The purpose of this activity is to observe, document, and classify changes in visibility and sky color over time and to understand the relationship between sky color, visibility, and aerosols in the atmosphere. The intended outcome is that students become aware of the changes in visibility and sky color due to particles suspended in the air. Educational levels: Primary elementary, Intermediate elementary, Middle school, High school

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GLOBE® 2005 Observing Visibility and Sky Color Learning Activity - 1 Atmosphere
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Introduction
Protocols
Learning A
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!
?Observing Visibility and  
Sky Color
Purpose
To observe, document, and classify changes 
in visibility and sky color over time and to 
understand the relationship between sky color, 
visibility, and aerosols in the atmosphere
Student Outcomes
Students become aware of the changes in 
visibility and sky color due to particles 
suspended in the air.
Overview
Science Concepts
Earth and Space Science
The atmosphere is composed of different 
gases and aerosols.
Geography
Human activities can modify the physical 
environment, specifically air quality and 
the composition of the atmosphere.
Atmosphere Enrichment
Aerosols decrease the amount of solar 
energy reaching Earth’s surface. 
Aerosols in the atmosphere increase haze, 
decrease visibility, and affect air quality. 
Scientific Inquiry Abilities 
Identify answerable questions. 
Observe and describe sky conditions. 
Develop descriptions and explanations 
using evidence.
Recognize and analyze alternative 
explanations. 
Communicate procedures and 
explanations. 
Time
Initial observations: 20 minutes 
Continued observations: 10 minutes
Level
All
Frequency
Initial observations: for five to ten days, days with 
limited cloud covered preferred
Continued observations: throughout the year, days 
with limited cloud covered preferred
Materials and Tools
Colored pencils or water-colored paint and 
brushes
White paper
Optional: camera or paint sample cards 
(from a local paint store)
Visibility and Sky Color Data Sheet
Visibility and Sky Color Summary Chart
Scissors and tape
Preparation
None
Prerequisites
Cloud Cover Protocol
GLOBE® 2005 Observing Visibility and Sky Color Learning Activity - 2 Atmosphere
Background
Why is a clear sky blue? The atmosphere consists 
primarily of molecules of oxygen and nitrogen. 
Sunlight bounces off these molecules, a process 
called scattering. Light with shorter wavelengths, 
at the blue end of the visible light spectrum, is 
scattered more efficiently than longer wavelengths. 
To an observer on the ground, this scattered light 
fills the entire sky and a clear sky appears blue.
However, there are also liquid and solid particles 
called aerosols suspended in the atmosphere. 
When there are relatively few aerosols, the sky 
appears clear. For example, a distant building 
or mountain peak appears clearly defined, with 
colors similar to what you would see if you were 
much closer to that distant object. On a very clear 
day, you would report the sky color as blue or deep 
blue and the sky condition as clear or unusually 
clear. Aerosols come from natural sources such as 
condensation and freezing water vapor, volcanoes, 
dust storms, and salt crystals evaporated from sea 
spray. They also come from human activities such 
as burning fossil fuels and biomass (e.g., wood, 
dung, dried leaves) and plowing or digging up 
soil. Aerosols are much bigger than gas molecules 
(they range in size from about 10-6 m (1 micron) 
to 10-7 m) and they scatter light from all visible 
wavelengths. Individual aerosols are too small to 
be visible to the human eye, but their presence 
affects the sky’s appearance. As the aerosol 
concentration, and therefore scattering of sunlight, 
increases, the sky appears less blue. Haze is the 
visible effect of aerosols on the atmosphere; it is 
a qualitative condition you can observe. When 
aerosol concentrations are high, we say that the 
sky looks hazy. Aerosol concentrations can also be 
measured quantitatively. 
Hazy skies appear pale blue or almost white. 
Depending on the type of aerosols present in the 
atmosphere, the sky may also appear brownish 
or yellowish. Scattering of visible light through 
a hazy sky affects horizontal visibility, so distant 
objects appear less distinct, with washed-out or 
distorted colors. Distant objects that are visible 
on a clear day may actually disappear on a hazy 
day. Aerosols, probably produced by urban smog, 
cause the haze evident in this picture of the Empire 
State Building in New York City. Over the past 
few decades, horizontal visibility has declined 
around the globe, on average, due to increasing 
aerosol concentrations. As a result, scenic vistas 
throughout the world have been obscured.
Photograph © Forrest M. Mims III. Used by permission. May be 
freely reproduced with acknowledgment. 
GLOBE® 2005 Observing Visibility and Sky Color Learning Activity - 3 Atmosphere
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Introduction
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Teacher Preparation
In this activity, your students will carefully 
observe the atmosphere over a period of days and 
record their observations. Through these direct 
observations they will develop an understanding 
that visibility and sky color are related and that 
both are due to the relative presence or absence 
of aerosols. 
The students will classify the sky color using 
standard categories and will represent the sky 
color using paints or colored pencils. They also 
will record the visibility based on observation of 
a distant object such as a mountain or a building. 
It is not important that they observe every day, 
but they should try to sample a wide range of the 
visibility and sky conditions that occur at your 
location. They should try to observe on some 
very clear days, on some hazy days, and on some 
intermediate days. After they have observed and 
recorded examples of very clear days, very hazy 
days, and various conditions in between, the class 
will record their observations in a summary table 
and see whether or not a pattern emerges that 
relates visibility to sky color.
Visibility
By “visibility” we mean the clarity with which 
objects can be viewed through the intervening 
atmosphere. In order to judge visibility or the 
clarity of the atmosphere, students need to be able 
to look out at a distant scene, such as a distant 
building or a mountain or hillside. By looking 
at the same scene or object every day students 
will gradually develop a sense of whether the 
day is unusually clear, clear, somewhat hazy, very 
hazy, or extremely hazy. Only practice, lots of 
different examples, and discussion will make these 
categories clear. (No pun intended!)
Sky Color
Students are also asked to observe, classify, and 
represent the sky color. They will classify the sky 
color using the categories listed at the bottom 
of the data sheet. They represent the sky color 
in a drawing using paints or colored pencils. 
They could also try using photographs or color 
paint chips. As they make more observations, 
the students will become more confident of their 
classifications and more skillful in drawing the 
sky color.
Students may notice that the sky is often a different 
color in different parts of the sky. Near the horizon 
it is typically lighter due to the presence of aerosols. 
The darkest part of sky can often be seen about half 
way between the horizon and directly overhead, 
in the “anti- sun” direction – that is, when you 
look at the sky with your shadow in front of you. 
Students should try to locate the darkest (bluest) 
color of the sky and record it.
Correlation Between Visibility and 
Sky Color
One of the purposes of this activity is for students 
to realize that on the clearest days with the highest 
visibility the sky is a deep blue color, while on 
hazy days it appears milky. Changes in visibility 
and sky color are both due to changes in aerosol 
concentrations in the atmosphere. Because aerosols 
scatter sunlight, high aerosol concentrations make 
it harder to see distant objects and make the sky 
appear lighter. On clear days when aerosols are 
low, visibility is high and the sky is deep blue. But 
do not TELL students this; let them discover it by 
pooling the class observations in the Visibility and 
Sky Color Summary Chart. It should be the case that 
most of their observations will tend to fall along 
the main diagonal from upper left to lower right.
What To Do and How To Do It
1. Lead students through a discussion of 
aerosols, visibility, and sky color. Begin by 
asking them what they recall about times 
when the sky was very hazy. How was the 
visibility? How did they recognize that 
visibility was low? What color was the sky? 
When did this occur? What do they think 
caused it?
2. Continue by asking them to recall a time 
when the sky was very clear. What did 
it look like? What color? How was the 
visibility through the atmosphere? When 
did this very clear event occur? What was 
the weather like at that time? What do 
GLOBE® 2005 Observing Visibility and Sky Color Learning Activity - 4 Atmosphere
they think caused the air to be so clear at 
that time?
3. If it has not already come up in discussion, 
discuss the role of aerosols in the creation 
of haze. Discuss local and regional sources 
of aerosols. Discuss, also, how aerosols 
such as dust can be transported from long 
distances and affect local conditions.
4. Explain that they will undertake an 
investigation of sky color and visibility. 
Introduce the Visibility and Sky Color Data 
Sheet and discuss how to use it. Take 
observations for as many days as necessary 
to obtain a full range of sky conditions in 
the data.
5. After the class has made a large number 
of observations, covering the entire 
range of sky conditions that occur in 
your area, bring the class together for a 
group discussion of the data. Engage the 
students in a discussion of the conditions 
that existed when they observed the 
clearest and their haziest skies. What was 
the weather like? What do they think 
accounted for the clearest and the haziest 
skies? When they had hazy skies, was the 
haze created by local, regional, or long-
distance factors?
6. On the blackboard or on chart paper, 
create a chart similar to the Visibility and 
Sky Color Summary Chart shown. Invite 
students to contribute their data to the 
chart by placing a mark in the appropriate 
cross-classification cell to represent each of 
their observations.
7. When the chart has been populated with 
all of the student observations you should 
observe a diagonal trend in the data, from 
upper left to lower right. Ask students to 
explain why this trend exists. What is the 
common element that causes both low 
visibility and milky skies?
8. (Optional) Have each student or team 
create a “key” to help them make future 
observations. Select one sky color example 
for each level of visibility/sky color from 
“unusually clear” to “extremely hazy.” Use 
these keys to standardize your observations 
of haze conditions. Students can continue 
to take observations throughout the year 
and note relationships to season, storms, 
time of day, temperature, wind direction 
and other conditions. Depending on 
students’ ages, these color keys can be 
sky paintings, photographs, or paint color 
chips that can be obtained from stores that 
sell interior paint.
Student Preparation for Observing 
Visibility and Sky Color
Make these observations only on days when you 
can see the sky. Do not attempt to observe visibility 
and sky color on days that are overcast. For each 
day that you make an observation, record the date, 
the local time, your estimate of the visibility and 
your estimate of the sky color.
Both visibility and sky color are subjective 
classifications. That means you should expect 
some variation among observers and changes in 
your own classifications as you gain experience. As 
you gain experience in observing the atmosphere 
and the sky you may change your mind about 
some of your initial classifications. You may 
decide that what you originally classified as a deep 
blue sky you now consider to be merely “blue.” 
Or, you may decide that what you thought was 
“somewhat hazy” was really “very hazy.” Do not 
worry about this and do not go back and change 
your original observations. You can expect your 
skill in classifying to evolve and change. Gradually, 
you should gain confidence in your ability to 
classify consistently. 
GLOBE® 2005 Observing Visibility and Sky Color Learning Activity - 5 Atmosphere
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Introduction
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Learning A
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1. Estimate the visibility.
 Select some distant object – a mountain 
range, a building, or other object several 
kilometers away. Use this object as your 
“reference object” to judge visibility every 
day you make an observation. Take note 
of how distinctly you can see it and select 
one of the visibility categories below and 
record it on the Visibility and Sky Color 
Data Sheet.
  Unusually clear
  Clear
  Somewhat hazy
  Very hazy
  Extremely hazy
2. Observe the sky color.
 Now look at the sky and find the part of 
it that is the darkest color. When you do 
this activity, be sure not to look directly at 
the sun even if it is partially obscured by 
clouds. Select a category for the sky color 
from the list below and record it on the 
Visibility and Sky Color Data Sheet.
  Deep blue
  Blue
  Light blue
  Pale blue
  Milky
3. Paint or draw with colored pencils your 
best representation of the sky color in the 
“picture” box. You can also use paint color 
chips or photographs to represent sky 
color.
Questions for Understanding
1. When you see blue skies, what other 
weather conditions are likely to exist? 
What else would you observe on very clear 
days?
2. Are you aware of any daily patterns in sky 
color and visibility in your location? Is 
it usually hazier at certain times of day? 
What causes this?
3. How are sky color and haze related to 
weather?
4. Are sky color and haze at your location 
related to the amount of wind and the 
wind direction? If so, why?
5. Are sky color and haze at your location 
related to the time of the year? That is, are 
there seasonal patterns in your data?
GLOBE® 2005 Observing Visibility and Sky Color Learning Activity - 6 Atmosphere
Visibility and Sky Color Data Sheet
Observer(s):    
          
 Date Local Visibility                      Sky Color  Comments: 
  Time (use categories Category Draw the weather conditions,   
   below) (see below) Color * wind direction, etc.
*Alternatively, you can give the name of a paint color, or you can refer to a photograph you have taken 
previously.
 
   
Visibility Categories Sky Color Categories
Unusually clear Deep blue
Clear Blue
Somewhat hazy Light blue
Very hazy Pale blue
Extremely hazy Milky
GLOBE® 2005 Observing Visibility and Sky Color Learning Activity - 7 Atmosphere
Visibility and Sky Color Summary 
Chart
Make a mark or an “x” in the cell of the chart where each observation falls.
 Visibility/Sky Color Deep blue Blue Light blue Pale blue Milky
 Unusually clear      
 Clear      
 Somewhat hazy      
 Very hazy      
 Extremely hazy      
What do you notice about the pattern of observations?
How can you explain this pattern?


Observing Visibility and Sky Color

http://www.globe.gov/tctg/atla-hazyskies.pdf?sectionId=21

Observing Visibility and Sky Color

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