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1
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2
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- Over 70% of the planet is covered by water
- Water is unique in that it can simultaneously exist in all three states
(solid, liquid, gas) at the same temperature
- Water is able to shift between states very easily
- Important to global energy and water cycles
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3
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- On average, 1 meter of water is
evaporated from oceans to the atmosphere each year.
- The global averaged precipitation
is also about 1 meter per year.
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4
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- Earth’s surface lost heat to the atmosphere when water is evaporated
from oceans to the atmosphere.
- The evaporation of the 1m of water causes Earth’s surface to lost 83 watts per square meter,
almost half of the sunlight that reaches the surface.
- Without the evaporation process, the global surface temperature would be
67°C instead of the actual 15°C.
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5
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- Latent heat is the heat released or absorbed per unit mass when water
changes phase.
- Latent heating is an efficient way of transferring energy globally and
is an important energy source for Earth’s weather and climate.
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6
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- Evaporation: the process whereby molecules break free of the liquid
volume.
- Condensation: water vapor molecules randomly collide with the water
surface and bond with adjacent molecules.
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7
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- by mass
- by vapor pressure
- by temperature è
Dew Point Temperature
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8
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9
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- The air’s content of moisture can
be measured by the pressure exerted by the water vapor in the air.
- The total pressure inside an air parcel is equal to the sum of pressures
of the individual gases.
- In the left figure, the total pressure of the air parcel is equal to sum
of vapor pressure plus the pressures exerted by Nitrogen and Oxygen.
- High vapor pressure indicates large numbers of water vapor molecules.
- Unit of vapor pressure is usually in mb.
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- Saturation vapor pressure describes how much water vapor is needed to
make the air saturated at any given temperature.
- Saturation vapor pressure depends primarily on the air temperature in
the following way:
- è
- Saturation pressure increases exponentially
with air temperature.
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11
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- Specific Humidity: How many grams of water vapor in one kilogram of air
(in unit of gm/kg).
- Relative Humidity: The percentage of current moisture content to the
saturated moisture amount (in unit of %).
- Clouds form when the relative humidity reaches 100%.
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- Three ways:
- Increase (inject more) water vapor to the air (Aà B).
- Reduce the temperature of the air (A à C).
- Mix cold air with warm, moist air.
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13
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- Dew point temperature is another measurement of air moisture.
- Dew point temperature is defined as the temperature to which moist air
must be cool to become saturated without changing the pressure.
- The close the dew point temperature is to the air temperature, the
closer the air is to saturation.
- Dew points can be only equal or less than air temperatures.
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14
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- When air reaches saturation at
temperatures below freezing the term frost point is used.
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15
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- The easiest way to measure humidity is through use of a sling
psychrometer - A pair of thermometers one of which has a wetted cotton
wick attached to the bulb.
- The two thermometers measure the wet and dry bulb temperature.
- Swinging the psychrometer causes air to circulate about the bulbs.
- When air is unsaturated, evaporation occurs from the wet bulb which
cools the bulb.
- Once evaporation occurs, the wet bulb temperature stabilizes allowing
for comparison with the dry bulb temperature.
- The wet bulb depression is found with a greater depression indicative of
a dry atmosphere.
- Charts gauge the amount of atmospheric humidity.
- Aspirated and hair hygrometers are alternatives.
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16
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17
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18
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19
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20
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21
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- Saturation vapor pressure describes how much water vapor is needed to
make the air saturated at any given temperature.
- Saturation vapor pressure depends primarily on the air temperature in
the following way:
- è
- Saturation pressure increases exponentially
with air temperature.
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22
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23
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- Radiation Fog: radiation cooling è condensation è fog
- Advection fog: warm air advected
over a cold surface è fog
- Upslope fog: air rises over a
mountain barrier è air expands and cools è fog
- Evaporation fog: form over lake
when colder air moves over warmer water è steam fog
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24
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- If a material changes its state (pressure, volume, or temperature)
without any heat being added to it or withdrawn from it, the change is
said to be adiabatic.
- The adiabatic process often occurs when air rises or descends and is an
important process in the atmosphere.
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25
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- Involve the direct addition or
removal of heat energy.
- Example: Air passing over a cool surface loses energy through
conduction.
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26
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- Air pressure decreases with
elevation.
- If a helium balloon 1 m in
diameter is released at sea level, it expands as it floats upward
because of the pressure decrease. The balloon would be 6.7 m in diameter
as a height of 40 km.
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- Air molecules in the parcel (or
the balloon) have to use their kinetic energy to expand the
parcel/balloon.
- Therefore, the molecules lost energy and slow down their motions
- è The temperature of the air parcel (or balloon)
decreases with elevation. The lost energy is used to increase the
potential energy of air molecular.
- Similarly when the air parcel descends, the potential energy of air
molecular is converted back to kinetic energy.
- è Air temperature rises.
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28
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29
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30
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- Dry adiabatic lapse rate is
constant = 10ºC/km.
- Moist adiabatic lapse rate is NOT
a constant. It depends on the temperature of saturated air parcel.
- The higher the air temperature, the smaller the moist adiabatic lapse
rate.
- èWhen warm,
saturated air cools, it causes more condensation (and more latent heat
release) than for cold, saturated air.
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- Static stability is referred as to air’s susceptibility to uplift.
- The static stability of the atmosphere is related to the vertical
structure of atmospheric temperature.
- To determine the static stability, we need to compare the lapse rate of
the atmosphere (environmental lapse rate) and the dry (moist) adiabatic
lapse rate of an dry (moist) air parcel.
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33
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- The environmental lapse rate is referred to as the rate at which the air
temperature surrounding us would be changed if we were to climb upward
into the atmosphere.
- This rate varies from time to time and from place to place.
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- Ge = environmental
lapse rate
- Gd = dry adiabatic
lapse rate
- Gm = moist
adiabatic lapse rate
- Absolutely Stable
- Ge < Gm
- Absolutely Unstable
- Ge > Gd
- Conditionally Unstable
- Gm < Ge < Gd
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35
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36
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37
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38
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- At the end of a sunny day, warm air near the surface, cold air aloft.
- In the early morning, cold air near the surface, warm air aloft.
- The later condition is called “inversion”, which inhibits convection and
can cause sever pollution in the morning.
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39
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40
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- The potential temperature of an air parcel is defined as the temperature
the parcel would have if it were moved adiabatically from its existing
pressure and temperature to a standard pressure P0 (generally
taken as 1000mb).
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41
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- In the atmosphere, air parcel often moves around adiabatically.
Therefore, its potential temperature remains constant throughout the
whole process.
- Potential temperature is a conservative quantity for adiabatic process
in the atmosphere.
- Potential temperature is an extremely useful parameter in atmospheric
thermodynamics.
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42
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43
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44
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45
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46
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47
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48
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49
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Notes
