Chapter 16 Section 1: part 1
Water in the Air
Water can exist in three forms: solid, liquid, and gas. An example of water as a solid would be ice found in
clouds as snowflakes. As a liquid, water exists in clouds as water droplets. And water as a gas is water vapor. Water
in the air affects the weather. Weather is the condition of the atmosphere at a particular time and place.
Water Cycle
Water amount
Vs.
Temperature
Water in all three forms is constantly changed through a process known as
the water cycle. The water cycle is the continuous movement of water from
sources on the ground like lakes and oceans, into the air, onto and into the ground,
and back to the original water sources from evaporation, condensation,
precipitation, and runoff.
Humidity:
Humidity is the water vapor or moisture in the air. As water
evaporates from various sources, the amount of humidity increases. But
the air’s ability to hold water vapor depends on temperature. As
temperature increases, the air’s ability to hold water vapor also increases.
The summer is when we usually complain that is hot and humid outside.
See the chart at the left and/or figure 3 on page 423.
The relative humidity is the amount of moisture the air contains
compared with the amount it can hold at a particular temperature. It is
given as a percentage and is the one you have been recording in your
weather journal. When the air holds as much as it can at a given
temperature, the air is said to be saturated. Saturated air has a relative
humidity of 100%. How do you find the R.H. of the air if the air is not
saturated? If you know how much it can hold, and how much it is
presently holding, you can calculate the R.H. Divide the amount of
moisture currently in the air by how much it can hold and then multiply
that by 100 . See below.
R. H. formula = amt. of water at present
/
amt. of water at saturation
x 100%
= R.H. %
If you don’t know the moisture levels in the air, measuring the R.H. can be done with a devise called a
sling psychrometer. This instrument consists of two thermometers. One thermometer is called the dry-bulb, while
the other (with a wet cloth over it) is called the wet-bulb. The two thermometers are swung around in the air. As air
passes over the one with the wet cloth, the water in the cloth begins to evaporate. Evaporation causes cooling. If there
is very little humidity in the air, evaporation off the cloth happens readily. If the air is already humid, there is less
evaporation and less temperature change. A larger difference in the two readings indicates a lower humidity. Along
with the two temperature readings, a special chart is used to determine the relative humidity. To see an actual
psychrometer and chart, and learn how to use it, turn to page 424.
Condensation:
You have probably seen water droplets form on the outside of a glass of ice water. Where did the water come
from? It came from the air, and it formed because of condensation. Condensation is the process where a gas, like
water vapor, turns into a liquid. Before this can happen, however, the temperature of the air must be cooled to a point
beyond where it becomes saturated (R.H.=100%). The temperature beyond which saturation occurs is called the dew
point. The dew point is the temperature to which air must be cooled to be completely saturated. With additional
cooling, fog will develop or some form of precipitation will fall.
1
Chapter 16 Section 1-1:
Answer the following questions searching the reading assignment when appropriate.
__________1. The water vapor or moisture in the air is called ___ .
__________2. The ____ is the temperature to which air must be cooled to be completely saturated.
__________3. A devise to help measure the relative humidity is called a _____ .
__________4. When the air holds as much water vapor as it can, the air is said to be _____ .
__________5. The _______ is the continuous movement of water from the ground, to the air, and back to the
ground through the processes of evaporation, condensation, and precipitation.
__________6. The condition of the atmosphere at a particular time and place is called the _____ .
__________7. Name the three forms that water comes in.
__________8.
__________9.
_______%_10. If the air is holding as much water vapor as it can, what is the relative humidity?
__________11. Calculate the R.H. of the air if it is presently holding 5 g. of water per m3, but can hold 10 g.
Write out the formula, substitute the numbers into the formula, divide, and times by 100.
R.H. = ____________ / _____________ x 100% = __________ / __________ x 100 % = ______ %
Finish writing out the formula from the previous page
Substitute the numbers in and divide
Times by 100
__________12. In the summer is when it usually gets hot and ____ outside.
_______%_13. What is the R.H., using the chart below, for a dry-bulb of 600 and a wet-bulb of 500 ?
To determine relative humidity with the chart: (see also pg. 424 for additional directions on how to use the chart)
1 Subtract the wet-bulb temperature from the dry-bulb temperature.
2 Find that number (the difference in degrees) at the top of the chart and place your right index finger on it.
3 Find the dry-bulb temperature in the first column on the left. Place your left index finger on it.
4 Bring your fingers down the column and across the row.
The relative humidity (R.H.) percentage appears where the column and row intersects within the chart.
2
Chapter 16 Section 1: part 2
Clouds:
A cloud is made of millions of tiny water droplets or ice crystals. Clouds form as warm, moist air rises and
cools with altitude. At cooler temperatures, the air’s ability to hold moisture decreases. Cooling continues to a certain
temperature where the amount of moisture in the air reaches full capacity, called the dew point. At this temperature,
the invisible water vapor particles join and grow into a visible liquid or solid depending on the season and the air
temperature. At higher temperatures, the water vapor condenses to form droplets, clouds, and possibly raindrops. At
temperatures below freezing, the water vapor changes to ice crystals that eventually combine to make snow flakes.
Clouds are classified by altitude and by shape. There are three cloud types based on altitude: low, middle, and
high. Low clouds are made up of water droplets and so are usually gray in color. The prefix strato- is commonly used
in the names of these clouds. Middle levels clouds can be made up of water droplets and ice crystals and so can be
gray and white as a result. The prefix alto- is commonly used when naming these clouds. High clouds, because of the
cold temperatures at high altitudes, are made up of ice crystals and are usually white in color. The prefix cirro- is
commonly used when naming these clouds.
There are also three cloud types based on shape. Cumulus is Latin for “heap”. Cumulus clouds are white,
puffy clouds. Usually, the weather is nice when these clouds are wondering around. Stratus is Latin for “spread out”.
Clouds that form in long layers and spread out over the entire sky are called stratus. This situation would seem as
though a dark, gray blanket has been pulled across the sky. You would not be able to locate the sun. When a stratus
cloud forms on the ground, it is fog. Cirrus is Latin for “curly”. Thin, feathery clouds are called cirrus clouds. They
appear wispy or have curled up ends because of strong winds blowing the ice crystals around. When you see cirrus
clouds, it won’t be long before the weather turns for the worse, however.
Cloud names are combinations of shape and altitude:
cirrus-thin, feathery wisps, cirrocumulus-small,high puffs, cirrostratuslooks like a fine white veil pulled across the sky-can still see the outline of
the sun, altocumulus-small, higher level puffs, altostratus-looks like a
gray/white sheet pulled over the atmosphere-can see where the sun might
be, stratus-looks like a gray blanket pulled over the atmosphere-not able to
locate the sun, stratocumulus-some breaks in the stratus clouds,
nimbostratus-if snowing or raining. When nimbus- or nimbo- is used in
the name, it is associated with precipitation falling.
A cumulus cloud looks like large, lower level white puffs. It is
also called “fair weather cumulus”. It can grow vertically into a huge pile
of clouds or “towering cumulus” or “thunderhead”, however. When a
thunderstorm is produced, the clouds making the cold, pouring rain and
lightning are called cumulonimbus.
Precipitation:
Water vapor, that condenses to form cloud droplets and clouds, can eventually grow larger and heavy enough
to fall to the ground as precipitation. Precipitation is water in solid or liquid form, that falls from the air to the Earth.
There are four major forms-rain, snow, sleet, and hail.
Rain is the most common form of precipitation. A cloud droplet must become 100 times its normal size to
become large enough the fall as precipitation. The most common form of solid precipitation is snow. Snow forms
when the temperature in the cloud is so cold that the moisture changes directly to a solid. Snow falls as individual ice
crystals or combine to form flakes. Sleet, also called freezing rain, forms when rain falls through a layer of air below
freezing. Hail is solid precipitation that falls as clumps or balls of ice after raindrops get thrown around and into the
upper part of a thunderhead. The drop freezes, falls, collects more moisture, and gets raised upward to freeze again. If
the upward movement within the cloud is strong enough, a hailstone can accumulate many layers of ice. Eventually, it
will become too heavy and fall to the ground.
Precipitation is measured in a device called a rain gauge. If the precipitation is in the form of snow, the gauge
would have to be heated to melt the snow into water first. See figure 13 on page 429.
3
Snow forms when the temperature in the cloud is so cold
that the moisture changes directly to a solid. Snow falls as
individual ice crystals or combine to form flakes.
Precipitation is measured in a device called a rain gauge.
If the precipitation is in the form of snow, the gauge would have
to be heated to melt the snow into water first.
A cloud droplet must become 100 times its normal size to become large enough
the fall as rain. If, before falling, the rain drops are lifted into the upper parts of the
thundercloud and freeze, it is called hail. Repeated lifting makes the hailstones bigger.
Chapter 16 Section 1-2:
Answer the following questions searching the reading on the previous page.
__________1. Cooling continues to a certain temperature where the amount of moisture in the air reaches
full capacity, called the ____ ____.
__________2. A ____ is made of millions of tiny water droplets or ice crystals.
__________3. Clouds are classified and by ____ and by shape.
__________4. There are ___ cloud types based on shape.
__________5. Thin, feathery clouds are called ____.
__________6. Clouds that spread out and form long layers covering the entire sky are called ____.
__________7. ____ clouds are white, puffy clouds.
__________8. When a stratus cloud forms on the ground, it is called ___.
__________9. During a thunderstorm, the clouds making the rain and lightning are called ____.
__________10. ______ is water falling in either solid or liquid form.
__________11. A cloud droplet must become ___ times its normal size to become large enough the fall.
__________12. The most common form of solid precipitation is ____.
__________13. Precipitation is measured in a device called a ____ gauge.
4