Transcript Relative humidity - Solon City Schools

UNDERSTANDING
WEATHER
The Water cycle
The amount of water vapor in the air is called
humidity. As water evaporates and becomes air
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.
Relative humidity is the amount of water vapor in the air
compared with the maximum amount of water vapor
that the air can hold at a certain temperature.
Given at a percentage (%)
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%.
To calculate unsaturated air
actual water vapor content (g/m3)
saturation water vapor content (g/m3)
X 100 = relative humidity (%)
2 factors affect relative humidity:
1) Amount of water vapor
2) Temperature
The more water vapor
there is in the air, the
higher the relative humidity.
The relative humidity
decreases as the
temperature rises
and increases as
the temperature drops.
The outside air might have a
comfortable level of humidity, but
when that air is heated, the relative
humidity drops, causing the air to be
very dry inside the house.
During the dry months, a humidifier
can help maintain a comfortable level
of humidity. Let's take a look at a
simple humidifier.
A psychrometer is an instrument that is used to measure
relative humidity.
This consists of 2 thermometers, one which is a wet-bulb
thermometer and one thermometer is a dry-bulb
thermometer.
The wet-bulb thermometer
is covered with a damp cloth.
The difference in temperature readings between the
wet and dry-bulb thermometers indicates the amount
of water vapor in the air. The larger the difference is
between the two readings, the less water vapor the air
contains and thus the lower the humidity is.
A wet-bulb thermometer works differently
than a dry-bulb thermometer.
Dry-bulb thermometer only measure air temperature.
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 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 wet-bulb thermometer and the
change in temperature will be small.
Condensation is the process by which a gas, such as
water vapor, becomes a liquid.
Before condensation can occur, the air must be
saturated.
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 temperature of the air at a particular location at
which saturation occurs and water vapor
begins to condense forming Dew.
Dew is commonly observed on the leaves
of plants in the early morning.
Do Now For Friday, November 2, 2012
• What is the difference between humidity
and relative humidity?
• How do you calculate (find) relative
humidity?
Do Now For Wednesday, Nov. 7, 2012
• We recorded relative humidity values in
three locations throughout periods 1 – 5
yesterday.
• Choose any portion of the data from
yesterday’s lab and explain how there
could be differences in values.
Do Now For Thursday, Nov. 8, 2012
• Pick a cloud, any cloud and describe it in
terms of its altitude and the type of
weather it brings.
CLOUDS
A cloud is a collection of millions of tiny water droplets
or ice crystals.
Clouds form as warm
air rises and cools.
As the rising air cools
it becomes saturated.
When the air is saturated,
the water vapor
changes to a liquid or
solid, depending on
the air temperature.
Cumulus Clouds
•puffy, white clouds that
• tend to have flat
•bottoms
•form when air rises
•generally indicate fair weather
•when get larger – produce thunderstorms
•Thunderstorms come from cumulonimbus clouds
•If a cloud includes “nimbo-” are likely to produce
precipitation
Stratus Clouds
•Clouds that form in layers
•Cover large areas of
the sky and often block
out the sun
•Can be caused by a
gentle lifting of a large
body of air into the atmosphere
•Nimbostratus clouds = dark clouds that usually
produce light to heavy, continuous rain.
•Fog is a stratus cloud that has formed near the
ground
Cirrus Clouds
•thin, feathery, white clouds found
at high altitudes
•form when the wind is strong
•if they get thicker =
a change in
the weather is
coming
Clouds and Altitudes
“cirro-” = clouds that form at high altitudes
Because of the cold temperatures at high altitude,
high clouds are made up of ice crystals.
“alto-” = clouds that form at middle altitudes
Middle clouds can be made up of both water
drops and ice crystals.
Low clouds do not have a special prefix to
describe them.
Low clouds are made up of water drops.
Precipitation
When water from the air returns to Earth’s surface, it is
returned as precipitation.
5 major forms of precipitation:
* rain
* sleet
* freezing rain
* snow
* hail
RAIN
•the most common form of precipitation
•clouds produce when the water
drops become large enough to
fall
•a water drop in a cloud begins
as a droplet smaller than a
period at the end of a sentence –
it must increase its size 100 times
before it will be large enough to fall
Freezing Rain
*Raindrops falling through cold air freeze when they
touch a cold surface (like roads & sidewalks).
*This thick layer of ice can break branches and snap
power lines.
Sleet forms when
rain falls through a
layer of freezing air,
which produces
falling ice.
Snow forms when
temperatures are so
cold that water
vapor changes
directly into a solid.
Snow can fall as
single ice crystals or
can join to form
snowflakes.
SLEET AND SNOW
HAIL
•balls or lumps of ice that
fall from clouds
•form in cumulonimbus
clouds
•when updrafts of air in
clouds carry raindrops
high in the clouds, the
raindrops freeze
•As hails falls, water drops
coat it, another updraft
sends the hail up again
(over and over) until it
becomes too heavy
How to Measure Rain!
• We use a rain gauge to measure rain.
• Most give the calculated rainfall amounts.
Some require calculations.
Snow Calculations
• For snow: 10 in. of snow = 1 inch of rain
•
10 cm. of snow = 1 cm of rain
Controlling Precipitation
• A DROUGHT is a long period with very
little or no rain.
• Effects of drought range from loss of crops
to loss of life. Causes famine in many
areas.
Cloud Seeding
• To combat droughts, scientists have tried
different techniques to create rain.
• Cloud seeding involves sprinkling clouds
with super cooled solid CO2 or silver
iodide crystals.