Reasons Snow Important

•10% of the Earth's surface is covered by glacial ice, with snow covering the glacial ice.

* Small changes in climate can have very large effects on precipitation as snow, the amount of water stored as snow, and the timing and magnitude of snowmelt runoff.

Hydrologic Cycle

Water movement from the atmosphere to the oceans and continents occurs as precipitation, including rain, snow, sleet, and other forms.

Hydrologic Cycle

On the continents, water may be stored temporarily, but eventually returns to the oceans through surface/groundwater runoff or to the atmosphere through evapotranspiration.

Hydrologic Cycle

For this class, we concentrate on the contributions to the hydrologic cycle from snow and snowmelt. Additionally, we will investigate some of the unique properties of snow that result in avalanches.

Phase diagram – H20

The phase diagram is divided into three regions, each of which represents a pure phase.

The line separating any two regions indicates conditions under which these two phases can exist in equilibrium.

Phase diagram – H20

The point at which all three curves meet is called the triple point. For water, the triple point is at 0.01° C and 0.006 atmospheres.

Phase diagram – H20

Lets change pressure, and see how the boiling point and freezing (melting), point of water deviate from normal magnitudes

Air Masses and Fronts

Air mass: regional scale volume of air with horizontal layers of uniform temperature and humidity.

m = maritimec = continentalT = tropicalA = arctic P = polar

Air Masses and Fronts

m = maritimec = continentalT = tropicalA = arctic P = polar

As these air masses move around the Earth, they can begin to acquire additional attributes.

In winter, an arctic air mass (very cold and dry air) can move over the ocean, picking up some warmth and moisture from the warmer ocean and becoming a maritime polar air mass (mP) - one that is still fairly cold but contains moisture.

If that same polar air mass moves south from Canada into the southern U.S. it will pick up some of the warmth of the ground, but due to lack of moisture it remains very dry. This is called a continental polar air mass (cP).

Mountain Climates of Western North America

•Four mountain ranges parallel west coast of North America

Coast RangesAlaska RangeCascades RangeSierra Nevada

• Ranges lie perpendicular to the prevailing westerly winds of the mid-latitudes

Mountain Climates of Western North America

Each mountain range varies w/respect to elevationCoast Range: Elevation varies from N to S. Olympic Mtns are highest portion of Coast Range in the USA

Cascades: Highest peaks are volcanoes. The mean crest elevation is considerably below the elevations of these isolated volcanoes.

North Cascades - somewhat higher elevations and heavy winter snowfalls produce extensive glaciation

• Significant barriers to maritime air masses moving into the continent from the Gulf of Alaska and northern Pacific• Moist air carried inland from the Pacific Ocean is lifted:

• First over the Coast Range• Then over the Cascades

Range

Mountain Climates of Western North America

Heavy precipitation on the windward slopes Amount of precipitation varies with elevation of crest of range

• Low precipitation or rain shadows on the lee side of mountain ranges

West slope of Olympic Mts: 150” (381cm)Sequim, WA = 16 “ (41 cm)

•Supports different ecosystem, semi-arid

shrub/steppe.

Mountain Climates of Western North America

Coast Mountains: 0.3 SWE (1-7-2007; Hurricane Ridge, Olympic National Park, Washington)

Cascades Crest: 2.85 SWE (1-7-2007; Alpental Ski Area, Washington)

Eastern Washington: 0.12 SWE (1-7-2007; Mission Ridge Ski Area, Washington)

• Precipitation is highly seasonal

• Influenced by Aleutian Low & Pacific High

•These two semi-permanent pressure systems move in tandem..

• Northward shift in summer

• Southward shift in winter

• Summer = intensified high• Winter = intensified low

Mountain Climates of Western North America

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• Winter

• Storms develop in the area of the Aleutian Low & bring near continuous drizzle, rain and moderate coastal winds along the west coast of N. America

• Southwestery winds to the south of low pressure storm systems bring the heaviest precipitation

•Maritime influence moderates the temperatures •Rain = low elevation•Snow = higher elevations

Mountain Climates of Western North America

• Spring

• Pacific High moves northward and intensifies

• Brings milder, drier weather to the Coast and Cascades Ranges

• Clockwise circulation exposes the coast to winds out of the NW

• High pressure suppresses cloudiness and precipitation

Mountain Climates of Western North America

•Synoptic weather systems• Monsoons• High & low pressure centers• Fronts

Introduction to the Atmosphere

•Composition• A. Three gases

• Nitrogen (78%)• Oxygen (21%)• Argon (1%)

•These gases are well-mixed in the lower atmosphere

Introduction to the Atmosphere

• A. Other gases• Water vapor (0 - 4% vol)• Carbon dioxide (0.034% vol)

• Water vapor and CO2 absorb radiation emitted by the Earth’s surface and reradiate it back towards the Earth

• B. Aerosols: natural or man-made• Rain, snow, ice, dust, pollen,• Carbon, Acids

Affect transmission of light - visibility

Serves as a nuclei for condensation of water vapor

Introduction to the Atmosphere

•C. Humidity• Water content varies

over time and space• Amount of water vapor

depends on air temperature

• Warm air holds more water vapor than cooler air

• High humidity areas are found in warm equatorial regions

• Relative humidity [Room example]

Introduction to the Atmosphere

C. Relative humidity

Ratio of actual water content of air to the water vapor content of saturated air at the same temperature

e = actual water vapor pressure

es= water vapor pressure that would have if it were saturated at its current temperature

Introduction to the Atmosphere

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RH =100e

es

C. Relative Humidity• Percentage value (20%..)• Water vapor content at

saturation rises with T, but actual vapor content does not

• Diurnal variations are present

• Relative humidity reaches max just before sunrise when temp is lowest.

• Relative humidity reaches min in mid-afternoon, when temp is highest.

Introduction to the Atmosphere

D. Water phase changes in atmosphere

Water vaporLiquid waterSolid waterWater changes between phases

• Phase changes release or store large quantities of heat, called latent heat• Heat must be supplied to change solid to liquid or solid to vapor• Heat is liberated with reverse phase

• Amount of heat required to evaporate water is equal to the amount of heat liberated when water vapor condenses.

Introduction to the Atmosphere

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• Vertical Structure

• Troposphere• Stratosphere• Mesosphere• Thermosphere

• Defined by air temperature w/r to height• Troposphere = lowest layer

• 7 mi (11 km) above MSL

• Mean temperature decease with height of about 3.5°F/1000’ or 6.58°/km

Introduction Atmospheric Structure

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Vertical structure, exponential decrease of air density and pressure with height.

• Air pressure: Mass per unit volume of atmosphere

• Millibars or pounds per sq inch

• Air pressure is the measure of weight of a column of air above that level

• Temperature, density, and pressure are closely related.• Gas laws exponential decrease of air density and

pressure with height.

Introduction Atmospheric Structure

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p = ρR*T P = pressure

R = air density

R* = gas constant

T = absolute temperature

• Atmospheric stability: resistance to vertical motion.

• Stable atmosphere = horizontal clouds

• Unstable atmosphere = vertical clouds

• Determine a parcel of air taken from a point in layer, lift it a small distance and release.

Introduction Atmospheric Structure

Before lifting, air temperature is constant.New term: Adiabatic process = no heat exchange

•If parcel lifts, it encounters lower pressure, it expands and cools

•If parcel drops, it encounters higher pressure, it compresses and warms

Introduction Atmospheric Structure

• If after lifting the parcel is warmer (and lighter and less dense), it continues to rise.• This indicates that the atmosphere is unstable.

• If the parcel is colder (heavier and denser) than its new surroundings, it sinks back to its point of origin.•This indicates that the atmosphere is stable.

• If the parcel is the same temperature as its new surroundings, it remains at the point where it is released, indicating the atmosphere is neutral.

Introduction Atmospheric Structure

• Lapse Rate• Rate at which temperature

decreases with height• Lapse rate depends on

whether the parcel is saturated (RH=100%) or unsaturated (RH<100%)

• Unsaturated parcel: dry adiabatic lapse rate (DALR)

5.4°F/1000’ or 9.88°/km

• Saturated parcel: moist adiabatic lapse rate

MALR is not constant, but dependent on parcel’s T & P

Introduction Atmospheric Structure

• Troposphere is generally stable

•Mean lapse rate is about 3.5°/1000’ or 6.5°C/km

Relative changes in temperature across layers when horizontal winds bring colder or warmer air into part of the layer or

•When air sinks and warms or rises and cools•Vertical motions associated with high and low pressure systems

Introduction Atmospheric Structure

• Atmospheric boundary layer (ABL)

•Lowest part of the atmosphere where temperature and humidity after by the transfer of heat and moisture from the Earth’s surface.

• The ABL is warmed by upward transfer of heat during the day, or cooled by downward transfer of heat during the night.

• ABL is also defined as the layer of the atmosphere affected by frictional drag along the Earth’s surface.

Introduction Atmospheric Structure

Surface Energy Budget

• Amount of heat and moisture transferred between the SBL and Earth’s surface.

Net solar and terrestrial radiation at the Earth’s surface (R) must be transformed into:1) Latent heat flux (L) used to

evaporate or condense water

2) Ground heat flux (G), used to warm or cool the ground

3) Sensible heat flux (H), used to warm or cool the atmosphere

R + H + L + G = 0

Introduction Atmospheric Structure

Net all-wave radiation term, RAll objects emit radiation.

The wavelength depends on the temperature of the radiating body

The Sun (6,000°C, 11,000°F) emits most radiation in the wavelength range of .015-3 micrometers.

Human vision responds to the visible spectrum (0.36-0.75 micrometers).

Terrestrial objects have much lower temperatures and radiate energy at 3-100 micrometer wavelength.

Introduction Atmospheric Structure

Hot objects emit at short wavelengthsCold objects emit at long wavelengths

Net all-wave radiation term, R

• The intensity of solar and terrestrial radiation are comparable, buy there is no overlap in the wavelength between the two.

• Radiation from the Sun is called shortwave radiation.

• Radiation emitted from objects or gases at normal terrestrial temperatures are called longwave radiation.

Introduction Atmospheric Structure

Net all-wave radiation term, R• Both shorwave and longwave radiation can be directed

upward from the ground or downward from the atmosphere

• Thus there are four components of the net all-wave radiation term R

a) incoming shortwave radiationb) outgoing shortwave radiation (fraction of the incoming

shortwave radiation)c) incoming longwave radiation emitted by gases and clouds in

the atmosphere

d) outgoing longwave radiation emitted by the Earth’s surface and

objects on it.

Introduction Atmospheric Structure

Diurnal Variations in R and SEB• Strong variations in 4 components of the net all-wave

radiation term R

Introduction Atmospheric Structure

Diurnal Variations in R

Both solar terms begin at sunrise and end at sunset.

Solar term peaks at midday

At night, solar radiation is zero

Introduction Atmospheric Structure

Diurnal Variations in R

At night, R is driven by net loss of longwave radiation

Why:Outgoing longwave radiation

is larger than incoming longwave radiation

Introduction Atmospheric Structure

Diurnal Variations in R

R changes sign about 1 hr after sunrise when the net loss of longwave radiation is overpowered by a net gain in shortwave radiation

R changes sign again about 1 hr. before sunset, when the amount incoming solar radiation is reduced.

Introduction Atmospheric Structure

Diurnal Variations in R

When R changes, all other terms of the surface energy budget changes.

Spatial variations in energy fluxes can change reducing differences between sunny and shaded areas, or amt. of precipitation.

Introduction Atmospheric Structure