Relative humidity - Blue Valley Schools

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Transcript Relative humidity - Blue Valley Schools

+
Moisture and Stability
Chapter 4
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The Hydrologic Cycle
 Hydrologic
Cycle: the circulation of Earth’s water
supply
 The
cycle illustrates the continuous movement of water
from:



The oceans to the atmosphere
The atmosphere to the land
The land back to the sea
+ The Water Cycle
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The Importance of Water
 Water Vapor:
gas that can change from one state of
matter (solid, liquid or gas) to another at the
temperatures and pressures experienced on Earth
 The
processes that change that state of matter of
water include:






Evaporation (liquid to gas)
Condensation (gas to liquid)
Melting (solid to liquid)
Freezing (liquid to solid)
Sublimation (solid to gas)
Deposition (gas to solid)
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States of Matter
 Latent
Heat: as water changes from one state to
another latent heat is either released or absorbed
 Calories: measurement
of heat energy involved in
the change of state of water


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Melting: Heat absorbed (80 calories)
Freezing: Heat released (80 calories)
Evaporation: Heat absorbed (600 calories)
Condensation: Heat released (600 calories)
Sublimation: Heat absorbed (680 calories)
Deposition: Heat released (680 calories)
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Humidity
 The
general term used to describe the amount of
water vapor in the air
 Methods




to express humidity
Absolute humidity: mass of water vapor in a given volume of
air
Vapor pressure: pressure attributable to water vapor content
Relative humidity: ratio of the air’s actual water vapor content
compared with the amount of water vapor required for
saturation at a specific temperature
Dew Point: temperature needed in order for air to reach
saturation.
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Relative Humidity
 RH: relates
the amount of water vapor in the air to the
maximum possible at the current temperature
 More
water vapor exists in warm air than in cold air
 RH
= (specific humidity/saturation specific humidity)
x 100%
 RH
depends on both moisture content and the air
temperature

Adding moisture to the air while keeping the temperature constant
increases the relative humidity

Removing moisture lowers the relative humidity
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Water Vapor Content Changes
 When
water vapor content of air remains at a constant
level, a decrease in air temperature results in an
increase in relative humidity and an increase in
temperature causes a decrease in relative humidity
 In
nature, there are three major ways that air
temperature changes to cause changes in RH


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Daily changes in temperature
Temperature changes that result as air moves horizontally from
one location to another
Changes caused as air moves vertically in the atmosphere.
+ Relative Humidity changes
throughout the day
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Relative Humidity

In (a), the temperature is 14oC and
has a saturation specific humidity of
10g of water vapor per kilogram of
air. If the actual specific humidity is
6g the RH is 60%

In (b) the specific humidity is still
6g, but the higher temperature of
25C results in a greater saturation
specific humidity.

The RH is less in (a) even though the
amount of water vapor is the same
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Relative Humidity
+
Relative Humidity
Relative
Humidity
Practice
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
If a parcel of air at 25oC contains 10
grams of water vapor per kilogram
of air, what is its relative humidity?

Step One: determine how much
water 25oC can hold. (check
chart on page 105)


20 grams
Step Two: take the amount of
water vapor in the air and divide
it by the amount it can hold

10g/20g = 50% relative
humidity
Temp oC (oF)
Saturation Mixing
Ratio
-40
0.1
-30
0.3
-20
0.75
-10
2
0
3.5
5
5
10
7
15
10
20
14
25
20
30
26.5
35
35
40
47
Relative
Humidity
Practice
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
35oC
If a parcel of air at
contains 5
grams of water vapor per
kilogram of air, what is its relative
humidity?

Step One: determine how much
water 35oC can hold.


35 grams
Step Two: take the amount of water
vapor in the air and divide it by the
amount it can hold

5g/35g = 14% relative humidity
Temp oC (oF)
Saturation Mixing
Ratio
-40
0.1
-30
0.3
-20
0.75
-10
2
0
3.5
5
5
10
7
15
10
20
14
25
20
30
26.5
35
35
40
47
Temp oC (oF)
Saturation Mixing
Ratio
-40
0.1
-30
0.3
-20
0.75
-10
2
0
3.5
5
5
10
7
15
10
20
14
25
20
30
26.5
35
35
40
47
Relative
Humidity
Practice
+

If a parcel of air at 15oC
contains 5 grams of water
vapor per kilogram of air, what
is its relative humidity?


Step One: determine how much
water 15oC can hold.
Step Two: take the amount of
water vapor in the air and divide
it by the amount it can hold
Temp oC (oF)
Saturation Mixing
Ratio
-40
0.1
-30
0.3
-20
0.75
-10
2
0
3.5
5
5
10
7
15
10
20
14
25
20
30
26.5
35
35
40
47
Relative
Humidity
Practice
+

If a parcel of air at 10oC
contains 5 grams of water
vapor per kilogram of air, what
is its relative humidity?


Step One: determine how much
water 10oC can hold.
Step Two: take the amount of
water vapor in the air and divide
it by the amount it can hold
Temp oC (oF)
Saturation Mixing
Ratio
-40
0.1
-30
0.3
-20
0.75
-10
2
0
3.5
5
5
10
7
15
10
20
14
25
20
30
26.5
35
35
40
47
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Water Saturation
 When
air is saturated the pressure exerted by the water
vapor increased.
 Saturation
produces a balance between the number of
water molecules leaving the surface of the water and
the number of returning
 Because
the saturation vapor pressure is temperature
dependent, at high temperatures more water vapor is
required for saturation to occur.
+ Saturation vs. Temperature.
+
Dew Point

The temperature to which the air must be cooled to become
saturated

An expression of water vapor content

The closer the air temperature is to dew point indicates lots of
water in the air…humid day
Where in the US is it most
humid?
Where is it most dry?
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Measuring Humidity
 Sling
Psychrometer: instrument for measuring
humidity


Wet Bulb Depression: depends on the moisture of the air
Dry Bulb Depression: ambient air temperature
The value corresponding to the row for the dry bulb temperature and the
column for the wet bulb depression yields the dew point temperature.
The value corresponding to the row for the dry bulb temperature and the
column for the wet bulb depression yields the relative humidity.
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Measuring Humidity
 Amount
of coolness that takes place is directly
proportional to the dryness in the air
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Dryer the air, the greater the cooling
Larger the difference between the wet and dry bulb the
lower the humidity; the smaller the difference the higher
the humidity
If the air is saturated, no evaporation will occur and the 2
thermometers will have identical readings
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Adiabatic Temperature Change

As air is heated it expands becoming less dense, and as a
result, lighter.

Because it is lighter, it rises upwards above the cooler air.
As it does so, this air continues to expand.


This is because there is less pressure higher in the atmosphere,
allowing the air molecules to spread out more.

Temperature changes but no heat is added to or removed from a
substance are said to be adiabatic
Condensation occurs when water vapor is cooled enough to
change to a liquid – produces dew, fog, or clouds
+
Adiabatic Temperature Change
 Dry
Adiabatic Lapse Rate (DALR): The rate at which
a rising parcel of of unsaturated air cools

1.0oC/100m (5.5oF/1000ft)
 Saturated
Adiabatic Lapse Rate (SALR): the rate at
which saturated air cools

0.5oC/100m (3.3oF/1000ft)
 Lifting
Condensation Level (LCL): the altitude at
which a parcel of air cools sufficiently and
condensation/deposition commences.
+
Dry Adiabatic Lapse Rate
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Processes that Lift Air
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Air resists vertical movement.

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Air near the surface stays near the surface, air up high tend to
remain up high
Four mechanisms that cause air to rise

Orographic Lifting

Frontal Wedging
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Convergence
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Localized convection lifting
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
Orographic Lifting
Occurs when elevated terrains such as mountains act as barriers
to the flow of air

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As air ascends, adiabatic cooling often generates clouds and
precipitation
Rain Shadow Desert: as air reaches the leeward side of the mountain,
much of the moisture has been lost. If the air descends, it warms
adiabatically making condensation and precipitation unlikely
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Orographic Lifting
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Orographic Lifting
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Rainshadow
Effect
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Rainshadow Effect
California’s Elevation, Great Basin
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Frontal Wedging

Masses of warm and cold air collide to produce fronts
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The cooler, denser air acts as a barrier over which the
warmer less dense air rises (this is frontal wedging)
Mid-latitude cyclones
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Frontal Wedging
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Convergence
Whenever air in the lower
troposphere flows
together, lifting happens.

When air flows in from
more than one direction, it
has to go somewhere
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Convergence
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Localized Convection Lifting

Unequal heating of Earth’s surface may cause pockets of
air to be warmed more than surrounding air.
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Thermals: pockets of rising hot air
Warm parcels of air rise above the lifting condensation level,
clouds form, which can bring about mid-afternoon showers.
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Convection lifting
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Atmospheric Stability
Chapter 4
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Critical Weathermaker

Stable Air: A parcel of air that is cooler than its surrounding
environment, it would be more dense; and it would sink to its
original position

Resists vertical movement
+

Critical Weathermaker
Unstable Air: A parcel of air that
is warmer than its surrounding
environment, it would be less
dense; and it would rise until its
temperature was the same as its
surroundings
+
Critical Weathermaker

Stability is a property of air that describes its tendency to
remain in its original position (stable) or to rise (unstable)
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Absolute Stability

When environmental lapse rate is less than the wet adiabatic

Even if this stable air were forced above the condensation level, it
would remain cooler and denser than its environment and would
have a tendency to return to the surface
+
+ Absolute Stability

Temperature Inversion: the most
stable conditions occur when
the temperature in a layer of air
actually increases with altitude
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Absolute Instability

The environmental lapse rate is greater than the dry
adiabatic rate

Occurs most often during the warmest months and on clear days
when solar heating is intense
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Conditional Instability

Moist air has an environmental lapse rate between the dry
and wet adiabatic rate

The atmosphere is conditionally unstable when it is stable with
respect to an unsaturated parcel of air, but unstable with respect
to a saturated parcel of air

Depends on whether or not the rising air is saturated
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Daily
Weather
+

When stable air is forced aloft, the clouds that form are
widespread and have little vertical thickness


Light to moderate precipitation
Clouds associated with unstable air are towering and
are usually accompanied by heavy precipitation
+
Daily Weather

Any factor that causes air near the surface to become
warmed in relation to the air aloft increases the air’s
instability

Any factor that causes the surface air to be chilled results in
the air becoming more stable