Transcript Weather Introduction

Bell work
Observe two glasses of water. One is filled with ice
water, and one is filled with warm water.
1. Why do water droplets form on the outside of the
cold glass?
2. Where do the water beads come from?
3. Why don’t the water beads form on the warm
glass?
4. Have you seen this happen before on other
containers? On your soda can? On a coffee cup or
a soup bowl?
Weather Introduction
Objectives for today!
• Explain how water moves through the water cycle.
• Describe how relative humidity is affected by
temperature and levels of water vapor.
• Describe the relationship between dew point and
condensation.
A quick tongue twister!

Whether the weather be hot, or
whether the weather be cold. We’ll
weather the weather, whatever the
weather whether we like it or not.
What is weather?


Weather is the condition of the
atmosphere at a certain time and place.
The condition of the atmosphere is
affected by the amount of water in the
air.
How is that different from
Climate?



Climate is the condition of the
atmosphere over a long period of time.
Like a season.
Remember weather is a one particular
time and place.
What is the Water Cycle?

The water cycle is the continuous
movement of water from sources on
Earth's surface—such as lakes, oceans,
and plants—into the air, onto and over
land, into the ground, and back to the
surface.
The Water Cycle- A water cycle is the way
water moves from the air to land and back
again.
Condensation
• The water vapor rises into the air, cools,
and condenses into tiny droplets of water.
• The drops of water come together to form
clouds.
Water Cycle
Evaporation is when a
liquid changes to a
gas.
The sun’s heat makes
water evaporate into
water vapor.
Precipitation
The water drops
become heavy and
fall as rain, sleet,
snow, or hail.
The rain falls or snow melts
and flows to rivers, lakes,
and oceans.
What is Humidity?
Humidity



As water evaporates from lakes, oceans, and
plants, it becomes water vapor, or moisture
in the air.
Humidity refers to the amount of moisture
(water vapor) in the surrounding air.
As water evaporates and becomes water
vapor, the humidity of the air increases.
• The air's ability to hold water vapor changes as
the temperature of the air changes.
• As the temperature of the air increases, the air's
ability to hold water vapor also increases.
Objectives for today!
• Explain how water moves through the water cycle.
• Describe how relative humidity is affected by
temperature and levels of water vapor.
• Describe the relationship between dew point and
condensation.
Relative Humidity
Relative Humidity is a measure of the

amount of moisture in the air compared with
the amount of moisture the air can hold.
Relative humidity is expressed as a
percentage of how much moisture the air
could possibly hold at the temperature it
happens when you measure it.
Relative Humidity


When air holds all of the water that it
can at a given temperature, it is said to
be saturated. Saturated air has a
relative humidity of 100%.
If air holds all the water vapor it can, it
is at 100% RH and is said to be
“saturated.”



We are very sensitive to humidity. Sweating keeps
our body cool and maintain its current
temperature.
If the air is at 90% relative humidity, sweat will
not evaporate into the air. As a result, we feel
much hotter than the actual temperature when the
relative humidity is high.
If the relative humidity is low, we can feel much
cooler than the actual temperature because our
sweat evaporates easily, cooling the body.
Relative Humidity





Two factors that affect relative humidity are amount
of water vapor and temperature.
At constant temperature and pressure, as the
amount of water vapor in air changes, the relative
humidity changes.
The more water vapor there is in the air, the higher
the relative humidity is.
If the amount of water vapor in the air stays the
same but the temperature changes, the relative
humidity changes.
The relative humidity decreases as the temperature
rises and increases as the temperature drops.
Measuring Relative Humidity



A psychrometer (sie KRAHM uht uhr) is an
instrument that is used to measure relative
humidity.
A psychrometer consists of two
thermometers, one of which is a wet-bulb
thermometer. The bulb of a wet-bulb
thermometer is covered with a damp cloth.
The other thermometer is a dry-bulb
thermometer.
Measuring RH


Using the difference
between the two (“wet
bulb depression”) and
charts allow calculation
of the relative humidity.
“Hygrometers” also
measure RH
[Sources:www.novalynx.com and www.accumall.com]
Measuring Relative Humidity




As air passes over the wet-bulb thermometer, the
water in the cloth evaporates.
As the water evaporates, the cloth cools.
If the humidity is low, the water will evaporate
more quickly and the temperature reading on the
wet-bulb thermometer will drop.
If the humidity is high, only a small amount of
water will evaporate from the cloth of the wetbulb thermometer and the change in temperature
will be small.
Measuring Relative Humidity


The difference in temperature readings
between the thermometers indicates
the amount of water vapor in the air.
The larger the difference between the
two readings is, the less water vapor
the air contains and thus the lower the
humidity is.
Wet- and Dry Bulb Psychrometer
Psychrometric Chart.
If the difference between
wet and dry bulb is 6º F
and the temperature is
72º F (dry bulb), then the
RH is 54%.
Objectives for today!
• Explain how water moves through the water cycle.
• Describe how relative humidity is affected by
temperature and levels of water vapor.
• Describe the relationship between dew point and
condensation.
What is Condensation?




The water came from the surrounding air,
and droplets formed as a result of
condensation.
Condensation is the process by which a
gas, such as water vapor, becomes a liquid.
The air must be saturated, which means that
the air must have a relative humidity of
100%.
Condensation occurs when saturated air
cools.



Air can become saturated when water
vapor is added to the air through
evaporation.
Air can also become saturated when it
cools to its dew point.
The dew point is the temperature at
which a gas condenses into a liquid. At
its dew point, air is saturated.


The ice in the glass
of water causes the
air surrounding the
glass to cool to its
dew point.
Before water vapor
can condense,
though, it must have
a surface to
condense on.
Objectives for today!
• Explain how water moves through the water cycle.
• Describe how relative humidity is affected by
temperature and levels of water vapor.
• Describe the relationship between dew point and
condensation.