Transcript Relative Humidity

Chapter 2:
“Understanding Weather”
Humidity
Humidity – the amount of water vapor in air
 Air’s ability to hold water vapor changes as the
temperature of the air changes.
 Increased temperature means the air can hold
more water.
 Decreased temperature means the air can hold
less water.
Relative Humidity
 Relative Humidity –ratio of water vapor in the
air to the maximum amount the air can hold
 2 factors that
affect relative
humidity:
1. Temperature
2. Humidity
Measuring Relative Humidity
 How to measure relative humidity?
 Psychrometer – a tool to measure relative humidity;
contains a wet and a dry bulb.
1) Wet-bulb thermometer –bulb covered with wet cloth
2) Dry-bulb thermometer –bulb is dry
The difference in
temperature readings
between the
thermometers can be
used to determine the
relative humidity.
Determining Relative Humidity
Dew Point
Dew point is the temperature at
which air can no longer hold
any more water
PRECIPITATION
Relative humidity = 100%
Clouds
Clouds are groups of millions of tiny water
droplets or ice crystals
 2 Steps to Cloud formation:
1. Hot air rises
2. Air cools and condenses - water droplets form
Clouds Are Made Of…
Ice crystals – form
when water condenses
below 0 C = 32 F
Water droplets – form
when water condenses
above 0 C = 32 F
Clouds
Clouds are classified by:
1) Altitude
• Low clouds
• Middle clouds
• High clouds
*NIMBO or NIMBUS = clouds producing precipitation*
Low Clouds
Found below 2,000 meters & made of only
water droplets
Three Types:
1)Stratus Clouds
2)Nimbostratus Clouds
3)Stratocumulus Clouds
The lowest clouds
“Overcast”
Individual clouds have ill-defined edgesLayered
Cover large areas
Can block the sun
Dark stratus cloud, overcast
Produces continuous rain
Gray, rainy day
Lumpy & Low lying
Cover much of the sky
Form patches or rows of clouds with some blue
sky in-between.
Flat; don’t look like popcorn
Middle Clouds
Found between 2,000 meters and 6,000 meters
They are made of water droplets or ice crystals
Alto- indicates most middle clouds
Four Types:
1) Cumulus
2) Cumulonimbus
3) Altocumulus
4) Altostratus
White & Puffy
Usually have flat bottoms
Distinct edges
Popcorn-like; NOT flat
Big & Dark cumulus
cloud
Thunderstorm clouds
Hail, tornadoes
Tallest clouds
Usually have a anvil
shaped top because
off strong winds at
higher altitudes
Grayish, white
Layered cumulus clouds
Distinct cloud elements
patchy, and scattered OR in bands
Uniform & Scattered Coverage
Difficult to detect individual features
Grayish blue
High Clouds
Found above 6,000 m
Temperature are very cold and made of ice
crystals
Cirro- is used to indicate all high clouds
Three Types:
1) Cirrus Clouds
2) Cirrocumulus Clouds
3) Cirrostratus Clouds
Thin & feathery, wispy
White
Very high altitudes
Made of ice crystals
Thick white wisps/layers
Extremely high in the air
Distinct patchy or wavelike appearance
.
Sheet-like & cover entire sky
Composed of ice crystals
Usually transparent (sun/moon can easily be seen)
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Water in the Air
Precipitation – solid or liquid water that falls
to Earth’s surface from clouds
More common =
Rain & Snow
Less common =
Sleet & Hail
Recap!!!
What is weather? What does it greatly
depend on?
Weather is the condition of the atmosphere at
a certain time and place. It depends greatly on
the amount of water in the air.
What is the difference between humidity and
relative humidity?
Humidity is the amount of water vapor in the
air. Relative humidity is the ratio of the
amount of water vapor in the air to the
maximum amount the air can hold.
A
B
Stratus
Cumulus
Which two main cloud shapes are seen in this
picture?
C
D
Cirrus
Cumulus
Which two main cloud shapes are seen in this
picture?
Cirrus
Clouds
Stratus
Clouds
Recap!!!
What does nimbo or nimbus mean for clouds?
It produces precipitation
What does stratus mean for clouds?
They are layered
What are each of the following clouds made of?
1) Low Clouds
1) Water droplets only
2) Middle Clouds 2) Water droplets/ice crystals
3) High Clouds 3) Ice crystals only
What are the two most common forms of
precipitation?
Rain and snow
Air Masses
Air Masses – a very large volume of air that
has a certain temperature & moisture content
Fronts – the place where two or more air
masses meet
When air masses
meet, the less
dense air mass
rises over more
dense air mass.
Since warm air is
less dense, warm
air masses usually
rise above cold air
masses
Fronts
 The four main kinds of fronts are:
1) Cold Fronts
2) Warm Fronts
3) Occluded Fronts
4) Stationary Fronts
Cold Fronts
Cold air mass moves under a warm air mass
and pushes the air up
Cold air mass replaces warm air mass
Moves quick & brings heavy precipitation
Warm Fronts
Warm air mass moves in over a cold air mass
leaving an area.
Warm air mass replaces cold air as it moves
away
Bring light rain & followed by clear, warm
weather
 Cold Front = cold air mass pushes warm air mass away
 Warm Front = warm air mass replaces a cold air mass
Occluded Fronts
Warm air mass is caught between 2 cold air
masses.
 Brings cool temperatures, rain, and snow
Stationary Fronts
Cold air mass & a warm air mass move toward
each other.
 Neither has enough energy to push the
other away.
 Remain in same place causing cloudy weather
 Occluded Front = warm air mass is trapped between two cold air
masses, which move together and push war air away
 Stationary Front = air masses stay in one place
Air Pressure and Weather
Cyclone – air mass formed that has lower pressure than
surrounding air.
 Air rises and cools
 Forecast: Clouds can form & cause rainy or stormy
weather
Anticyclone – air mass that has higher pressure than the
surrounding air
• Air sinks and gets warmer
• Relative humidity decreases
• Forecast: Warm sinking air brings dry, clear weather
Cyclones and anticyclones can affect each other. Air
moving out from anticyclones move towards areas of low
pressure. This movement can form a cyclone.
Recap!!!
 What does air mass depend on?
Water Content (Moisture) & Temperature
 What are the four different kinds of
fronts?
Cold Fronts, Warm Fronts, Occluded Fronts,
Stationary Fronts
Recap!!!
 What is the difference between Cyclones
and Anticyclones?
•Cyclones  lower pressure than surrounding air, so
air cools as it rises.
•Anticyclones  higher pressure than surrounding air
& warms as it sinks.
 How are front and air masses related?
A front is where two or more air masses meet.
Therefore, without air masses…there would be no
fronts.
Severe Weather
 What are the different types of severe
weather?
1. Thunderstorms
2. Hurricanes
3. Tornadoes
Thunderstorms
 Thunderstorms –intense storm with strong
winds, heavy rain, & thunder
 Many happen along cold fronts
Conditions required:
1) Warm, moist air near Earth’s surface
2) Unstable area of the atmosphere
Thunderstorms
 4 Steps to a Formation of a Thunderstorm
1. Body of cold air is found above body of warm
air
2. Warm air rises & cools as it mixes with cool air
3. When warm air reaches its dew point, water
vapor condenses & forms cumulus clouds
4. If warm air keeps rising, clouds may become
dark cumulonimbus clouds
What is the difference between a severe
thunderstorm watch and a severe
thunderstorm warning?
• A severe thunderstorm watch is issued by the
National Weather Service when the weather
conditions are such that a severe thunderstorm is
likely to develop.
• A severe thunderstorm warning is issued when a
severe thunderstorm has been sighted or indicated
by weather radar. At this point, the danger is very
serious and everyone should go to a safe place, turn
on a battery-operated radio or television, and wait
for further information
Thunderstorm Safety
•Avoid touching metal objects
•Install metal lightening rods
•Find a low area away from trees
•Stay away from water
Number of days each year in which thunderstorms
occur. Note maximum (90) in Florida.
Lightning
5 steps to the formation of lightning:
1. As a cloud grows bigger, parts of it
begins to develop electrical charges
2. Upper parts = positively charged
3. Lower parts = negatively charged
4. When one charge gets big enough,
electricity flows from one area another
or the ground
5. These electrical currents are lightning
http://www.usatoday.com/weather/resources/graphics/2008-05-18-lightning-flashgraphic_N.htm
Thunder
3 Steps to the Formation of Thunder
1. When lightning moves through the air, the
air gets very hot and expands rapidly
2. As it expands, it makes air vibrate
3. These vibrations release energy in the
form of sound waves, called thunder.
Tornado
• Rapidly whirling, funneled-shaped cloud that
reaches down from a storm cloud to touch E’s
surface
• Most frightening & disruptive storm
• Usually brief, touching the ground approx. 15
minutes or less
• Wind speeds may reach 480 km/hr (230 mph)
• Counterclockwise in both the Northern and
Southern Hemispheres
Tornadoes
1. Wind moving in 2 directions causes a lyaer of
air in the middle to being to spin like a roll of
toilet paper.
2. The spinning column of air is turned to a
vertical position by strong updrafts of air in
the cumulonimbus cloud. The updrafts of air
also begin to spin.
3. The spinning column of air moves to the bottom
of the cumulonimbus cloud and forms a funnel
cloud.
4. The funnel cloud becomes a tornado when it
touches ground.
Tornadoes
 Fewer than 1% of thunderstorms produce
tornadoes
 About 75% of the world’s tornadoes happen
in the United States
 Most happen in the spring and early summer
Most last for only a few minutes
 Average wind speeds are between 120 km/h
and 180 km/h
Tornado Alley
 Where is Tornado Alley??
An area of the United States where
tornadoes are most frequent
Between Rocky & Appalachian Mountains
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Tornado Alley
Why is Central United States considered
Tornado Alley?
90% of tornadoes hit this region of the US
Cold, dry air from Canada/Rocky Mountains
meets warm, moist air from the Gulf of Mexico
and hot, dry air from deserts
They are combined with atmospheric
instability to produce intense tornadoes
Tornado Alley
The core of Tornado Alley consists of:
 Texas Panhandle
 Oklahoma
 Kansas
 Nebraska
 South Dakota
 Colorado Eastern Plains
 Ohio Valley
 The southern Great Lakes
 The Tennessee Valley
 The lower Mississippi Valley
A map Tornado Alley outlined. This area has been defined as
the area where there has been approximately at least one
significant tornado per day, per decade.
Tornado Safety
•Listen for watches & warnings
•Basement level, or to the middle room on
ground floor
•Stay away from windows
•Lie in a ditch
Hurricanes
 Hurricanes – large, rotating tropical weather
systems
 Wind speeds over 120 km/h
 Can be 160 km to 1,500 km in diameter and
travel thousands of miles
Most powerful storms on Earth
 Also called  typhoons & cyclones
 Form over warm, tropical oceans
Hurricanes
7 Steps in the Formation of a Hurricane
1. Begins as a group of thunderstorms
traveling over topical ocean waters
2. Winds traveling in two different directions
meet and cause the storm to spin – Tropical
Depression
Tropical Depression
3. Because of the Coriolis Effect, they
rotate counterclockwise in the North
and clockwise in the South
Tropical Storm
• 4. A group of thunderstorms form and
produce a large, spinning storm –
Tropical Storm
Hurricane Formation continued
• 5. The hurricane forms as the storm
gets stronger
• 6. It will continue to grow as long as it is
over warm ocean water
• 7. When they move over cold water or
land they lose energy which is why they
aren’t common in middle of continents
Hurricane Damage
A storm surge is a
rise in sea level
that happens
during a storm
A storm surge
from a
hurricane can
be up to 8 m
high
Hurricane Safety
•Evacuate
•Move into the interior room and stay away from
windows
Recap!!!
Why do thunder and lightning usually happen
together?
When lightning moves through the air, the air gets very hot
and expands rapidly. This makes the air vibrate which
releases energy in the form of sound waves, called thunder.
Where do most tornadoes happen. Why?
90% of tornados occur in Tornado Alley. This is a
result of when the cold, dry air from Canada and the
Rocky Mountains meets warm, moist air from the Gulf
of Mexico and hot, dry air from deserts, which
combines with atmospheric instability to produce
intense tornadoes.
Recap!!!
Which two conditions are required for
thunderstorms to form:
1) warm, moist air near Earth’s surface
2) an unstable area of the atmosphere
What type of energy powers hurricanes? How
do hurricanes lose energy?
They are powered by solar energy. The sun’s energy causes
ocean water to evaporate. When they move over cold water
or land they lose energy.
What is a storm surge? Which type of severe
storm is most commonly associated with this?
A storm surge is a rise in sea level that happens during a
storm. A hurricane can cause a storm surge to be 8 m high.