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Weather Factors
Table of Contents
Energy in Earth’s Atmosphere
Heat Transfer
Winds
Water in the Atmosphere
Precipitation
Weather Factors - Energy in Earth’s Atmosphere
Energy From the Sun
• Electromagnetic wave – waves that transfer
electric and magnetic energy through the vacuum
of space.
– Most of the energy from the sun travels to Earth in
the form of visible light and infrared radiation. A small
amount arrives as ultraviolet radiation.
– Radiation – the direct transfer of energy by
electromagnetic waves.
Weather Factors
Energy From the Sun
• Visible light – includes all of the
colors of the rainbow. Each of
which has it’s own wavelength (λ).
– Red = longest λ; violet = shortest λ
• Infrared radiation –
electromagnetic waves with
wavelengths that are longer than
visible light but shorter than
microwaves.
– Felt as “heat”
Weather Factors
Energy From the Sun
• Ultraviolet radiation –
Electromagnetic waves
with wavelengths that are
shorter than visible light
but longer than x-rays.
– Can cause sunburns and
skin/eye cancers
Weather Factors - Energy in Earth’s Atmosphere
Energy in the Atmosphere
– Some sunlight is absorbed or reflected by the
atmosphere. The rest passes through to the
surface.
Weather Factors
Energy in the Atmosphere
• Ozone layer of the atmosphere absorbs most UV radiation
• Water vapor and CO2 absorb some infrared (heat)
• Scattering – reflection of light in all
directions.
– Occurs mostly by clouds
– Gas scatters shorter λ (blue) more than
longer λ (red)
• Reason the sky looks blue and
sunsets/sunrises look orange/red
(you are looking through a greater
thickness of gases)
Weather Factors - Energy in Earth’s Atmosphere
Energy at Earth’s Surface
• When Earth’s
surface is heated, it
radiates most of the
energy back into the
atmosphere as
infrared radiation.
• Much of this energy
is held by the
atmosphere,
warming it.
Weather Factors
Energy at Earth’s Surface
• Greenhouse effect – the process by which heat is
trapped in the atmosphere by gases that form a
“blanket” around Earth.
– natural process that keeps Earth’s atmosphere at a
temperature that is comfortable for most living
things
Weather Factors - Energy in Earth’s Atmosphere
Links on Energy in Earth’s Atmosphere
• Click the SciLinks button for links on energy
in Earth’s atmosphere.
Weather Factors
Thermal Energy and Temperature
• Remember….all matter is made of atoms.
– Atoms combine to form molecules
– Molecules move around, but how much they
move depends on how much energy they have.
– The average amount of energy of motion of each
particle of a substance is it’s temperature.
• Temperature – a measure of how hot or cold an
object is compared to a reference point.
• Thermal energy – the total energy of motion in the
particles of a substance.
Weather Factors - Heat Transfer
Thermal Energy and Temperature
• The iced tea is cold, so its molecules move slowly. The
herbal tea is hot, so its molecules move faster than the
molecules in the iced tea.
Weather Factors
Thermal Energy and Temperature
• Temperature is a major factor effecting
weather.
• Thermometer – an instrument used to
measure temperature.
– Glass tube containing mercury or colored
alcohol
– Liquids expand when heated and condense
when cooled
– Measured in “degrees” on two scales
• Celsius – (0° freezing, 100°boiling)
• Fahrenheit – (32° freezing, 212° boiling)
Weather Factors - Heat Transfer
Converting Units
• Temperatures in weather reports are usually given in
Fahrenheit scale, but scientists use the Celsius scale.
Temperature readings can be converted from the Fahrenheit
scale to the Celsius scale using the following equation:
• If the temperature is 68ºF, what is the temperature in degrees
Celsius?
ºC = 20ºC
Weather Factors - Heat Transfer
Converting Units
• Practice Problem
• Use the equation to convert the following temperature from
Fahrenheit to Celsius.
• 35.0ºF
• 1.67ºC
Weather Factors - Heat Transfer
Converting Units
• Practice Problem
• Use the equation to convert the following temperature from
Fahrenheit to Celsius.
• 60.0ºF
• 15.6ºC
Weather Factors - Heat Transfer
Converting Units
• Practice Problem
• Use the equation to convert the following temperature from
Fahrenheit to Celsius.
• 72.0ºF
• 22.2ºC
Weather Factors - Heat Transfer
How Heat Is Transferred
• Heat - the transfer of thermal energy from a hotter object
to a cooler one.
• Heat is transferred in three ways: radiation, conduction,
and convection.
Weather Factors
How Heat Is Transferred
• Radiation - the direct transfer of energy by
electromagnetic waves
– Ex. Sunlight
• Conduction – the direct transfer of thermal
energy from one substance to another that it
is touching.
– Ex. Metals
• Convection – the transfer of thermal energy
by the movement of a fluid.
– Ex. Boiling water
Weather Factors
How Heat Is Transferred
Radiation, conduction, and convection work together to
heat the troposphere.
Weather Factors
How Heat Is Transferred
• Convection currents – the
circulation of a fluid as it
alternately heats up and cools
down.
– Controls much of the weather in the
troposphere
– Ground heats air molecules.
– They expand and rise.
– After they move away from the heat
source, they cool, condense, and fall.
Weather Factors - Heat Transfer
Links on Heat Transfer
• Click the SciLinks button for links on heat
transfer.
Weather Factors - Heat Transfer
Heat Transfer
• Click the Video button to watch a movie
about heat transfer.
Weather Factors
What Is Wind?
• Wind – the horizontal movement of
air from an area of high pressure to
an area of lower pressure
– Caused by differences in air
pressure which is caused by
unequal heating of the
atmosphere
– Warm air is less dense and rises
– Cool air is more dense and sinks
Weather Factors
What Is Wind?
• Winds are described by their direction and speed.
– The direction tells you which way the wind is coming from
• Ex. South wind blows from the south to the north
– A wind vain points into the wind, showing the direction the
wind is coming from
• Anemometer – an instrument used to
measure wind speed.
– has three or four cups mounted at the ends
of spokes that spin on an axle
– Force of wind turns the axle and a meter
measures wind speed.
Weather Factors
What Is Wind?
• Wind blowing over the body feels cool/cold
because it is removing body heat.
• Wind-chill factor – measure of cooling
combining temperature and wind speed.
– Weather reports may state: “The temperature
outside is 20 degrees Fahrenheit. But with a wind
speed of 30 miles per hour, the wind-chill factor
makes it feel like 1 degree above zero.”
Weather Factors
Local Winds
• Local winds – winds that blow over short distances.
– Caused by the unequal heating of Earth’s surface
within a small area
• Sea breeze – the flow of cooler air from over an
ocean or lake toward land.
– Caused by land heating faster than water
Weather Factors
Local Winds
• Land breeze – the flow of air from land to a
body of water.
– Caused at night by land cooling faster than water
Weather Factors
Global Winds
• Global winds – winds
that blow steadily from
specific directions over
long distances.
– Caused by unequal
heating over a large area
– Due to sun’s rays striking
the equator more directly
than the poles
Weather Factors
Global Winds
• Temperature differences between the equator
and the poles produce giant convection
currents in the atmosphere.
– Air pressure tends to be lower at the equator due
to warmer temperatures and higher at the poles
due to cooler temperatures.
– Surface winds tend to blow from the poles to the
equator
– High altitude winds tend to blow from the equator
to the poles…creating global winds.
Weather Factors - Winds
Coriolis Effect
• Coriolis effect – the
change that Earth’s
rotation causes in the
motion of objects that
explains how winds curve.
– Winds in the Northern
Hemisphere curve
toward the right.
– Winds in the Southern
Hemisphere curve
toward the left
Weather Factors - Winds
Global Wind Belts
• A series of wind
belts circles
Earth. Between
the wind belts
are calm areas
where air is
rising or falling.
Weather Factors
Global Wind Belts
• Calm areas consist of:
– Doldrums
– Horse Latitudes
• Wind belts consist of:
– Trade winds
– Polar easterlies
– Prevailing westerlies
Weather Factors
Global Wind Belts
• Doldrums – regions near the equator with little or no
wind
– Due to low pressure, more vertical movement than horizontal
movement
• Horse Latitudes – occur at about
30° north and south latitudes, the
air stops moving toward the poles
and sinks creating a belt of calm air.
– Name comes from sailors who became
stuck in these areas and had to throw
their horses overboard due to lack of
food and water.
Weather Factors
Global Wind Belts
• Trade winds – occur between
the equator and 30° North
latitude and 30° South
latitude.
– Due to the Coriolis effect,
winds from the equator to
30° North latitude tend to
blow from the northeast
– Winds from the equator to
30° South latitude tend to
blow from the southeast
Weather Factors
Global Wind Belts
• Prevailing westerlies – occur in
the mid-latitudes (30°60°north and south) where
winds blow from west to east
– Also caused by the Coriolis effect
– Generally blow from the
southwest in the northern
latitudes
– Play a big role in the weather
patterns of the United States.
Weather Factors
Global Wind Belts
• Polar easterlies – start at
60°north and south
and move from east to
west.
– The polar easterlies meet
the prevailing westerlies
around 60° latitude and
create the polar front
which plays a major role
in our weather.
Weather Factors - Winds
Global Wind Belts
• Jet streams - high-speed bands of winds occurring at
the top of the troposphere.
– generally blow from west to east at speeds of 200
to 400 kilometers per hour and travel a wavy path
Weather Factors
Graphic Organizer
Earth’s winds
include
include
Local winds
Global
winds
two types
three types
Sea breeze
Land breeze
Trade winds
Prevailing
westerlies
Polar
easterlies
Weather Factors - Winds
Global Winds Activity
• Click the Active Art button to open a
browser window and access Active Art
about global winds.
Weather Factors - Winds
Convection Currents
• Click the Video button to watch a movie
about
convection currents.
Weather Factors
Water in the Atmosphere
• Water Cycle – the continual movement of water
among Earth’s atmosphere, oceans, and land surface
through evaporation, condensation, and
precipitation.
Weather Factors
Water in the Atmosphere
• Evaporation – the process by which water molecules
in liquid water escape into the air as water vapor.
– As air cools water vapor condense into clouds
– If the water droplets in clouds become heavy
enough they fall as precipitation (ex Rain)
Weather Factors
Humidity
• Humidity – the amount of water vapor in a
given volume of air.
• Relative Humidity – the percentage of
water vapor in the air compared to the
maximum amount of water vapor that air
can contain at a particular temperature.
– Relative humidity can be measured with
an instrument called a psychrometer.
• Uses a wet-bulb and a dry-bulb
thermometer that is “slung” around. If the
wet-bulb reads lower than the dry-bulb, it is
evaporating faster indicating lower humidity
Weather Factors - Water in the Atmosphere
Determining Relative Humidity
• Relative humidity is affected by
temperature. Use the data table
to answer the following
questions. First, find the dry-bulb
temperature in the left column
of the table. Then find the
difference between the wet- and
dry-bulb temperatures across
the top of the table. The number
in the table where these two
readings intersect indicates the
relative humidity in percent.
Weather Factors - Water in the Atmosphere
Determining Relative Humidity
• Interpreting Data:
At noon, the reading on a sling
psychrometer are 18ºC for
the dry-bulb thermometer
and 14ºC for the wet-bulb
thermometer. What is the
relative humidity?
64%
Weather Factors - Water in the Atmosphere
Determining Relative Humidity
• Interpreting Data:
At 5 p.m., the psychrometer is
used again. The reading on
the dry-bulb thermometer is
12ºC and the reading on the
wet-bulb thermometer is
11ºC. Determine the new
relative humidity.
88%
Weather Factors - Water in the Atmosphere
Determining Relative Humidity
• Interpreting Data:
How did the
temperature change
between noon and
5 P.M.?
It decreased from 18
degrees to 12 degrees.
Weather Factors - Water in the Atmosphere
Determining Relative Humidity
• Interpreting Data:
How did the relative
humidity change
during the course of
the day?
It increased.
Weather Factors - Water in the Atmosphere
Determining Relative Humidity
• Drawing Conclusions:
How was the relative
humidity affected by air
temperature? Explain your
answer.
For the same amount of
water in the air, as the
temperature decreases,
the relative humidity
increases. Warm air can
hold more moisture than
cool air can.
Weather Factors - Water in the Atmosphere
How Clouds Form
• Clouds form when warm, moist air rises and cools.
Water vapor condenses on tiny particles to form
liquid water or ice crystals.
Weather Factors
How Clouds Form
• Condensation – the process by which molecules of
water vapor in the air become liquid water.
– Condensation is affected by cooling of the air and
presence of particles in the air
• The warmer air is, the more water vapor it can hold.
As the air cools water vapor condenses into liquid
water.
• Dew Point - the temperature at which condensation
begins.
– Will form water droplets or ice crystals depending
on temperature
Weather Factors
How Clouds Form
• Water vapor in the air will
condense on solid surfaces.
– Clouds form because tiny liquid
droplets condense on particles
of salt crystals, dust, and
smoke in the air.
– If condensation forms on
earth’s surface it is called dew.
– If it is forms ice crystals it is
called frost.
Weather Factors - Water in the Atmosphere
Types of Clouds
• Scientists classify
clouds into three
main types based
on their shape:
cirrus, cumulus,
and stratus.
• Clouds are further
classified by their
altitude.
Weather Factors
Types of Clouds
Cirrus Clouds
Cirrocumulus
Clouds
• Cirrus – wispy, feathery clouds
made of ice crystals that form at
high levels.
– form only at high levels, above
about 6 kilometers, where
temperatures are very low
– have feathery “hooked” ends are
sometimes called mare’s tails
• Cirrocumulus clouds – look like
rows of cotton balls
– indicate that a storm is on its way
Weather Factors
Types of Clouds
• Cumulus – fluffy, white clouds, usually
with flat bottoms, that look like
rounded piles of cotton.
– form less than 2 km above the
ground, but can grow in height to
Cumulus Clouds
reach 18 km high
– Short cumulus clouds indicate fair
weather
• Cumulonimbus clouds – extend high in
the atmosphere and have flat bottoms
Cumulonimbus Clouds
– Produce thunderstorms
Weather Factors
Types of Clouds
• Stratus – clouds that
form in flat layers and
often cover much of the
sky.
– Typically a dull gray color
Stratus Clouds
• Nimbostratus clouds –
stratus clouds that
thicken to produce
drizzle, rain, or snow
Nimbostratus Clouds
Weather Factors
Types of Clouds
• “Alto” – means high
– Altocumulus and Altostratus – are “middle-level”
clouds (2-6km)
– are higher than regular cumulus and stratus
clouds, but lower than cirrus and other “high”
clouds.
Altocumulus Clouds
Altostratus Clouds
Weather Factors
Types of Clouds
• Fog – clouds that form at or near the
ground
– Often forms when the ground cools
at night after a warm, humid day
– Heat of the morning sun “burns”
the fog off as its water droplets
evaporate
– More common in areas near bodies
of water or low-lying marshy areas.
– Can also form in mountains as
warm moist air moves up a slope
Weather Factors
Precipitation
• Precipitation – any form of
water that falls from clouds
and reaches Earth’s
surface.
– cloud droplets or ice crystals
must grow heavy enough to
fall through the air
– droplets grow by colliding
and combining with other
droplets
Weather Factors - Precipitation
Precipitation
• Droplets come
in many sizes.
• A raindrop has
about one
million times as
much water in it
as a cloud
droplet.
Weather Factors
Types of Precipitation
• Common types of precipitation include rain, sleet,
freezing rain, snow, and hail.
• Rain – drops of water are called
rain if they are at least
0.5 millimeter in diameter
– Most common type of precipitation
• Sleet – raindrops fall through a
layer of air that is below 0°C
causing the droplets to freeze into
ice particles
– Smaller than 5 millimeters
Weather Factors
Types of Precipitation
• Freezing rain – falls as rain, but freezes
on cold surfaces
– Can make roads and walkways slick
– Can bring down branches and power
lines
• Snow – water vapor in a cloud is
converted directly into ice crystals called
snowflakes
– have an endless number of different
shapes and patterns, all with six sides
or branches
Weather Factors - Precipitation
Types of Precipitation
• Hail - round pellets of
ice larger than
5 millimeters in
diameter are called
hailstones
– start as small pellets of
ice in cumulonimbus
clouds.
– grow larger as they are
repeatedly tossed up
and down until they
become so heavy they
fall to the ground.
Weather Factors
Types of Precipitation
• Droughts – Long periods of low precipitation.
– Caused farmers hardship for centuries
– Scientists looking into perfecting the process of
“cloud seeding” in order to produce rain during
droughts
Weather Factors - Precipitation
Measuring Precipitation
• Scientists
measure
precipitation
with various
instruments,
including:
– rain gauges
– measuring
sticks.
Weather Factors
Measuring Precipitation
• Snowfall measurements
– On average, 10 centimeters
of snow contains about the
same amount of water as
1 centimeter of rain
– Light fluffy snow contains less
water than heavy wet snow
Weather Factors - Precipitation
Links on Precipitation
• Click the SciLinks button for links on
precipitation.