Transcript The Atmosphere - Valhalla High School

The Atmosphere
Structure of the Atmosphere
 The atmosphere is divided into four layers
– Troposphere
 The lowest layer
is where most of the
weather occurs
– Stratosphere
– Mesosphere
– thermosphere
Weather
 The short-term (a few hours or days)
condition of the atmosphere at a given
location
– Temperature, sky conditions,
precipitation, atmospheric
pressure, humidity, wind
speed, and wind direction
 Meteorologists are
scientists who study
and predict the weather
Air Temperature
 In the daily cycle, temperature
is usually lowest in the early
morning and warmest at mid-afternoon
 In the season cycle, winters are generally
cold, while summers tend to be hot
 Short term factors such as cloud cover and
regional weather systems affect
temperatures
– Clouds reduce daytime temperature by reflecting
sunlight back into space
– At night, clouds help hold heat energy to Earth
Air Temperature
 Measured with a thermometer
– A bulb that contains liquid that
expands into a narrow,
calibrated neck when it is
heated and moves down the
neck when the temperature
decreases
 When meteorologists record
official air temperature, the
thermometer is kept in a special
weather shelter to protect the
instruments from direct sunlight
Temperature Scales
 A temperature of zero
on the Fahrenheit
scale is the
temperature of a
mixture of equal parts
ice, water, and salt
 The freezing point of
water is what sets the
zero point on the
Celsius (centigrade)
scale
 The point at which all
particle motion stops
is defined as zero on
the Kelvin scale
Air Pressure
 Is caused by the weight of the atmosphere
 Above each square inch of Earth’s surface
is a column of air the weighs 14.7 pounds
Measuring Air Pressure
 A barometer is an instrument used to
measure air pressure using the dense liquid
metal mercury
 Meteorologists measure
air pressure in millibars
 Standard sea level pressure
is 1013.2 millibars
 On a weather map, isobars
connect places that have the
same air pressure
Air Pressure Factors
 If air is cooled, it contracts and becomes
denser
– This causes pressure to rise
 If air is heated, it expands and becomes less
dense
– This causes pressure to fall
 Humid air is lighter than dry air
– This is because water
molecules are lighter
than the gasses they
displace in the air
Moisture in the Atmosphere
 When the air is holding as much moisture as
it can, the air is saturated
– The air’s ability to hold water vapor depends
upon the temperature
– The warmer the air, the more moisture the air
can hold
 The dew point is the temperature
to which the air must be cooled to
become saturated
– If the temperature falls below the
dew point, condensation occurs as
water vapor changes to liquid water
Measuring Moisture
in the Atmosphere
 Meteorologists use a sling psychrometer and a
dew-point temperature table to determine the dew
point
– The psychrometer consists of two thermometers mounted
side by side which can be swung through the air
– One thermometer measures the air temperature
– The bulb of the other thermometer is covered by a wet
cloth
– As the thermometers are swung through the air,
evaporated cooling causes the wet-bulb thermometer to
register a lower temperature
– When you subtract the wet-bulb temperature from the drybulb temperature, you can use the dew-point table to
determine the dew point
Psychrometer
Relative Humidity
 Compares how much moisture the air is
actually holding with how much moisture it
could hold if the air were saturated
– It is expressed as a percent of saturation
 Air is saturated if it is holding all the moisture
it can hold at its present temperature
 Determined with a psychrometer and a
relative humidity table
The Wind
 Wind is heat flow by convection within
the atmosphere
 Winds are the result of uneven heating of
the Earth’s surface
– This uneven heating
causes differences
in air pressure
to develop
The Wind
 Winds always blow from areas of high
pressure to areas of low pressure
– Winds blow fastest where the gradient in air
pressure is greatest, where the isobars are close
together
Measuring the Wind
 To measure the wind, you need to determine
both the wind speed and the wind direction
 Wind speed is measured with an
anemometer
– The cups catch the wind,
causing it to spin
 Wind direction is indicated
by a wind vane, which
points into the wind
The Coriolis Effect
 The Earth’s
rotation causes
winds to curve
– to the right in
the Northern
Hemisphere
– to the left in
the Southern
Hemisphere
The Coriolis Effect
 Winds move in a clockwise outward spiral
around high-pressure systems
 Winds move in a counterclockwise inward
spiral around low-pressure systems
The Coriolis Effect
Zones of
Convergence and Divergence
 Rising warm, moist air at the center of the
low causes winds and air masses to blow
into the low-pressure system
 The rising air cools, which causes cloud
formation and precipitation
 The descending air turns a high-pressure
system into a single mass of cool, dry air
that spreads across the surface of Earth
Cloud Formation
 Clouds form when rising air is cooled below
its dew point
 Tiny particles
called
condensation
nuclei in the
air allow a
cloud to form
Precipitation
 Rain and snow are the most common forms
of precipitation
 Drizzle is small raindrops that fall slowly
 Sleet is a partially frozen mixture of rain and
snow that occurs when the temperature is
just above freezing
 Hail is in the form of ice balls, which usually
occurs in violent thunderstorms
– Hailstones begin as snowflakes that start to
melt and gather more moisture as they fall
Precipitation