Download Atmospheric Moisture and Precipitation

Atmospheric Moisture
and Precipitation
Water is life!
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71% of the earth is covered in water
70% of our body weight is water
A human can survive 50-60 days without food; only 2-3 days
without water
Characteristics of H20:
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Pure water is odorless, colourless and tasteless
Water is a solvent (pure water rarely exists in nature)
Water weighs 1g/cm3 or 1kg/L
Water has three states: solid, liquid and gas
 Although there is evidence of water on other planets (Mars) earth is
the only planet known to have water in all three phases
The Hydrosphere: the hydrologic reservoir
Hydrosphere – water in the earth-atmosphere system
 Earth’s hydrosphere contains 1.36 billion cubic km
 97.22 % (1.321 billion km3) is located within the oceans
(saltwater)
 2.78% is freshwater
 77.14% of freshwater and 99.357% of surface fresh water is
within glaciers and ice sheets
 Less than 1% of freshwater is within lakes, rivers and streams
Water
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Water is unique in that it exists in all three states (solid, liquid, gas)
Consists of 2 H atoms and 1 O atom that bond easily → hard to separate
Water is a versatile solvent
Water is slightly weird as it’s greatest density occurs in the liquid phase
Water is able to shift between states very easily
Water exhibits polarity
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Polarity –
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The 2 H atoms are positively charged and located on one side of
the molecule
The 1 O atom is negatively charged
This polarity causes water molecules to attract other molecules
Hydrogen bonding
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This is why water tends to stick to things
It also leads to surface tension; allows things with greater density
to float
Capillarity = water will move into other substances b/c one water
molecule will draw other molecules into it
Heat Properties of Water
For water to change from on phase to another it either
absorbs or releases heat energy:
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Heat energy is needed to change from phase to another to
break the hydrogen bonds between water molecules
Phase Change – when water changes from one state to
another
 Melting/freezing describe the change between liquid and solid
 Condensation/evaporation describe the change between liquid and
gas
 Sublimation/deposition describe the direct change between solid
and gas
Solid Phase:
- Unlike other substances that contract went cooled, water expands
as a solid
- Ice will increase it’s volume up to 9% as it cools to -29 C
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Reason for potholes, and major landforms/weathering
- Because water expands when it’s a solid (increasing volume) its
density decreases (fewer water molecules per unit volume)
Ice has 0.91 times the density of liquid water
- That’s why ice floats
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Melting/Freezing:
- Melting/freezing occurs at 0 C
- For water to move from a solid (ice) to a liquid, heat energy must
be supplied to increase the motion of water molecules to break the
strong bond achieved as a solid
- There is no change in sensible temperature between ice and water
at 0 C
Latent heat of melting: heat energy absorbed when water goes from
solid to liquid form
Latent heat of fusion: heat energy released when water goes from liquid
to solid form
Liquid phase –
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Water attains its greatest density at 4 C
Liquid water has no shape → water assumes shape of its container
Liquid water exists under temperatures between 0 and 100 C
Evaporation/Condensation phase –
- Evaporation/condensation occurs at 100 C
- More latent heat energy is absorbed and released when water
changes phase between liquid and gas than between solid and
liquid
- Greater latent heat energy is needed to increase the motion of
water molecules to break the surface tension created by hydrogen
bonding
Gaseous Phase – water vapor is an invisible gas and each
molecule moves independently
Sublimation/Deposition –
 Water can move directly from a solid to a gas and back without
entering the liquid phase
Latent heat of sublimation – heat absorbed when water moves from a
solid to gaseous state directly
Happens in your freezer
 When gas moves from a gas to solid phase heat is released; ex frost
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In Summary:
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Energy is absorbed by processes that result in water
changing phase from a solid to liquid to gas
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Energy is released by processes that result in water
changing phase from a gas to liquid to solid
Humidity
Humidity: the water vapor content of the air
The capacity of air to hold water is primarily a function of air
temperature
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Warmer air has a greater capacity to hold water vapor than
cooler air
Relative Humidity:
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After air temperature and air pressure, relative humidity is the most
common piece of information in local weather broadcasts
Relative Humidity - the ratio (expressed as a percentage) of the actual
amount of water vapor in the air (content) to the amount of water vapor
the air can hold at that temperature (capacity)
RH = (water vapor content / maximum water vapor capacity) * 100
Relative Humidity:
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Varies b/c of evaporation/condensation, or temperature changes
Condensation and evaporation happen continuously
Consider warm air to be a large sponge that soaks up large
quantities of water vapor
 Think of cooler air as a small sponge that can only soak up small
quantities of water vapor
The amount of water it takes to fill up cooler air may only partially fill
up warmer air
Saturation - occurs when a particle of air contains all the
water vapor it can hold (it is filled to capacity)
 RH is at 100% (the ratio of water vapor content equal the air’s
capacity to hold water vapor)
 The net transfer of water molecules between the surface and the
air reaches equlibrium (rates of condensation and evaporation are
equal)
 To go past saturation will result in condensation
You can pass saturation by decreasing air temperature (decrease the
air’s capacity to hold water vapor)
 It may not rain when RH = 100% but if the air goes past it , precipitation
will occur
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Dew Point Temperature - the temperature at which a given mass of
air will become saturated given the amount of water vapor contained
within that air parcel
 Air is saturated when the dew point temperature and the air
temperature are equal
 At this point RH = 100%
Relative Humidity Patterns
b/c the capacity of air to hold water vapor increases as air temperature
increases, the daily values of air temperature and relative humidity are
inversely related
- air temperature rise from just after sunrise to mid-afternoon
- Given a constant water vapor content as air temperature rises,
relative humidity will decrease
Daily RH:
- Relative humidity is highest at dawn when air temperature is lowest
- Relative humidity is lowest around mid-afternoon when temperature
and the capacity to hold water vapor is highest
Vapor Pressure – the portion of the air pressure exerted by water
molecules within the atmosphere
Saturation Vapor Pressure – the maximum pressure that water
vapor molecules can exert
 Occurs when air has reached the max capacity to hold water vapor at the
given temperature
 Saturation vapor pressure increases exponentially with increasing
temperature → higher temp can hold more water vapor
 Saturation vapor pressure is higher over water than ice
Specific Humidity – measure of the actual amount of water vapor in
the air
S.H. = mass of water vapor (g) / mass of air (kg)
 it takes into account the actual mass of water vapor in the air
 It is not affected by changes in temperature
 Maximum specific humidity – is the maximum mass of water
vapor that a kg of air can hold at any specified temperature
Measuring Humidity:
Hair hygrometer – measures humidity based on the change in length
of a piece of human hair
 Human hair changes by as much as 4% in length between 0 and
100% relative humidity
Sling Psychrometer – measures the rate at which water evaporates
from a wet wick
 One thermometer measures air temp (dry bulb)
 The bottom of one thermometer is wrapped in a wet cloth
That thermometer measures the temperature relative to the rate of
evaporation from the wick
 Evaporation cools the wick
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Atmospheric Stability
When talking about atmospheric stability, we will use the
term parcel of air
Parcel – term used to describe a body of air with specific
temperature and humidity characteristics.
Two forces determine the motion of an air parcel:
Bouyant force: moves an air parcel up
An air parcel of lower density will rise
 As it rises it will expand as external pressure decreases
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Gravitational force: moves an air parcel down
A parcel of higher density will fall
 As it falls it compresses as external pressure increases
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Stability – the tendency of a parcel to remain in place or to
change vertical position by rising or falling
 A parcel is stable if it resists vertical motion
 A parcel is unstable if it continues to rise until it reaches an
elevation where the surrounding atmosphere has a density or air
temperature similar to that of the parcel
Changes in a parcel’s vertical position will affect the amount of
moisture it can contain
Adiabatic Processes:
Determining atmospheric stability requires the temperature of
the parcel and surrounding air:
Ex: a parcel that is warmer than the surrounding air will rise until it is
cooled to the same temperature as the surrounding air, at which point
it will cease to rise
Normal lapse rate – average decrease in air temperature with
elevation (6.4 C/km)
Environmental Lapse Rate – actual decrease in air
temperature with elevation
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A rising parcel will cool by expansion resulting from a decrease in
pressure with increasing elevation
 As pressure decreases, a parcel will expand and it’s internal temperature
and density will decrease
 Temperature decreases b/c sensible heat is consumed in the process of
expansion
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A falling parcel will warm by compression resulting from an increase in
pressure with decreasing elevation
 As pressure increases, a parcel will compress and its internal temperature
and density will increase
 Temperature increases b/c sensible heat is produced in the process of
compression
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These temperature changes within individual parcels of air occur
without any significant heat exchange between surrounding
environment and the parcel
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Temperature changes within a parcel that occur b/c of expansion and
compression are Adiabatic
Adiabatic processes – describe the cooling and warming rates of
individual parcels of air due to expansion and compression of the
parcel rather than energy exchange between the parcel and
surrounding air
There are two adiabatic rates for a vertically moving parcel, depending on
moisture conditions of the parcel:
Dry Adiabatic Lapse Rate (DAR or Γd) – the rate at which rising dry air cools
by expansion or falling dry warms by compression
 Used for air that is not saturated (has a RH < 100%)
 Has an average value of 10 C/km
Moist Adiabatic Lapse Rate (WAR/ Γm) – rate at which rising moist air cools by
expansion or falling moist air warms by compression
 used for saturated air (RH = 100%)
 Has an average value of 6 C/km
 This is lower that the DAR b/c of the latent heat of condensation
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Rising air will cool and it is possible that it will reach the dew
point temperature (a relative humidity of 100%)
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At that point, or the lifting condensation level, moisture will
condense from the air and the air temperature will follow the
moist adiabatic lapse rate which ranges from between 4°C
and 9°C per kilometer (avg 6 C/km)
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The wet adiabatic lapse rate will be lower for warmer air since
there is more moisture to condense
Stable vs. Unstable Atmospheric
Conditions
Temperature relationships with rising and falling air parcels
causes three atmospheric conditions:
2. Unstable
3. Conditionally stable
4. Stable
Unstable – atmospheric conditions that occur when an air parcel is
warmer than the surrounding air and continues to rise
Unstable:
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the environmental lapse rate is greater than the DAR and MAR
The parcel cools at the DAR making it warmer than the surrounding air
As long as the parcel is warmer it continues to rise
Eventually the parcel will cool until it’s temperature is equal to the dew
point temperature → RH = 100%
Water vapor in parcel will condense, forming clouds and possible
precipitation
Conditionally Unstable – atmospheric conditions that occur when a
parcel rising according to the DAR will resist upward movement but a
parcel rising according to the MAR will continue to rise
Conditionally Stable:
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The environmental lapse rate is greater than the MAR but lower than
the DAR
Unsaturated air will rise according to the DAR
Saturated air will rise according to the MAR
 The surrounding air cools at a rate of 7 C/km and the parcel cools at 6
C/km
 At 1000m the parcel will still be warmer than the surrounding air
 As long as it’s warmer it will continue to rise under unstable conditions
Stability is dependent on the moisture content of the air parcel
Stable – atmospheric conditions that occur when a parcel rising
according to the DAR and MAR will resist upward motion
Stable:
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The environmental lapse rate is lower than the MAR and the DAR
Parcel will not rise and clouds will not form
Clouds
Clouds and Cloud Formation
Cloud – an aggregation of tiny moisture droplets and ice
crystals that are suspended in air, with great enough
volume to be visible
 Form when air becomes saturated with water
 Fog – a cloud in contact with the ground
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Clouds form when water vapor within a saturated air parcel condense
 A parcel will rise until its internal temperature is cooled to the dew point
temperature
 As the condensation occurs it warms the parcel and the parcel continues to
rise and cool
 Condensation continues as the parcel rises
 Liquid water will condense around cloud condensation nuclei
 If there is enough condensation and the moisture droplets are large
enough, eventually the force of gravity will pull them toward the surfece
(precipitation)
Cloud Types:
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Clouds are classified by altitude and shape:
Strat → horizontal
Cumulo → puffy
Nimbus → precipitation
Name:
Altitude:
Shape:
Stratus
Low altitude
Horizontal
Cumulus
Low altitude
Globular
Nimbostratus
Low altitude
Horizontal
Precipitation
Stratocumulus
Low altitude
Lumpy
Grey
Middle altitude
Contain water and
Many styles ice
Altocumulus
Vertical
Cumulonimbus
Cirrus
High altitude
Wispy
Characteristics
thunderstorms
Frozen water
molecules
Cirrostratus
When viewed through a
layer of cirrostratus, the
Moon or Sun has a
whitish, milky appearance
but a clear outline. A
characteristic feature of
cirrostratus clouds is the
halo, a circular arc around
the Sun or Moon formed by
the refraction (bending) of
light as it passes through
the ice crystals.
altocumulus standing
lenticularus
Lenticular Clouds
Stratiform Clouds
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Fog – stratus cloud at the ground
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Advection Fog – moving air over a colder surface
 (e.g., the Grand Banks of Newfoundland)
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Radiation Fog – motionless air that cools below the dew point
due to longwave radiation loss at night
Radiation Fog in the Central Valley of California
Stratiform Clouds
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Fog – stratus cloud at the ground
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Sea Fog – forms when a cool moist air parcel comes in contact
with cold ocean
Occurs along the coasts of continents where cold currents move
toward the equator (e.g. California, Peru)