Transcript Weather

Properties of the
Atmosphere
Composition
Structure and Function
Air Pressure
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What Elements are in our
Atmosphere?
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Find this graphic in your ESRT now.
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Layers of the Atmosphere
• 4 main layers--from lowest to highest
– Troposphere
– Stratosphere
– Mesosphere
– Thermosphere
 What is the upper limit of each of
these layers?
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Each layer of the
Atmosphere absorbs
energy and protects
the Earth from
harmful radiation.
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Air Pressure
• Caused by the
weight of the air
above you
– Greatest at
Earth’s surface
– Lower at higher
elevation
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Air Pressure
• Because air pressure is exerted in all
directions we are seldom aware of it
• Can be measured with one of two types
of barometers
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Mercury Barometer
• Uses a
column of
mercury in a
vacuated
tube
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Aneroid Barometer
• Uses a thin
metal airtight
box
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Air Pressure
• Measured in
millibars
• Standard sea level
pressure is 1013.2
mb
– From your ESRT
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•  Why do
Air Pressure
people often
use oxygen
tanks when
they climb
high
mountains?
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Factors that affect air pressure
• Temperature
• Moisture
• Altitude
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Temperature as a factor
• Cold air
–Molecules closely packed
• Weighs more
• Warm air
–Molecules further apart
• Weighs less
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How does pressure affect
temperature?

What happens when air is pressurized?

What happens when air is expanded?
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Increase temperature – decrease pressure
PRESSURE
T
What weighs more????
Water vapor replaces other air molecules that are heavier
MOIST AIR WEIGHS LESS
Moisture as a factor
Increasing water vapor makes air
less dense
–Nitrogen weighs more than the
water vapor that replaces it
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Dry Air
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Moist Air
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Air Pressure
• Air with most pressure is
cold and dry
• Air with least pressure is
warm and moist
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Pressure and Density
Decrease with
ALTITUDE
Properties of the
Atmosphere
Wind
Coriolis Force
Clouds
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Air Pressure
• In general, the poles are areas of high
pressure and the equator is an area of
low pressure (ESRT)
• Air moves from the poles to the
equator then back to the poles
– Cools the tropics and warms the poles
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Wind
• Wind velocity is
measured with
an anemometer
• Is named by the
direction it is
coming from
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Wind
• Caused by the
uneven heating
of Earth’s
surface
• Creates areas
with different air
pressures
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Wind
• Air always moves from high pressure
areas to low pressure areas
– Just like air in a balloon
• Warm, rising air creates low pressure
areas
– Less dense
• Cold falling air creates high pressure
areas
– More dense
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Wind--sea breezes
• During the
day the land
is heated
more than
the water
causing low
pressure
over the land
• Remember-air moves
from high
pressure to
low pressure
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Wind--land breezes
• Just the
opposite
happens at
night
– The land cools
down more
than the water
causing high
pressure over
the land
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Wind speed is DIRECTLY related
to the pressure gradient
Closer the isobars
gradient
Greater the pressure
Greater the wind speed
Prevailing winds and ocean
currents
• This is what
air movement
would look
like if the
Earth were
not spinning
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Actual Prevailing Winds and Ocean
Currents
• Due to the Earth
spinning.
 What is the
name we give
motion due to the
earth’s rotation?
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Coriolis Force
• Because Earth is spinning wind curves
to the right in the northern hemisphere
and to the left in the southern
hemisphere
– As seen from space
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Coriolis Force
Air moves away from a high
pressure area like this
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Air moves into a low
pressure area like this
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WIND MOVES
CLOCKWISE
OUT OF THE
HIGH
WIND MOVES
COUNTERCLOCKWISE
INTO THE LOW
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Wind
–Wind always blows from
areas of high pressure to
areas of low pressure
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Wind
• High pressure areas are zones of
divergence (ESRT)
– Where air pushes away
• Low pressure areas are zones of
convergence (ESRT)
– Where air moves together
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Wind--zones of convergence
and divergence
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Air pressure systems
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3 convection cells due to
rotation, and unequal heating
due to tilt
WINDS AND CONVECTION CELLS
CAUSED BY DIFFERENCES IN AIR
PRESSURE
WINDS NAMED FOR THE DIRECTION FROM WHICH THEY
COME
Isobars
• Isobars are drawn on maps to show
atmospheric pressure
– Isobars are isolines that connect points of equal
atmospheric pressure
– Similar to contour lines on a topographic map
• The closer the isobars, the greater
the difference in pressure and
therefore the stronger the wind
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Isobars of a hurricane
Fast wind
Slow wind
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Evaporation of Water
• Three factors that affect the rate at which
water evaporates (vaporizes)
– Wind
– Heat
– Surface area
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Measuring the
Atmosphere’s Moisture
Dew Point
Relative Humidity
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Measuring moisture in
the atmosphere
• Can be measured 2 ways
–Dew point
–Relative humidity
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Moisture in the Atmosphere
• Saturated
– When air is holding as much moisture as it
can
• The warmer the temperature, the more
moisture air can hold
• For every 10 °C rise in temp. the air can
hold approx. twice as much moisture
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Dew point
• When air temperature is lowered it
reaches a point of saturation
– Where it can hold no more water vapor
• This is known as the dew point
• When a mass of air is cooled, the air
cannot hold as much water vapor. This
water vapor “falls” out of the air as dew.
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 Write a concise statement or draw a
graph that describes how temperature
affects the amount of water air can hold:
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Dew point
• Dew point
temperature is
determined with a
sling
psychrometer
– A wet
thermometer and
a dry thermometer
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are used
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Dew point
• You need 2
measurements:
– Wet bulb temp
– Dry bulb temp
• Find the
difference
• Use the dew point
temperature chart
in your ESRT
– Read the chart!
– Follow the lines!
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Dew point
• The difference between the dry bulb
and wet bulb temperatures is known as
the wet bulb depression
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DPT & Relative Humidity
Charts
Handy Dandy Earth Science
Reference Tables Page 12
The “Dry Bulb”
• Don’t let it fool you. It
is just a thermometer.
• It measures the air
temperature.
• Duh!
20°C
The “Wet Bulb”
• Has a little wet
booty tied to the
bottom.
• Gets cool when
water evaporates.
Wet Booty
12°C
A Dry Day…
• A lot of moisture will
evaporate.
• The wet bulb will be
a lot cooler than the
dry bulb.
20°C
Difference between wet bulb
& dry bulb is 12 °C.
20°C
14°C
8°C
A Humid Day…
• A little bit of
moisture will
evaporate.
• The wet bulb will not
be much cooler than
the dry bulb.
20°C
Difference between wet bulb
& dry bulb is 6 °C.
14°C
Page 12 of your Handy Dandy
Earth Science Reference
Tables
• Warning #1: Be sure to READ the correct chart:
DPT or RH
• Warning #2: Dew Point Temperature IS NOT
“Difference between wet bulb and dry bulb”.
• Warning #3: The wet bulb temp IS NOT the DPT.
Dry-bulb temperature is your air temperature.
20 °C
Subtract (the difference) between the dry bulb and wet bulb
Put it all together
20°C
8°C
20-8=12
Relative Humidity = 11%
Dew point
Your dry bulb temperature is 10°C
and your wet blub depression is 3°C.
What is the dew point?
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Dew point
 Using a sling psychrometer, you
measure the dry bulb temperature to be
20°C and the wet bulb temperature to
be 15°C. What is the dew point?
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Dew point
 If the dew point is below zero, what
happens when the dew point is
reached?
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Relative Humidity
• Relative humidity expresses how full of
moisture the air is
• Compares how much moisture the air is
holding compared to what it could hold
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Relative Humidity
• Relative humidity is also
determined by comparing the dry
bulb temperature with the
difference between the wet and
dry bulb temperature
• You use a different chart in your
ESRT.
– Be careful. Read the title!
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Relative Humidity
 Your dry bulb temperature is 23°C and
your wet bulb is 20°C. What is the relative
humidity?
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 Draw a line to complete this graph:
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 Where
on this line
would you
expect
rain?
0
Relative Humidity
Relative Humidity vs. wet bulb depression
0
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Wet Bulb Depression
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The influence of temperature on relative humidity
• Raising the
temperature
without adding
more water
vapor makes
the relative
humidity go
down
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The influence of increasing moisture on relative humidity
• Adding more
moisture without
increasing the
temperature will
also cause a rise
in relative
humidity
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What causes precipitation?
• Cloud formation
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Ingredients Required for Clouds:
Water vapor (water as a gas)
Conditions favoring the change of state
(from gas to liquid or ice)
CONDENSATION
A surface for water vapor to
condense on (condensation nuclei)
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What causes cloud formation?
• When a parcel of air rises it expands
• When air expands it cools
– Called adiabatic cooling
• When air cools it reaches the dew point
– 100% relative humidity
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Cloud Formation
• When air reaches the dew point it
condenses
– Forming clouds if conditions are right
• The water vapor has to have something to
condense on
– Called condensation nuclei
• The rate at which a parcel of air cools as it
rises is known as the adiabatic lapse rate
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Condensation Nuclei
• Raindrops are
typically millions of
times larger than
their condensation
nucleus
– Condensation
nucleus
– Raindrop
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Cloud droplets can survive by latching onto microscopic solid
particles, or condensation nuclei in our atmosphere. These
solid particles can be dust, smoke, and salt particles.
From
volcanoes
From Forest
Fires
The Ocean
Pollution
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(First three pictures are not by the author)
Salt water droplets from
the ocean are carried by
updrafts into the
atmosphere. When the
water evaporates, the salt
is left behind.
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Clouds--what makes air rise?
• Air moving over mountain
ranges
–Orographic effect
• When low density air is pushed
up by high density air
–Happens when a cold front
pushes up a warm mass of air
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Air rising and cooling to the dew point by expansion (adiabatic
cooling)
By forced lifting—such as when air is forced over a mountain:
Pictures from the National Audubon Society Field Guide to Weather
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Air is forced to rise over mountains and cools
on the windward side and warms on the
leeward side (rain shadow)
Clouds
• Names are based on shape and altitude
• Can tell the observer something about
atmospheric conditions
– Ex. Cirrus clouds indicate high pressure
and clear conditions
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Cloud Height
• You can find the
elevation of the
base of a
cumulus cloud by
knowing
– Dew point
– Dry bulb
temperature
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• The lapse
rate chart is
used to find
the altitude
of the bottom
of cumulus
clouds
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The second way to get air to rise
is for less dense air to be force
up and over more dense air.
• This happens at the boundaries between AIR
MASSES
• AIR MASS: A region of air that has similar
temperature and moisture.
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What is an Air Mass?
• Air masses are large bodies of air which have
similar temperature and moisture
characteristics.
• Air masses form when air stays over a region
(called the source region) for several days.
• Air masses that form over water will be moist.
• Air masses that form over land will be dry.
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AIR MASSES
• Air takes the characteristics of the surface over
which it formed.
• An air mass that forms over the ocean will be
moist. These air masses are called MARITIME
air masses
• An air mass that forms over land will be
relatively dry. These air masses are called
CONTINENTAL air masses
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Air Masses
• An air mass that forms over a cold
portion of the Earth is called a POLAR
air mass.
• An air mass that forms over a warm
portion of the Earth is called a
TROPICAL air mass.
• An air mass that forms over an
extraordinarily cold portion of the Earth
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is called an ARTIC air mass.
Open your ESRT to page 13!
Air Masses
Air Mass Classification
Source Region
Identifiers:
• A for Arctic,
• P for Polar,
• T for Tropical;
Moisture Content
Identifiers:
• c for continental
(meaning the air is
relatively dry),
• m for maritime (meaning
the air is relatively moist);
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•
•
•
•
cT
cP
mT
mP
Continental Tropical – dry & warm
Continental Polar – dry & cold
Maritime Tropical – humid & warm
Maritime Polar – humid & cold
• cA Continental Artic – very dry & very
cold
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What happens when air masses
meet?
• The boundary between two air masses
is called a FRONT.
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Fronts
• Occur when different air masses collide
• Types include:
– Cold front
– Warm front
– Stationary front
– Occluded front
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Cold Front
• Where cold, dense
air pushes beneath
warm, less dense
air
– Where powerful
storms occur
– Cold fronts move
quickly
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Warm Front
• Where a mass of
warm air slides
over a retreating
mass of cold air
– Often producing
slow steady rain
– Warm fronts
move slowly
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Occluded Front
• When one front
catches up to
another
• Produces large
areas of rainy,
unsettled
weather
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Stationary Front
• Front does not
move
• Winds blow in
opposite
directions
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Causes of Precipitation
• LOW PRESSURE
• FRONTS
• MOUNTAINS
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Precipitation Types
• Rain: water droplets leave cloud and fall to
earth
– Drizzle, mist, etc.
• Snow: water droplets freeze inside cloud and
fall to earth
• Hail: water droplets fall, are swept up and
freeze forming a ball of ice that falls when the
wind is not strong enough to support it
• Sleet: water droplet leaves cloud, moves to
cold area and freezes
• Freezing rain: water droplet freezes on
contact with frozen surface
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Synoptic Weather Maps
• Symbols are used on weather
maps to show current weather
conditions
–Allows you to predict future
weather
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Synoptic Weather Maps
• Symbols for present weather
– Called a station model
– This graphic is in your ESRT!
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Synoptic Weather Maps
• Symbols used for fronts:
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