1

Snow

Precipitation composed of white or Precipitation composed of white or translucent ice crystals, chiefly in translucent ice crystals, chiefly in complex branched hexagonal form and complex branched hexagonal form and often agglormerated into snowflakes.often agglormerated into snowflakes.

Dendrites -- Fernlike, branchedDendrites -- Fernlike, branched Columns -- Taller than thinColumns -- Taller than thin Plates -- FlatPlates -- Flat Other typesOther types

2

Snow

Snow shapes, or habits, are Snow shapes, or habits, are temperature dependent.temperature dependent.

Temperature (C) Crystal Habit

0 to -4 Thin Plates-4 to -10 Columns-10 to -12 Plates-12 to -16 Dendrites-16 to -22 Plates

Much of the snow we see are aggregates of thesecrystal habits.

3

Snow

The warmer the air, but still below freezing, The warmer the air, but still below freezing, often the heavier the snowfall.often the heavier the snowfall. Warmer temperatures have a higher Warmer temperatures have a higher

saturation vapor pressure so there are saturation vapor pressure so there are more vapor molecules that can allow more vapor molecules that can allow crystals to grow.crystals to grow.

Greater chance of large aggregates of Greater chance of large aggregates of crystals forming in warmer temperatures.crystals forming in warmer temperatures.

4

Snow

Snow ShowerSnow Shower: A form of precipitation : A form of precipitation from a convective cloud that is short in from a convective cloud that is short in duration, starts and stops suddenly, and duration, starts and stops suddenly, and often has a rapid change in intensity.often has a rapid change in intensity.

FlurriesFlurries: A very light and usually brief : A very light and usually brief show shower.show shower.

Snow SquallSnow Squall: A sudden wind : A sudden wind associated with snowfall that occurs associated with snowfall that occurs suddenly and is usually intense and suddenly and is usually intense and short lived.short lived.

5

Freezing Rain

Precipitation that falls to the ground as a Precipitation that falls to the ground as a liquid then freezes upon impact with the liquid then freezes upon impact with the ground or exposed objects. Forms a ground or exposed objects. Forms a glazeglaze of ice on the objects and ground. of ice on the objects and ground.

6

Freezing Rain

Warm:T > 0C

Cold:T < 0C

Rain freezesupon contactwith belowfreezingsurface.

Shallowfreezinglayer

7

Sleet -- Ice Pellets

Warm:T > 0C

Cold:T < 0C

Deepfreezinglayer

Rain freezesbefore reachingthe surface.

8

Measuring Precipitation

Rain GaugeRain Gauge StandardStandard Weighing GaugeWeighing Gauge Tipping BucketTipping Bucket RadarRadar Optical Rain GaugeOptical Rain Gauge

9

State Variables

State variables are quantities that tell us State variables are quantities that tell us the state of a gas -- the atmosphere. the state of a gas -- the atmosphere. Several of these deserve special Several of these deserve special attention:attention: TemperatureTemperature DensityDensity PressurePressure

10

TemperatureTemperature: The temperature of a : The temperature of a substance is a measure of the average substance is a measure of the average kinetic energy of the molecules in that kinetic energy of the molecules in that substance.substance.

Thus Thus atmospheric temperature atmospheric temperature isis

proportional to the speed of the airproportional to the speed of the air

molecules.molecules.

State Variables -- Temperature

11

TemperatureTemperature

T ~ mVT ~ mV22, or, or

as T , Vas T , V

Records:Records:

Low: -127Low: -12700 F in Vostok ,Antarctica, F in Vostok ,Antarctica,

High: + 136High: + 13600 F in El Azizia, Libya. F in El Azizia, Libya.

12

Temperature

T1 > T2

m1v12 > m2v2

2 If m1 = m2 then:

v1 > v2

(Kinetic Energy)

13

Temperature ScalesTemperature Scales

Three (3) temperature scales and their Three (3) temperature scales and their units:units:

Fahrenheit (Fahrenheit (F) -- GermanF) -- GermanCelsius (Celsius (C) -- SwedishC) -- SwedishAbsolute (K) -- “Scientific”Absolute (K) -- “Scientific”

14

Density

The The densitydensity of a substance is defined as of a substance is defined as the amount of mass of a substance in a the amount of mass of a substance in a given volume.given volume.

It can also be defined by a It can also be defined by a number number densitydensity that tells us the number of that tells us the number of “things” in a given volume.“things” in a given volume. Number of students in M204Number of students in M204 Number of water drops in a cubic Number of water drops in a cubic

centimeter of cloudcentimeter of cloud

15

Density

V1 = V2

m1 < m2

m1/v1 < m2 /v2

1 < 2

Unit mass

16

Pressure

The air pressure is the force per unit The air pressure is the force per unit area that the atmosphere exerts on any area that the atmosphere exerts on any surface it touches.surface it touches.

The molecules of the air are in constant The molecules of the air are in constant rapid motion.rapid motion.

When a molecule collides with a When a molecule collides with a surface, such as your skin, the molecule surface, such as your skin, the molecule exerts a force on that surface.exerts a force on that surface.

17

Pressure

Pressure is the cumulative effect of these Pressure is the cumulative effect of these air molecules colliding with a surface.air molecules colliding with a surface.

Air Pressure Depends On:The mass of the moleculesGravityThe kinetic molecular activity

(temperature)

18

Pressure

Pressure is the force per unit area:Pressure is the force per unit area:

In meteorology we us the units millibarsIn meteorology we us the units millibars ““Typical” sea-level pressure is Typical” sea-level pressure is

approximately 1013 mb.approximately 1013 mb.

P = F

A

19

Ideal Gas Law

We have now investigated three state We have now investigated three state variables:variables: TemperatureTemperature PressurePressure DensityDensity

These variables change from place to These variables change from place to place and in time but they do not do so place and in time but they do not do so independently.independently.

This interrelationship is known as the This interrelationship is known as the Ideal Gas LawIdeal Gas Law..

20

Ideal Gas Law: P = R T If the density is constantIf the density is constant

If T increases then p increasesIf T increases then p increases If T decreases then p decreasesIf T decreases then p decreases

If the temperature is constantIf the temperature is constant If p increases then If p increases then increases increases If p decreases then If p decreases then decreases decreases

If the pressure is constantIf the pressure is constant If T increases then If T increases then decreases decreases If T decreases then If T decreases then increases increases

21

Ideal Gas Law: Reprise

We can substitute the definition for We can substitute the definition for density to put the ideal gas law into density to put the ideal gas law into terms that we can easily measure: terms that we can easily measure: temperature, pressure, volume.temperature, pressure, volume.

P = R T = m

V

PV = m R T

22

Ideal Gas Law: Reprise

We see that for a given mass of gas:We see that for a given mass of gas:

For a constant p, T increases, V increasesFor a constant p, T increases, V increases For a constant V, T increases, p increasesFor a constant V, T increases, p increases For a constant T, p increases, V decreasesFor a constant T, p increases, V decreases

PV

T= constant

23

Pressure

GravityBut, the airdoesn’t falldown!!!

Balanced byair pressure.

Pressure: Force due to gravity that the mass ofthe atmosphere exerts on a unit area of surface.