# GEOG 1200 SMU Module 2: HUMIDITY Module 2:HUMIDITY Objectives To study the concept of humidity.To understand use of the saturation curve graph.Learn about

GEOG 1200 SMU Module 2: HUMIDITY Module 2:HUMIDITY Objectives To study the concept of humidity.To understand use of the saturation curve graph.Learn about how relative humidity is measured using a sling psychrometer. SAINT MARY’S UNIVERSITY
GEOG 1200
DEPARTMENT OF GEOGRAPHY
FUNDAMENTALS OF PHYSICAL GEOGRAPHY
Module 2: HUMIDITY
Objectives
1.
To study the concept of humidity.
2.
To understand use of the saturation curve graph.
3.
Learn about how relative humidity is measured using a sling psychrometer.
Section 1: Understanding Humidity
Terminology
Air can hold up to a certain amount of water vapour (water in a gaseous state) but the amount varies depending on the
temperature. Humidity is a general term that refers to the amount of moisture in air. Some other important terms to
know when dealing with moisture in the atmosphere are:
Specific Humidity (SH): the actual quantity of water vapour in the air, in grams per kilogram (g/kg).
Maximum Specific Humidity (MSH): the maximum quantity of water vapour that could be held in the air at a given
temperature (g/kg). If the air is unsaturated, the SH is less than the MSH.
Relative Humidity (RH): the ratio of SH to MSH, expressed as a percentage:
Equation 1:
Specific Humidity
RH (%) = ———————————-Maximum Specific Humidity
x
100
Dew-Point Temperature (DT): the temperature at which air saturation and condensation occur for a given value of
specific humidity. Condensation is the change of water from a gaseous state to a liquid state.
Saturation Curve: a graph (on a separate sheet) showing the relationship between air saturation and temperature.
Once the saturation point is reached, the RH is 100% and no more water vapour can be evaporated into the air.
Example
For reference, an example using the Saturation Curve Graph is given. Follow the example of how to correctly read the
graph.
a.
A sample of air is collected and determined to lie at Point X on the graph.
b.
The air temperature is 30C.
c.
The SH is 10.0g/kg (grams of water vapour per kilogram of air).
d.
What is the MSH?
e.
What is the RH, rounded to the nearest %?
f.
What is the DT?
1
Air Temperature
(C)
X
30
Specific Humidity
(g / kg)
10
Maximum Spec.
Humidity
(g / kg)
Relative
Humidity
(%)
Dew-Point
Temperature (C)
27.5
36
12.5
2
Section 2: Using the Saturation Curve Graph

Recall from the previous section that relative humidity is the ratio between specific humidity and maximum specific
humidity expressed as a percentage.

It is possible for the specific humidity to be lower than or equal to, but not higher than, the maximum specific
humidity. A point on the graph can only lie on or below the line.

A series of values will be used to demonstrate the relationships between temperature and humidity of air.

In this example, the temperature of the air changes, forcing changes in the relative humidity values. A starting
point was selected with an air temperature of 20C and a specific humidity of 10 g / kg.
Air Temperature
(C)
Specific Humidity
(g / kg)
A
20
10
B
13
10
C
0
4
D
10
4
Maximum Spec.
Humidity
(g / kg)
Relative
Humidity
(%)
Dew-Point
Temperature (C)
Use the Saturation Curve Graph to complete the table (above or attached).
Section 3: Measuring Relative Humidity with a Sling Psychrometer
Background

A device for measuring relative humidity is called a sling psychrometer.

The paragraphs below describe the sling psychrometer and its operation – For the on-line course you will not be
using the instrument, however you will learn about it .
The Sling Psychrometer

The sling psychrometer contains two thermometers housed in a plastic casing attached to a handle.

The casing is designed to be spun around the handle, vigourously.

The upper thermometer is a standard mercury thermometer with a Celcius temperature scale – this is the dry-bulb
thermometer it measures the air temperature.

The lower thermometer is similar, except it is covered by a wick at the far end- this is the wet-bulb thermometer. If
the wick is dry, unscrew the plastic cap and add water to the reservoir.

The difference between the dry bulb and wet bulb temperatures is called the wet-bulb depression (or depression of
the wet bulb).
How the Sling Psychrometer Works

When the psychrometer is spun, evaporation causes the wet-bulb temperature to be lowered.
3

The amount of evaporation from the wick is related to the relative humidity (RH). The amount of evaporation is
determined by the amount of water vapour already in the air (the specific humidity) compared to the maximum
amount of water vapour that can be held at that temperature (maximum specific humidity).

If the RH is high, there will be relatively little evaporation and the dry and wet bulb temperatures will be close to
each other.

If the RH is low, there will be relatively more evaporation and the dry and wet bulb temperatures will be further
apart.
Using the Psychrometer

To use, spin the psychrometer vigourously for 30-40 seconds.

Read off the two temperatures to the nearest half degree and calculate the wet-bulb depression.

On a separate sheet is a chart that will tell you the relative humidity for the air temperature you measure (dry-bulb
temperature) and the corresponding wet-bulb depression.

Read the chart down to the dry-bulb temperature you recorded, and across to the wet-bulb depression to obtain
the relative humidity value.

For example, if you measured a dry-bulb temperature of 34C and a wet-bulb temperature of 24.5C, the wet-bulb
depression is 9.5C and the relative humidity is 46%. Use the chart to confirm how this value of relative humidity is
obtained.

On the chart, if the exact dry-bulb temperature you recorded is missing, you must interpolate between given
values.
Before Moving On

It is essential to understand the information provided to this point before moving on. Three points that
frequently require emphasis or clarification are:
o
Dry-bulb temperature on the psychrometer measures the air temperature.
o
Wet-bulb temperature is not the same as wet-bulb depression.
o
Wet bulb depression is the difference between dry-bulb and wet-bulb temperatures. (That is, how much
lower, or “depressed”, is the temperature of the wet-bulb compared to the dry-bulb?)
Section 4: Practice Exercises Using Given Sling Psychrometer Values
Work out the relative humidity assuming the following temperatures were measured off a sling psychrometer:
Dry-Bulb Temperature
(C)
Wet-Bulb Temperature
(C)
30
26
5
2
45
27.5
Wet-Bulb Depression
(C)
Relative Humidity
(%)
4
9.5
4.5
4
Section 5: Relative Humidity Measurements

I completed these measures for the class using the sling psychrometer to measure the relative humidity in three
locations on the campus:
1.
In room B205
2.
In the lobby of the Atrium, near the entrance to the library
3.
Outside, well away from any buildings or building entrances

Spaces are provided below to about the humidity conditions (using the relative humidity results and the saturation
curve graph).

Equation 1 – used to calculate RH when SH and MSH are known – can be rearranged into Equation 1a to calculate
SH, if RH and MSH are known.
Equation 1:
RH = (SH / MSH) x 100
Equation 1a:
SH = (RH / 100) x MSH
Make sure you include appropriate units for all values on p. 4 and 5.
Location 1: In Room B205
Dry-Bulb Temperature
_20.5 C ______________ Wet-Bulb Temperature
16.0 C _______
Location 2: In the lobby of the Atrium
Dry-Bulb Temperature
_19.5 C
Wet-Bulb Temperature
__18.5 C ______________
Location 3: Outside, away from buildings
Dry-Bulb Temperature
6 C ________________
Wet-Bulb Temperature
__2 C ______________
Location 1: In Room B205
Wet-Bulb Depression
Relative Humidity
________________
________________
Maximum specific humidity for this air temperature
________________
Specific humidity (SH = [RH / 100] x MSH)
________________
Dew-point temperature
________________
Location 2: In the Atrium, on the second floor beside the green living wall
Wet-Bulb Depression
Relative Humidity
________________
________________
Maximum specific humidity for this air temperature
________________
Specific humidity (SH = [RH / 100] x MSH)
________________
Dew-point temperature
________________
5
Location 3: Outside, away from buildings
Wet-Bulb Depression
Relative Humidity
________________
________________
Maximum specific humidity for this air temperature
________________
Specific humidity (SH = [RH / 100] x MSH)
________________
Dew-point temperature
________________
For each of the three locations, plot your values of specific humidity vs. air temperature on the Saturation Curve graph.

At which location is the relative humidity highest?
________________

Which location has the greatest amount of water vapour in the air?
________________

Is it possible for the highest relative humidity and the greatest amount of water vapour in the air to have occurred
at different locations?
6
Visualizing Physical Geography
by Timothy Foresman & Alan Strahler
Chapter 3
Air Temperature
Visualizing Physical Geography
Chapter Overview
Temperature and Heat Flow
Processes
Daily and Annual Cycles of Air
Temperature
Local Effects on Air Temperature
Guess whether the
World Patterns of Air Temperature eruption of Mt.
cooling or warming
The Temperature Record and
effect?
Global Warming
Visualizing Physical Geography
Temperature and Heat Flow Process
Measuring Temperature
• Temperature = level of internal
motion of atoms and molecules
that make up the matter
• Temperature scales
• Fahrenheit
• Celsius
• Kelvin
What is the freezing and boiling
point of water in oF and oC?
© John Wiley & Sons, Inc.
Visualizing Physical Geography
Temperature and Heat Flow Process
Surface Temperature

Air temperature is measured at 1.2 m (4 feet).
Daytime ground temperatures are usually warmer than 1.2 m.
Night ground temperatures tend to be cooler than at 1.2 m.
Ground temperatures are often more extreme than air
temperatures.
Visualizing Physical Geography
Temperature and Heat Flow Process
Wind Chill and Heat Index
• Wind chill index = the higher the wind speed, the faster
the rate at which heat leaves our bodies, and the colder
we feel.
If the air
temperature is
0o F and the
wind speed is
30 mph, what
is the wind
chill?
Visualizing Physical Geography
Temperature and Heat Flow Process
Wind Chill and Heat Index
• Heat index:
• Higher levels of humidity raise our perception of heat.
• Humid conditions reduce the amount of evaporative
cooling when we sweat.
If the relative humidity is
85%, at what temperature
should someone
exercising outdoors use
“extreme caution”?
Visualizing Physical Geography
Temperature and Heat Flow Process
Energy Transfer
• Heat = internal energy transferred from one substance to
another as a result of their temp differences.
• Heat flows by:
• Radiation = all objects emit heat (e.g., SW and LW).
• Conduction = transfer by particles (atoms or molecules).
• Convection = in gases or liquids (e.g., warm air rises).
• Advection = a mass of air moves to a new location,
bringing along its properties (temperature and moisture).
Visualizing Physical Geography
Temperature and Heat Flow Process
Energy Transfer
• Example:
• Sunlight hits earth via SW
• SW is absorbed by ground,
raising its temperature.
(LW) energy to the air, and
heats the soil below it
through conduction.
© John Wiley & Sons, Inc.
Where is convection
occurring in this image?
Visualizing Physical Geography
Temperature and Heat Flow Process
Latent Heat
• Sensible heat = flow of heat that results in a temperature
change of an object or its surroundings.
• Latent heat = flow of heat taken or released when a
substance changes states (solid, liquid, or gas) to another.
• Important energy transfer in the atmosphere/ocean:
• Water evaporating into water vapor is a cooling process as
heat is absorbed into the water vapor.
• The latent heat stored in water vapor is released in the
condensation process.
• Hurricanes are fueled by warm water and this process.
Visualizing Physical Geography
Daily and Annual Cycles of Air
Temperature
Four important controls of air temperature:
• Time of day
• Season
• Surface type (continental
or maritime)
• Latitude
Other local factors involved in determining temperature (see
the next section) include:
• Elevation
• Land use and urbanization
• Ocean currents
Visualizing Physical Geography
Daily and Annual Cycles of Air
Temperature
The Daily Cycle of Air Temperature
• Positive after sunrise
• Peaks at noon
• Decreases to negative by
sunset
What time of day would
you expect the high
temperature?
Visualizing Physical Geography
© John Wiley & Sons, Inc.
Daily and Annual Cycles of Air
Temperature
The Daily Cycle of Air Temperature
•Air temperature varies daily:
• Minimum is just after sunrise.
• Rises to a peak in mid-afternoon.
• Even after noon, incoming
continues to increase.
• Temperatures begin to decrease
Visualizing Physical Geography
© John Wiley & Sons, Inc.
Daily and Annual Cycles of Air
Temperature
Temperatures Close to
the Ground
• Temperatures on the
ground are usually more
extreme than
temperatures at standard
height.
• Soil, surface, and air
temperatures vary
throughout the day.
© John Wiley & Sons, Inc.
Visualizing Physical Geography
Daily and Annual Cycles of Air
Temperature
Annual Cycles of Insolation and Air Temperature
•Insolation varies by season:
• Day length longest at summer solstice = warmer temp.
• Day length shortest at winter solstice = colder temp.
© John Wiley & Sons, Inc.
© John Wiley & Sons, Inc.
Visualizing Physical Geography
Daily and Annual Cycles of Air
Temperature
Goddard Institute for Space Studies Surface
Temperature Analysis
• Calculating the Earth’s surface temperature from ground, air,
and satellite measurements is extremely complex.
• http://data.giss.nasa.gov/gistemp/
Courtesy NASA
Visualizing Physical Geography
Daily and Annual Cycles of Air
Temperature
Land and Water Contrasts
• Specific heat = amount of heat
required to raise the
temperature of a unit mass of a
substance by 1ºC.
• Rock and soil (inland areas)
have low specific heat, which
means less energy is needed to
raise the temperature.
© John Wiley & Sons, Inc.
Water has a much higher
specific heat capacity. An
extensive, deep body of water
heats more slowly and cools
more slowly than inland areas.
Visualizing Physical Geography
Daily and Annual Cycles of Air
Temperature
Inland climates have more
temperature extremes than
coastal climates:
1. Solar rays heat land
surface, but are distributed
deeper in water.
2. Water has higher heat
capacity than rock and
soil.
3. Water mixes.
© John Wiley & Sons, Inc.
4. Water evaporates,
removing latent heat.
Visualizing Physical Geography
Daily and Annual Cycles of Air
Temperature
Land and Water Contrasts
• Maritime = Coastal regions have smaller daily and annual
temperature ranges.
• Continental = Inland regions have greater daily and annual
temperature ranges.
© John Wiley and Sons Publishers Inc.
Visualizing Physical Geography
Daily and Annual Cycles of Air
Temperature
Temperature by Latitude
• Annual cycle of insolation affects→ net radiation, which
affects→ monthly mean air temperature.
• Higher latitudes experience large annual temperature range.
• Equatorial regions experience small annual temp ranges.
Courtesy David H. Miller
Courtesy David H. Miller
Visualizing Physical Geography
Daily and Annual Cycles of Air
Temperature
Temperature by Latitude
Courtesy David H. Miller
Is the annual temperature range for Manaus, Brazil,
small or large? Explain. In your explanation, describe the
latitude and annual net radiation for Manaus.
Visualizing Physical Geography
Daily and Annual Cycles of Air
Temperature
Temperature by Latitude
Courtesy David H. Miller
Is the annual temperature range for Yakutsk small or
large? Explain. In your explanation, describe the latitude
and annual net radiation for Yakutsk.
Visualizing Physical Geography
Daily and Annual Cycles of Air
Temperature
Temperature by Latitude
Although Yakutsk is only 9.5° further north
than Hamburg, the annual temp cycles of
these two cities are quite different. While
summer temp are similar, winter temp at
Yakutsk average –45°C compared to just
Which is the best explanation for this
observation?
a. The elevation is much higher in Yakutsk.
b. The elevation is much higher in
Hamburg.
c. Winds bring air from the Arctic Ocean to
Yakutsk.
d. Winds bring air from the Atlantic Ocean
to Hamburg.
© John Wiley & Sons, Inc.
Visualizing Physical Geography
Local Effects on Air Temperature
Local factors involved in determining temperature:
• Elevation
• Land use and urbanization
• Ocean currents
Microclimates = Local atmospheric zones where the climate
differs from surrounding areas
Visualizing Physical Geography
Local Effects on Air Temperature
Effects of Elevation on Temperature
• Temperature decreases with altitude in the troposphere,
then increases above the troposphere.
• Environmental temperature lapse rate = rate at which air
temperature drops with increasing due to pressure drop
and subsequently less carbon dioxide and water vapor to
absorb LW.
Visualizing Physical Geography
Local Effects on Air Temperature
Effects of Elevation on Temperature
• Temperature Structure of the Atmosphere
© John Wiley & Sons, Inc.
What gas absorbs ultraviolet radiation in the
stratosphere? How does this relate to the
temperature trend?
Visualizing Physical Geography
Local Effects on Air Temperature
Effects of Elevation on Temperature Variation
© John Wiley & Sons, Inc.
What happens to temperature as one increases in
elevation in the Andes mountains?
Visualizing Physical Geography