Download Atmospheric Moisture

Atmospheric Moisture
Earth is the Water Planet
• >70% of the surface is covered by liquid
water (oceans and lakes)
• Only planet in which water occurs in all
three states
– Solid
– Liquid
– Gas
Earth is the Water Planet
• >70% of the surface is covered by liquid
water (oceans and lakes)
• Only planet in which water occurs in all
three states
– Solid -ice, snow
– Liquid - oceans, lakes, rivers, rain
– Gas - water vapor in the atmosphere
Earth is the Water Planet
• >70% of the surface is covered by liquid
water (oceans and lakes)
• Only planet in which water occurs in all
three states
– Solid -ice, snow
– Liquid - oceans, lakes, rivers, rain
– Gas - water vapor in the atmosphere
Hydrologic cycle
• THydrologic cycle - the cycling of water
between the land, oceans and atmosphere
and includes the entire hydrosphere (all
forms and location of water).
• Processes involved in the hydrologic cycle
• Precipitation
• condensation
• Evaporation
• Infiltration
• Runoff
• Transpiration
Some characteristics of Water
• All life needs water
• Everything dissolves in water to some
extent - universal solvent.
• Water has a high specific heat capacity
– Specific heat capacity - amount of energy
needed to increase the temperature of a
substance by 1oC.
• Water expands upon freezing
Oxygen
Hydrogen
H 2O
O
H
H
Phase Diagram for Water
States of Water
• Evaporation - process whereby water molecules
break free of the liquid.
– Liquid to vapor
• Condensation - process whereby water
molecules bond with other water molecules.
– Vapor to liquid
• Saturation - maximum amount of water that can
exist in the atmosphere as vapor, at a given
temperature.
At saturation a balance exist between the rate of
evaporation and rate of condensation.
States of Water
(Water Vapor and Ice)
• Sublimation - phase change of ice (solid) directly
to vapor.
– Solid (ice) to vapor
• Deposition - phase change of vapor directly to
solid.
– Vapor to Solid (ice)
Latent heat
heat energy absorbed or released during
change of state
• Latent heat of vaporization - heat energy
absorbed during evaporation.
• Latent heat of condensation - heat energy
released when vapor changes to liquid.
• Latent heat of fusion - energy absorbed
during melting.
Measuring Water Vapor in the
Atmosphere
• Humidity - amount of water vapor in the air
• Can be expressed as:
– Density
– Pressure
– Amount (%)
Measuring Water Vapor in the
Atmosphere
1) Vapor Pressure - part of the total atmospheric
pressure due to water vapor.
Units of pressure - pascal (Pa) or millibar (mb)
increases with
temperature
abundance
Saturation vapor pressure - maximum vapor pressure
that can exist in the atmosphere at a given
temprature.
Not widely used in weather reports.
Mostly used in scientific applications
Measuring Water Vapor in the
Atmosphere
2) Absolute Humidity - density of water
vapor in the air
Expressed as: gwater vapor / m3air
(grams of water vapor per cubic meter of air)
Measuring Water Vapor in the
Atmosphere
3) Specific Humidity - mass of water vapor per unit
mass of air
Expressed as: gwater vapor / Kgair
(grams of water vapor per kilograms of air)
Specific humidity = Mass of water vapor
Mass of air
Saturation Specific humidity - maximum
specific humidity the air can have before it
will condense.
Measuring Water Vapor in the
Atmosphere
4) Mixing Ratio - mass of water per unit mass of
dry air
Expressed as: gwater vapor / Kgdry air
(grams of water vapor per kilograms of dry air)
Mixing Ratio = Mass of water vapor
Mass of dry air
Saturation Mixing Ratio - maximum ratio the
air can have before it will condense.
Measuring Water Vapor in the
Atmosphere
5) Relative Humidity - measure of the amount of
water vapor in the air relative to the maximum
amount the air can hold.
Most familiar method - used in weather reports
Relative humidity = Specific Humidity
X 100
Saturation Specific Humidity
Given as %
Measuring Water Vapor in the
Atmosphere
6) Dew Point - temperature to which the air needs to
be cooled to become saturated.
called dew point because dew forms
temperature of saturation
Important method used in weather reports and
by meteorologists.
The closer the dew point is to the air temperature, the
higher the relative humidity
Measuring Water Vapor in the
Atmosphere
6) Dew Point - temperature to which the air needs to be
cooled to become saturated.
Dew Point as Indicator of humidity
a) When the dew point is high there is abundant
moisture in the air.
b) Indicator of relative humidity
•
•
•
Dew point is much lower than the air temperature, RH is low.
Dew point is close to air temperature, RH is high.
Dew point equals air temperature and the air is saturated,
RH=100%.
Frost point - when the temperature is below 0 oC, the
term “frost point” is applied.
Measuring Humidity
1) Sling Pyschrometer - instrument used for
measuring humidity
– Consist of two thermometers
• Wet bulb
• Dry bulb
Wet bulb thermometer has a cotton wick wrapped
around the bulb that is saturated with water.
Dry measures air temperature
If the air is unsaturated, water evaporates from the wet bulb and
the temperature falls because latent heat is consumed to evaporate
the water from the cotton wick.
Measuring Humidity
Wet bulb depression - difference in
temperature between the dry and wet bulbs.
Depends on the moisture content (humidity)
of the air.
Measuring Humidity
2) Hair hygrometer - uses human hair attached
to a lever. Human hair will expand and
contract in response to humidity.
Hygrometer - any instrument used to measure
humidity.
Ways in which the air becomes
saturated
1. Adding water vapor to the air.
Example: rain drops evaporating produces precipitation
fog.
2. Mixing cold air with warm air.
Example: When warm moist air mixes with cool air, the
new air may be saturated. In steam fog cold air mixes
with warm moist air over a lake.
3. Lowering air temperature to the dew point.
Example: At night the air cools down. If it cools to the
dew point temperature the air will condense
producing dew or radiation fog.
Nucleation
Even of the air is saturated condensation may
not occur without the aid of solid particles.
• Effects of impurities (aerosols) on nucleation
• Homogeneous nucleation - water droplets
condense from chance collisions and bonding of
water molecules
• Heterogeneous nucleation - formation of water
droplets onto aerosols.
• Hygroscopic aerosols - particles in the
atmosphere that attract water and serve as nuclei.
Ice Nucleation
Even if the air temperature is below 0oC, it
may condense as water droplets and not ice.
• Supercooled liquids - water cooled to below the
ice point but existing as liquid.
0 to -10oC - supercooled liquids
Formation of ice at these temperatures requires ice nuclei.
Below -10 to -30oC - both ice crystals and
supercooled liquids may condense
Cloud Formation
Lowering the air temperature to the
dew point is an important mechanism
for cloud formation.
Adiabatic process - process in which
temperature changes but heat energy is
not added or removed.
Thermodynamics - science concerned
with the relationship between heat
energy and temperature.
First law of thermodynamics
Heat added to a system will result in some
combination of volume change (expansion of a
gas) and increase in its temperature.
∆ H = p . ∆V + Cv . ∆T
∆ H - Change in heat
p - pressure
∆V - Volume Change
Cv - Specific heat (energy needed to change the temperature of
one gram of a substance by 1oC.
∆ T - Temperature Change
For adiabatic systems ∆H = 0
∆ H = p . ∆V + Cv . ∆T
∆ H - Change in heat
p - pressure
∆V - Volume Change
Cv - Specific heat
∆ T - Temperature Change
0 = p . ∆V + Cv . ∆T
p . ∆V = - Cv . ∆T
For adiabatic systems gases cool as they
expand.
For adiabatic systems gases warm as they
contract.
As air rises, it encounters lower surrounding
pressure, it expands and therefore cools
adiabatically.
Adiabatic Lapse rate - rate at which air
cools as it rises.
dry adiabatic lapse rate (DALR) - rate at
which a rising of dry (unsaturated) air cools.
about 1oC/100m (5.5oF/1000 feet)
No heat is removed but rising air will cool 1oC for every 100m
that it rises into the atmosphere.
dry adiabatic lapse rate (DALR) - rate at which
a rising parcel of dry (unsaturated) air cools.
about 1oC/100m (5.5oF/1000 feet)
Lifting Condensation level (LCL) - the height at
which a rising parcel of air undergoes condensation
or deposition.
Saturated adiabatic lapse rate (SALR) - rate at
which a rising parcel of wet (saturated) air cools.
about 0.5oC/100m (3.3oF/1000 feet)
Environmental Lapse Rate (ELR) - rate at
temperature in the troposphere decreases with height.
Forms of Condensation
1) Dew - liquid condensation onto a surface.
Forms in the morning
Dew is most likely to form on cloudless and windless
nights.
2) Frost - similar to dew but saturation temperature is
below 0oC.
When the air is lowered to the frost point, the
water vapor in the air may go directly to ice
crystals.
Forms of Condensation
3) Frozen Dew - Differs from frost
Frozen dew is a continuous layer of ice.
Forms when liquid dew freezes
Frozen dew on roadways is called “black ice”
4) Fog - a cloud whose base is near the surface. Can
be a meter to tens of meters high.
Forms of Condensation
Types of fog:
(a) Precipitation fog - results from evaporation of
falling raindrops.
(b) Steam Fog - forms when cold air mixes with
warm moist air above a water surface.
Forms of Condensation
Types of fog:
(c) radiation fog (ground fog) - results from loss of
longwave radiation and cooling of the surface at night.
Forms on cloudless nights with gentle winds
(d) advection fog - forms when relatively warm moist air
moves horizontally over a cold surface. As the warm air
moves over the surface it transfers heat downward causing
it to cool. If it cools to the dew point, fog will form.
Examples: San Francisco Bay area - Warm
Pacific air moves over the cool California Ocean current.
London in winter - Warm air
from the Gulf Stream moves over England where it is
chilled to the dew point.
Forms of Condensation
Types of fog:
(e) Upslope Fog - formed by adiabatic cooling of
air as it moves up a slope.