Transcript Document

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 = maritime
c = continental
T = tropical
A = arctic
P = polar
Air Masses and Fronts
m = maritime
c = continental
T = tropical
A = 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 Ranges
Alaska Range
Cascades Range
Sierra Nevada
• Ranges lie
perpendicular to the
prevailing westerly
winds of the midlatitudes
Mountain Climates of Western North America
Each mountain range varies w/respect to elevation
Coast 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
Mountain Climates of Western North America
• 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
 Heavy precipitation on the
windward slopes
 Amount of precipitation varies
with elevation of crest of range
Mountain Climates of Western North America
• 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.
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)
Mountain Climates of Western North America
• 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
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Mountain Climates of Western North America
• 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
Introduction to the Atmosphere
•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 wellmixed 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
RH  100
es
e = actual water vapor pressure
es= water vapor pressure that
would
 have if it were saturated
at its current temperature
Introduction to the Atmosphere
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 vapor
Liquid water
Solid water
Water 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.
Qu ickTime™ and a
TIFF (Uncompressed) decompressor
are need ed to see this picture.
Introduction Atmospheric Structure
• Vertical Structure
• Troposphere
• Stratosphere
• Mesosphere
• Thermosphere
Qu ickTime™ an d a
TIFF (Unco mpressed) d ecompresso r
are ne eded to see this picture.
• 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
Vertical structure, exponential decrease of
air density and pressure with height.
Introduction Atmospheric Structure
• 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.
p  R * T
P = pressure
R = air density
R* = gas constant
T = absolute temperature
Introduction Atmospheric Structure
• 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, R
All 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.
Hot objects emit at short wavelengths
Cold objects emit at long wavelengths
Introduction Atmospheric Structure
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 radiation
b) 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.