t See Air, So What Can
We Measure?
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Temperature
Pressure
Wind direction and speed
Moisture—Humidity and Dew Point
Clouds—coverage and type
Precipitation—type and amount
Special Conditions and Changes
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Temperature
• Temperature is a measure of how much
heat energy the air molecules possess
• Measured in degrees Fahrenheit or
degrees Celsius.
• Thermometers can measure heat by
expanding and contracting metal coils
(left) or liquids (right)
[Source: www.weatheraffects.com]
Air Pressure
• The force that is applied on everything on
the Earth caused by the weight of the air.
• Air particles are mobile, the exert pressure
on objects.
• High air pressure brings warm, dry air.
Low air pressure brings rain or moisture
Air Pressure
• Air pressure can change quickly, ie- when
a storm comes in.
• There are three factors that affect air
pressure: altitude, air mass temperature
and the amount of moisture in the air.
• A Barometer is used to measure air
pressure.
Pressure
• Air molecules push down
on objects—including
you—with a force that we
call the “barometric
pressure.”
• Metal coils in barometers,
as in thermometers,
respond to differences by
expanding or contracting,
causing a dial to move or
change the LED display
[Source: www.accumall.com]
Wind Direction and Speed
• When air moves—
”wind”—we feel both its
direction and speed
(force.)
• Direction is measured
with a “vane”—an arrow
or other shape that
points into the wind.
• Speed is measured with
an “anemometer.” The
faster the cups spin, the
faster the wind.
[Source: www.fascinatingelecronics.com]
Atmospheric Moisture--Humidity
• Water vapor molecules evaporate and
enter air, mixing among the N2, O2, and
other gases.
• The amount of water in air compared with
the amount that it could hold at that
temperature is called “relative humidity.”
• If air holds all the water vapor it can, it is at
100% RH and is said to be “saturated.”
Measuring RH
• “Sling psychrometers”
have “dry bulbs” and
“wet bulbs.”
• Using the difference
between the two (“wet
bulb depression”) and
charts allow calculation
of the relative humidity.
• “Hygrometers” also
measure RH
[Sources:www.novalynx.com and www.accumall.com]
“Dew Point”
• As air cools, its relative humidity rises until
reaching 100% and saturation.
• You are familiar with this because when
you have an iced drink in the summer,
droplets form on the outside of the glass
as the air right next to the glass is cooled.
• The temperature at which saturation
occurs is called the DEW POINT.
Measuring Precipitation
• Rain is easy to
measure—all you
need is an “open tube
with a ruler”—a RAIN
GAUGE.
• “Tipping bucket” rain
gauges can measure
rainfall outside and
connect to show a
display inside.
[Sources: www.accumall.com and www.novalynx.com
Measuring Snowfall
• Measuring how much snow falls is trickier
than measuring rain, because wind can
carry it away or pile it up in drifts.
• Meter sticks or rulers in carefully selected
locations are used to record how deep the
snow lies.
• Meteorologists may also melt snow to find
its “water equivalent”—what it would have
been had it fallen as rain.
Clouds
• Clouds come in many shapes and forms.
• Some are high in the sky, while others are so
low they touch the ground.
• No matter what shape or elevation, clouds form
the same way, by having water vapor condense
onto small solid particles like dust, sea salt, and
pollution
Clouds
• Clouds serve several important functions.
• They provide rain and snow.
• They also help retain heat, so it doesn’t escape
quickly back into space.
• On hot days, clouds provide shade
Types of Clouds
• There are Four main types of clouds
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Cirrus- found high in the atmosphere
Cumulus- found in mid-atmosphere
Stratus- found in the low atmosphere
Nimbus- storm clouds.
Cirrus
• They are thin, wispy
clouds blown by high
winds into long
streamers.
• They usually mean
fair to pleasant
wheather.
Cumulus
• They are puffy clouds
that sometimes look
like pieces of floating
cotton
• They can develop into
a giant
cumulonimbus, which
is a thunderstorm
cloud
Stratus
• are uniform grayish
clouds that often cover
the entire sky.
• They resemble fog that
does not reach the
ground.
• Usually no precipitation
falls from stratus clouds,
but sometimes they may
drizzle.
Temperature
• Air temperature is shown in Fahrenheit
degrees by the number to the upper left of
the circle.
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Pressure
• Pressure—measured in mb (millibars)—is
given by the number to the upper right of
the circle
• For example, suppose it’s 1019.4 mb
• Only the 10s/1s/0.1s are shown—so it is
represented as 194
194
• Pressures below 1000 mb would start with
high numbers, such as 964 for 996.4 mb
Dew Point Temperature
• The dew point temperature in Fahrenheit
degrees is shown to the lower left of the
circle.
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Wind Direction and Speed
• Wind direction is show by an
“arrow” going into the circle.
The example shows wind
blowing from the lower right,
so this is a southeast wind.
• Each long “feather”
represents 10 mph and each
short “feather 5, so the wind
speed is 25 mph.
Cloud Cover
• The amount of the circle that is filled in
indicates how much of the sky is covered
by clouds.
• It may range from “clear” (left) to
“overcast” (right).
“Present Weather”
• When rain, snow, or
other forms of
precipitation,
lightning, and special
weather conditions
exist, these are
shown by symbols to
the left of the circle
between air
temperature and dew
point temperature
rain
showers
thunderstorm
*
snow
Try to interpret this station model
1. Temperature: 45 o F
2. Pressure: 1004.5 mb
6
o
3. Dew Point: 29 F
4. Clouds: overcast
5. Wind: from southeast at
15 mph
6. Precipitation: light rain
2
1
4
3
5
Air Masses
• An air mass is a defined as a large body of
air with very similar characteristics.
• Generally speaking, air masses are
generally defined by temperature and
dewpoints (moisture parameters.)
• Air masses are named based on the
source region of the air mass itself
Types of Air Masses
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Continental Polar, “cold and dry”
– Originates closer to the Poles over
land-locked regions.
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Continental Tropical, “warm and dry”
– Originates closer to the Tropics over
land-locked regions.
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Maritime Polar, “cold and damp”
– Originates closer to the Poles over
water.
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Maritime Tropical, “warm and humid”
– Originates closer to the Tropics over
water.
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Arctic, “very cold”
– Originates in the very cold land-locked
areas
Analyzing Air Masses
• An air mass is most easily identified by
comparing it to other air masses.
• Air masses can be modified with time,
most notably by days of sunshine or lack
thereof.
• Fronts are the dividing line between air
masses so understanding air masses,
means understanding where fronts are
located.
What is a Front?
• Definition: Place where two different air masses
meet and a boundary forms between them. A
narrow transition zone, or boundary, between air
masses with different densities.
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Cold
Warm
Occluded
Stationary
Cold Front
• Marked on a map with a blue line and blue
triangles pointing towards the warm air.
• Associated with cumulus & cumulonimbus
clouds ahead of the front in the warm air,
producing showers and thunderstorms.
Warm Front
• Marked on a map by a red line with red
semi-circles pointed towards the cool air (in
the direction the warm air is retreating to.)
• Generally associated with stratus type
clouds, overcast skies, fog, and general rain
or snow.
Stationary Front
• Marked by alternating blue lines & blue
triangles (pointed in the direction of the
warmer air) and red lines & red semi-circles
(pointed in the direction of the cooler air)
• Weather is similar to that of a warm front.
Occluded Front
• Marked by a purple line with alternating
purple triangles and purple semi-circles, all
pointing in the direction of the frontal
movement.
• Cold temperatures and lots of precipitation.