ACTIVITY #10: What is Relative Humidity?
One of the most deadly weather disasters in
New England history took place in the month of July,
1911. The temperature remained in the high 90’s and
low 100’s for over 15 days. The humidity soared as
well and ultimately the combination of temperature
and humidity caused 2,000 deaths during that month.
A similar heat wave killed 700 people during July 1995
in Chicago. More than 20,000 deaths have occurred
due to heat waves since the 1930’s.
 What is relative humidity?
 Does temperature and dew point temperature
have anything to do with relative humidity?
 Does the amount of moisture the air can hold
vary due to temperature?
 How do we calculate relative humidity?
 What instruments do we use to find relative
humidity?
GOALS:

In this lab activity, you will …
Observe, measure, and predict changes in weather using atmospheric
properties (wind speed and direction, cloud cover and type, temperature,
dew point, air pressure, and relative humidity).
ACTIVITY OVERVIEW:
A synopsis of this lesson is as follows…
The purpose of this activity is for students to calculate relative humidity
from dew point and temperature measurements. In Part 1, students will first
observe a graph showing the relationship between temperature and the amount
of water vapor a fixed volume of air can hold. This data will be used to discuss
how different parts of the world may have differences in relative humidity at
various seasons. Using the graph, students will next be given a sample
temperature and dew point and will calculate the relative humidity. Following the
sample relative humidity, the present day temperature and dew point will be used
to calculate the relative humidity. This can be compared to the recorded relative
humidity using a weather instrument or the internet data.
In Part B, students use a sling psychrometer to measure relative humidity.
The use of a wet and dry bulb thermometer to calculate relative humidity will be
conducted.
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CONNECTIONS
Scientific Content  Relative humidity is a measure of the amount of water vapor present in the
air compared to the amount needed for saturation at a specific
temperature.
 The amount of water vapor present in the air is called humidity.
 Air doesn’t always contain the same amount of water vapor.
 Particles of water vapor move faster in warm air and slower in cooler air.
 The slower the movement of the water vapor particles, the more particles
that stick together and form droplets of liquid water.
 When liquid water is formed from water vapor, condensation has occurred.
 If enough water vapor is present in the air to allow for condensation to
occur, the air said to be saturated.
 Relative Humidity is: The amount of water vapor air actually holds (wet bulb) (dew point temp.) x 100

Amount of water vapor air can hold at that temp.(present air temp) (dry bulb)
A sling pyschrometer and/or hygrometer may be used to measure relative
humidity.
Scientific Process –
 Students will be observing, measuring and calculating relative humidity.
Math/Graphing – Students will use mathematics in this exercise including
reading graphs, calculating relative humidity and percentages.
MAKING SENSE OF RELATIVE HUMIDITY…
Relative humidity is a measure of the amount of water vapor present in the
air compared to the amount of water vapor the air could hold at that particular
moment. Temperature can make a difference in the amount of water vapor the
air can hold. Relative humidity will increase if more water vapor collects in the
atmosphere, or if the temperature is lowered. If the amount of water vapor
remains constant, the humidity is dependent on temperature and any changes in
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temperature. If the temperature is lowered, the humidity increases and if the
temperature rises, the humidity is decreased.
The air does not always have the same amount of water vapor. More
water vapor can be present if the air is warm than when the air is cool. Warmer
temperatures cause water vapor particles to move faster and they do not easily
come together. At cooler temperatures, the particles move slower and begin to
stick together to form water droplets (condensation). If enough water vapor is
present in the air for condensation to take place, the air is saturated. If you hear
that the relative humidity is 60%, it means the air contains 60 percent of the
water needed for the air to be saturated. When saturation is reached, the
temperature and dew point are equal, and the relative humidity becomes 100%.
Therefore, increases in water vapor, or decreases in temperature, may result in
condensation.
The chart below shows the amount of water needed (grams of water vapor
per kilogram of dry air) for the air to become saturated at a given temperature.
Saturation Amounts Present at Different Temperatures
Amount of water vapor
present in air in grams/kg
Amount of water vapor
40
35
30
25
Amount of water
vapor
20
15
10
5
0
35 30 25 20 15 10
5
0
Temperature degrees C
It is evident that as temperature increases, the amount of water vapor necessary
for saturation also increases. The chart above helps to explain what relative
humidity means. The chart shows that 25 degree C air needs 20.44 grams of
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water vapor per kilogram of dry air to be saturated. If the air actually holds 10.5
grams of water vapor, the relative humidity can be calculated using this
information. Divide the actual water vapor in the air (10.5 g) by the amount the air
can hold at that temperature (20.44 g) and multiply by 100. The air has 51% of
the water vapor it could hold at its current temperature. The relative humidity is
51%.
Temperature ( º C)
0
Amount of Water Vapor
(grams per cubic meter)
3.5
5
5
10
7
15
10
20
14
25
20
30
26.5
35
35
40
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The psychrometer is used to determine relative humidity. The relative
humidity is the percent of water vapor in the air at a given temperature as
compared with the amount it could hold if it were saturated. For example, air with
a 50% relative humidity in just "half full" of water vapor. The psychrometer
consists of a dry and wet bulb thermometer placed side-by-side. The relative
humidity is determined by the difference between the dry and the wet
thermometer. In order to accurately determine the relative humidity, you will need
to use the relative humidity table.
Let’s Investigate …
Part 1: How Can Relative Humidity be Measured?
Part 1: How can humidity be measured?
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Look at the graph below.
1. What is the relationship between temperature and the amount of water vapor a
fixed volume of air can hold?
2. According to the graph can artic air hold more water vapor in July or February?
Why?
Perform the following procedure.
3. Use the graph to find the amount of water vapor the air can hold at 25
degrees Celsius.
4. Use the same graph to find the amount of water vapor the air actually holds if
the dew point is 15 degrees Celsius.
5. Find the ratio of the amount of water vapor the air can hold to the amount the
air actually holds and multiply this number by 100 to convert the number to a
percent.
The percentage just calculated is the relative humidity of the air.
Use today’s temperature and dew point and use the above graph to help
calculate relative humidity.
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6. What was your calculated Relative Humidity?
7. What is the reported Relative Humidity?
8. What would be the relative humidity if arctic air in July is 18 degrees Celsius
and the dew point is still 15 degrees Celsius?
9. What happens to the relative humidity if the temperature decreases and the
dew point stays the same? Why?
Part 2: What instrument can we use to measure relative humidity?
Use the sling psychrometer as directed by your teacher to measure the relative
humidity in your classroom.
1. What is the dry bulb reading? ___________
2. What is the wet bulb reading? ___________
3. Why is the wet bulb cooler?
4. What is the difference between the wet and the dry bulb readings?
Use the chart on the following page to find the relative humidity.
5. What is the relative humidity in your classroom?
Repeat this activity at another location. Your teacher may have you go outside to
take these measurements.
6. What is the wet bulb reading?
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7. What is the dry bulb reading?
8. What is the difference between the wet and the dry bulb readings?
9. What is the relative humidity outside your classroom?
9. Does the classroom or the outdoors have a higher relative
humidity and why there are differences?
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Find the dry bulb temperature in the left column (e.g. 20). Next
subtract the wet bulb reading from the dry bulb reading to find the
temperature difference (e.g. 20-18 = 2). Locate the number that
lies in the block intersecting the dry bulb reading and the dry-wet
bulb difference (e.g. 82). This number represents the relative
humidity
.
Investigating Further …
When the temperature is low and humidity high, most people
do not seem terribly uncomfortable. However, on a hot and
humid day, people are quite uncomfortable. Comfort level
appears to be a result of both temperature and humidity, with the
moisture content of the air playing a large role. The dew point is also an excellent
indicator of comfort. The dew point reflects the water vapor content in the air, and that
water vapor content affects the natural cooling mechanism of our bodies called sweating.
A gland in our hypothalamus tries to regulate our body temperature by setting off
millions of sweat glands when it becomes very hot. When the sweat evaporates, we cool
down because evaporation always removes heat. Why does alcohol cool your skin? In
very hot weather, a loss of water and salt may cause a chemical imbalance leading to heat
cramps. If the heat persists and the water loss continues heat exhaustion may occur.
What are some symptoms and possible results of heat exhaustion? What is heat
stroke?
Summary of Activity …
In your journal or notebook, write a concise summary of this activity.
Be sure to address the following questions and use your data to support your responses.





What is relative humidity?
Explain how temperature and dew point temperature influence relative humidity?
Explain how the amounts of moisture the air can hold vary due to temperature?
How do we calculate relative humidity?
How do we use a sling pyschrometer to measure relative humidity?
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Applying what you have learned …
Lisa, an 8th grade student, began studying evaporation and condensation in
Science class on Tuesday.
1. How could Lisa explain the difference between evaporation and
condensation to her friend Bruce? Bruce missed school on Tuesday and
needs help with these two terms.
2. Use the table below to answer the following questions: On Tuesday, the
temperature was 35 degrees Celsius. The temperature dropped 10
degrees on Wednesday. Use this information to answer the following
questions. The amount of water vapor in a cubic meter of air was 21
grams on Tuesday and Wednesday.
Temperature (degrees
Celsius)
15
25
35
Amount of Water Vapor Needed to
Saturate a Cubic Meter of Air (grams)
14
22
37
a. Is more water vapor possible in cooler or warmer air?
b. The air was almost saturated with water vapor on which day?
c. Lisa and Bruce calculated the relative humidity for Tuesday and
Wednesday. The formula is:
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Relative Humidity= The amount of water vapor actually in the air
x
The amount of water vapor that air can hold at that
Temperature
d. What was the humidity on Tuesday?
e. What was the humidity on Wednesday?
f. On which day would Lisa be the most comfortable (due to humidity)
playing field hockey?
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