Ideal Gas Lawand

Winter Weather

Lecture 3February 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

To convert from Z time to CST, subtract 6 hours. 05Z = 11 PM CST1020

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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

Pressure

Density

Temperature

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 temperatureWarmer than 0°C

→ all liquid dropsColder than 40°C

→ all ice crystalsBetween 0°C and 40°C

→ both ice and waterWater droplets are supercooled

Precipitation TypesIn 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 crystals3. Sleet frozen rain drops4. Freezing Rain rain the freezes on

contact with a cold surface5. 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 RadarBuffalo, 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 ininches (cm)

CT Fairfield 27.8 (70.6)

NJ Rahway 27.0 (68.6)

CT West Hartford 27.0 (68.6)

NY Manhattan (Central 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)