Transcript Understanding Weather

Water in the Air
Air Masses and Fronts
Severe Weather
Forecasting Weather
weather
water cycle
Humidity
relative humidity
Condensation
dew point
cloud
Precipitation
air mass
front
thunderstorm
lightning
thunder
tornado
hurricane
thermometer
barometer
windsock
wind vane
anemometer
isobars
Water in the Air
Although the Earth’s atmosphere contains only about
0.001 percent of the total volume of water on the planet
(about 1.46 x 109 km3), it is an essential link between
land masses and bodies of water on Earth.
The rate at which water evaporates into Earth’s
atmosphere is about 5.1 x 1017 L per year.
About 78% of all precipitation falls over Earth’s
oceans. Of the 22% that falls on land, about 65% returns
to the air by evaporation.
 Rainbows are evidence that air contains water. Water
in the air affects weather and weather is the condition of
the atmosphere at a particular time and place.
The Water Cycle
The
water
cycle
is
the
continuous movement of water
from water sources, such as
lakes and oceans, into the air,
onto and over land, into the
ground.
Physical Science
Connection
At the Earth’s surface and in the
atmosphere, energy from the sun moves
water thru the water cycle. Sunlight
provides the energy for evaporation,
transpiration,
condensation
and
precipitation.
Life Science
Connection
When air is humid, hair becomes
frizzy. Hair is made of a protein
called keratin. Each hair has a scaly
outer cuticle, which allow moisture
to enter the inner part of the hair
fiber. Humidity can cause hair length
to change as much as 2.5%.
Condensation
Condensation occurs when water vapor
cools and changes back into liquid
droplets. This is how clouds form. It is
the process by which a gas, such as
water vapor, becomes a liquid. Before
condensation can occur the air must be
saturated.
Evaporation
Evaporation occurs when liquid
water changed into a water vapor,
which is a gas.
Transpiration
Transpiration is the process by
which plants release water vapor
into the air through their leaves.
Precipitation
Precipitation occurs when rain,
snow, sleet, or hail falls from the
clouds onto the Earth’s surface.
4 Types of Precipitation
 Rain ( measured by a rain
gauge)
 Snow
 Sleet
 Hail
Runoff
Runoff is water, usually from
precipitation, that flows across
land and collects in rivers,
streams, and eventually the
ocean.
Humidity
Humidity is the amount of water vapor or
moisture in the air. It is the moisture in
the air that makes for bad hair days! Air’s
ability to hold water vapor depends on
the
temperature.
As
temperature
increases, the air’s ability to hold water
also increases. Page 423, figure 3 shows
the relationship between air temperature
and air’s ability to hold water.
Relative Humidity
Relative humidity is the amount of
moisture the air contains compared with
the maximum amount it can hold. When
air holds all the water it can at a given
temperature, the air is said to be
saturated. Saturated air has a relative
humidity of 100%. A psychrometer
measures relative humidity.
Math Concept
Relative humidity is the amount of moisture the air
is holding compared with the amount it can hold at
a particular temperature. The relative humidity of
air that is holding all the water it can at a given
temperature is 100%, meaning it is saturated. You
can calculate it with the following equation:
(present) g/m3
(saturated) g/m3
x 100 = relative humidity
How does Humidity
relate to the water cycle?
Evaporation occurs when liquid
water changes into water vapor and
returns to the air. Humidity is the
amount of water vapor in the air.
Dew Point
Dew point is the temperature to
which air must cool to be completely
saturated. It must also have a
surface to condense on. Have you
ever noticed the dew on the grass?
Clouds
A cloud is a collection of
millions of tiny water droplets or
ice crystals. They form as warm
air rises and cools becoming
saturated. At saturation the
water vapor becomes liquid.
Cumulus Clouds
Cumulus Clouds
Cumulus clouds form when
warm air rises. They tend to
have flat bottoms are white
and puffy
Cumulus Clouds
Larger
cumulus
clouds
form
thunderstorms. A cumulus cloud
that produces thunderstorms is
called a cumulonimbus cloud. When
–nimbus or nimbo- is used as part of
a clouds name, it means that it
might rain.
Stratus Clouds
Stratus clouds form in layers and
cover large areas. These clouds are
formed by a gentle lifting of warm air
into the atmosphere.
Cirrus Clouds
Cirrus clouds are thin, feathery
white clouds found at high altitudes
and form when the wind is strong.
They may indicate approaching bad
weather if they thicken and lower in
altitude.
Cirrus Clouds
Cloud Types Based on Form and
Altitude
 High Clouds- made of ice crystals and
the prefix cirro- is used to describe high
clouds.
 Middle Clouds- can be made of both
water droplets and ice crystals. The
prefix alto- is used to describe middle
clouds.
 Low Clouds- made of water droplets. The
prefix strato- is used to describe low
clouds.
Air Masses and Fronts
An air mass is a large body of air
that has similar temperature and
moisture throughout. It gets it
moisture
and
temperature
characteristics from the area in
which it forms. See page 430, figure
14.
Warm Air Masses
Four warm air masses influence
the weather in the United States.
As shown in figure 14.
Cold Air Mass
Most of the cold winter weather
in the United States is influenced
by the three polar air masses.
The cP is dry and the 2 mP’s are
moist.
Cold and Warm Fronts
Occluded and Stationary
Fronts
Quiz
1. If a cP air mass moves over Ohio in the
summer, what would the weather be
like?
2. Why does the cT air mass that forms
over northern Mexico bring clear, dry,
hot weather?
3. Explain how a cold front develops.
4. What kind of weather is associated with
a stationary front?
Answers
1. Cool and Dry
2. It forms over the desert, which is hot
and contains little moisture.
3. A cold air mass moves under a warm air
mass, forcing the warmer air upwards.
4. Will be cloudy and rainy as long as the
front lies over an area. After the front
passes, the weather will usually be
clear.
Severe Weather
Severe Weather is weather that
can cause property damage and
even death. Examples of severe
weather include thunderstorms,
tornadoes and hurricanes.
Thunderstorms
Thunderstorms
are
small,
intense
weather systems that produce strong
winds, heavy rain and lightning. They
occur when warm, moist air rises rapidly
in an unstable atmosphere. A typical
thunderstorm produces approximately
470 million liters of water and enough
electricity to power the “ENTIRE” United
States for 20 minutes.
Trees sometimes explode when
struck by lightning. Why? Lightning
causes the sap in the tree to
vaporize (turn from a liquid to a
gas). The steam expands rapidly as
it is heated, causing the tree to
explode.
Lightning and Thunder
Lightning is a large electrical discharge
that occurs between two oppositely
charged surfaces. When lightning
strikes, energy is released. The energy
is transferred to the air and causes the
air to expand rapidly and send out
sound waves. Thunder is the sound
that results from the rapid expansion of
air along a lightning strike.
Math & More
To find the distance
of a
thunderstorm in kilometers, count
the seconds between a lightning
flash and the thunder and divide the
number by 3. If you see a lightning
flash and then hear the thunder 21
seconds later, how far away is the
storm? 9 kms? 7 kms?
Tornadoes
Tornadoes are produced in only 1% of all
thunderstorms. A tornado is a small,
rotating column of air that has high wind
speeds and low central pressure and
touches the ground. A tornado starts out
as a small funnel cloud and hangs in the
air. It is called a tornado when it makes
contact with the Earth’s surface.
Tornadoes contd.
About 75% of tornadoes occur in
the United States. The average
tornado has wind speeds of 120180 km/hr, but rarer, more violent
tornadoes have spinning winds
up to 500 km/hr.
Hurricanes
Hurricanes are large, rotating
tropical weather systems with wind
speeds of at least 119 km/hr. They
are the most powerful storms on
Earth! They have different names
depending on where they form.
Western Pacific- Typhoons. Indian
Ocean- Cyclones.
Hurricanes generally form in the
area between 50 and 200 north
and south latitude over warm,
tropical oceans. At higher
latitudes, the water is too cold
for hurricanes to form. They vary
in size from 160 km to 1500 km
in diameter, and they can travel
thousands of miles.
Hurricane Formation
A hurricane begins as a group of
thunderstorms moving over tropical
ocean waters. Winds traveling in two
different directions collide, causing the
storm to rotate over the area of low
pressure. Because of the Coriolis effect,
the storm turns counterclockwise in the
Northern Hemisphere and clockwise in
the Southern Hemisphere.
Hurricane Energy
Hurricanes get their energy from the
condensation of water vapor. Once
formed, the hurricane is fueled through
contact with the warm ocean waters. As
warm moist air rises, water vapor
condenses, releasing large amounts of
energy. As long as the hurricane is over
it’s source of warm water, it will continue
to grow.
Hurricane Damage
High winds cause a great deal of
damage but most damage is caused by
flooding associated with the storm
surge. A storm surge is a wall of water
that builds up over the ocean due to
the heavy winds and low atmospheric
pressure. A storm surge can be 1-8 m
high and 65-160km long.
Forecasting Weather
Air temperature is measured by a
thermometer.
A barometer is used to measure air
pressure.
Wind direction can be measure by using a
windsock or wind vane.
Wind speed is measured by an
anemometer.
Eyes in the Sky
 Weather Balloons- carry electronic
equipment that can measure conditions
as high as 30 km.
 Radar- is used to find the location,
movement and intensity of precipitation.
 Weather Satellites- provide images of
swirling clouds, and can measure wind
speeds, humidity, and temperatures at
various altitudes.
Weather Maps
Meteorologists base their forecasts on
information gathered from many sources. In the
United States, the National Weather Service
(NWS) and the National Oceanic and
Atmospheric Administration collect and analyze
weather data. The NWS produces maps based
on information from about 1000 weather
stations across the country. This information is
presented on a map as a station model.
Isobars
Isobars are lines on a weather map,
similar to contour lines on a
topographical map. Isobars are lines
that connect points of air pressure
rather that elevation. These areas
are usually marked with an H or L.
Fronts are also labeled on weather
maps. See page 443, figure 32.