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Ideal Gas Law
and
Winter Weather
Lecture 3
February 8, 2010
Review
Review
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Review from last week
• Contouring helps gain a better sense of location
and strength of certain past or present weather
features
• Isopleths separate lower values from higher values
• Contours will never cross each other and never
branch or fork. They are always one continuous
line.
• Do not create contours where there is no data
• Contour at evenly-spaced intervals
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To convert from Z time to CST, subtract 6 hours. 05Z = 11 PM CST
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Common Meteorological Variables
• Density
• Pressure
• Temperature
• These variables are related by an equation known as
the Ideal Gas Law
IDEAL GAS LAW
p=ρRT
Temperature
Pressure
Density
Gas Constant
DENSITY (ρ)
• Density = mass/volume (kg/m3)
• Computed by determining the mass of air in a
given volume
Every side = 1 meter
= 1 kg
Which has the greater density?
• Box 1: 9 kg / (1 m * 1 m * 1 m) = 9 kg/m3
• Box 2: 3 kg / (1 m * 1 m * 1 m) = 3 kg/m3
Box 1 is more dense
• Air molecules
decrease
with height
away from
the surface
• Density
decreases
with height
PRESSURE (p)
• Air molecules are in constant motion and will
collide with other air molecules and objects
around it.
• With each collision, an air molecule exerts a
force.
• A force is a push or pull exerted from one
object to another
• Pressure = force / area
• If density and
temperature
decrease
with height,
pressure
must also
decrease
with height
• 90% of the
earth’s
atmosphere
is below 10
km
TEMPERATURE (T)
 Temperature is a measure of the average kinetic
energy of the molecules in a substance (or amount of
air)
 Kinetic Energy (KE) is a measure of an object’s ability
to do work via its motion
 KE=½*Mass*Velocity2
 Therefore, faster moving molecules have a higher
temperature
 Scales, starting with no molecule movement:
 Kelvin (K): begins at 0 K,
 Celsius (C): begins at -273.15 ºC
 Fahrenheit (F): begins at -459.67 ºF
Conversions:
K = ºC + 273.15
ºF = (9/5 * ºC) + 32
ºC = 5/9 * (ºF - 32)
Records:
Low Temp: -128.2 ºF
High Temp: 136.4 ºF
• Temperature
decreases w/
height in the
troposphere.
• Temperature
increases w/
height in the
stratosphere.
– This is known
as an
inversion.
– Stratosphere
warms due to the
presence of
ozone.
Ice-Crystal Process
• The Ice Crystal Process → The Bergeron Process
• Cold Clouds
– The cloud temperatures allow ice and liquid drops to coexist
Cloud temperature
Warmer than 0°C
→ all liquid drops
Colder than 40°C
→ all ice crystals
Between 0°C and 40°C
→ both ice and water
Water droplets are supercooled
Precipitation Types
In a cold cloud, all precipitation begins in the form
of snow (ice crystals)
• 5 Main Precipitation Types
1. Rain  drops of liquid water
2. Snow  ice crystals
3. Sleet  frozen rain drops
4. Freezing Rain  rain the freezes on
contact with a cold surface
5. Hail  large pieces of ice
How do we get this variety if the origin of the
precipitation is the same?
Temperature is Key
• The vertical distribution of temperature will
often determine the type of precipitation that
occurs at the surface
• As we have learned before, temperature
typically decreases with height in the
atmosphere
• But, in winter, temperature inversions can be
critical in determining the type or types of
precipitation
Snow
• The surface temperature is
25°F (-4°C) and increases with
height before decreasing.
• However, since the
temperature remains below
freezing at every height, any
precipitation that falls will
remain as snow.
http://www.srh.noaa.gov/jetstream//synoptic/precip.htm
http://okfirst.mesonet.org/train/materials/Winter/snow-sounding.jpg
Sleet
• Surface is below freezing
• As elevation increases, the
temperature increases to a
point where some of the
atmosphere is above freezing
• As snow falls into the layer of
air where the temperature is
above freezing, the snow
flakes partially melt.
• As the precipitation reenters
the air that is below freezing,
the precipitation will re-freeze
into ice pellets that bounce off
the ground, commonly called
sleet.
http://www.srh.noaa.gov/jetstream//synoptic/precip.htm
http://okfirst.mesonet.org/train/materials/Winter.html
Freezing Rain
• Will occur if the warm layer in
the atmosphere is deep with
only a shallow layer of below
freezing air at the surface.
• The precipitation can begin as
either rain and/or snow but
becomes all rain in the warm
layer.
• The rain falls back into the air
that is below freezing but
since the depth is shallow, the
rain does not have time to
freeze into sleet.
• Upon hitting the ground or
objects such as bridges and
vehicles, the rain freezes on
contact.
• Some of the most disastrous
winter weather storms are due
primarily to freezing rain.
http://www.srh.noaa.gov/jetstream//synoptic/precip.htm
http://okfirst.mesonet.org/train/materials/Winter.html
• Flurries - Light snow falling for short durations.
No accumulation or light dusting
• Showers - Snow falling at varying intensities for
brief periods of time. Some accumulation is
possible.
• Squalls - Brief, intense snow showers
accompanied by strong, gusty winds.
Accumulation may be significant. Snow squalls
are best known in the Great Lakes region.
• Blowing Snow - Wind-driven snow that reduces
visibility and causes significant drifting.
• Blizzard - Winds over 35 mph with snow and
blowing snow reducing visibility to less than ¼
mi.
Lake-effect Snow
• Lake-effect snow is produced when cold winds
move across warmer lake water and pick up
the lake’s moisture
• As the water vapor freezes and condenses into
snow, it is deposited on the windward shores.
• Produces narrow, but very intense bands of
precipitation
• Can snow at a rate of many inches per hour
• Occurs most frequently near the Great Lakes
Lake-effect Snow on Radar
Buffalo, NY (KBUF) - October 12, 2006
Blizzards
• Severe winter storm characterized by low
temperatures, strong winds greater than 35
mph, heavy blowing snow, and visibilities less
than ¼ mile, lasting for 3 hours or more
• Storm systems powerful enough to cause
blizzards usually form when the jet stream
dips far to the south.
• This allows cold air form the north to clash
with warm air from the south.
• Blizzard conditions usually develop on the NW
side of these storm systems
Nor’easters
• Among winter’s nastiest storms
• Derives its name from its continuously strong
northeasterly winds blowing in from the
ocean ahead of the storm
• Strong areas of low pressure that form in the
Gulf of Mexico or off the East Coast in the
Atlantic and move up the coast
• Produce heavy snow, rain, oversized waves,
and winds that sometimes exceed hurricane
force in intensity
Nor’easter of Feb. 11-13, 2006
State
City/location
Amount in
inches (cm)
CT
Fairfield
27.8 (70.6)
NJ
Rahway
27.0 (68.6)
CT
West Hartford
27.0 (68.6)
NY
Manhattan (Cen
tral Park)
26.9 (68.3)
CT
Danbury
26.0 (66.0)
NY
LaGuardia
Airport
25.4 (64.5)
NY
Bronx
24.5 (62.2)
NY
New Rochelle
24.5 (62.2)
NY
Brewster
24.0 (61.0)
NY
Yonkers
23.9 (60.7)
CT
Waterbury
23.0 (58.4)