Weather
Changes in our
Atmosphere
Why Do We Need To Know About
the Weather?
• Early humans were greatly affected by the
weather;
– Farming, hunting, and shelter to survive.
• Man is still greatly affected by the weather;
– Droughts, floods, winds, lightning, hail,
heat waves, freezing temperatures,
blizzards.
What is the Atmosphere?
• The atmosphere is a thin layer of gas, dust
and water vapor that surrounds the Earth.
– The atmosphere is held to the Earth by gravity.
– All weather happens in the atmosphere.
– The atmosphere has layers that each have
different characteristics.
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Troposphere
Stratosphere
Mesosphere
Thermosphere
What is Weather?
• Weather is the state of the atmosphere at
a particular place and time.
– Example: what it is like outside right here,
right now. “It is rainy and cold.”
What is Meteorology?
• Meteorology is the study of weather.
Electromagnetic Energy
Electromagnetic energy is energy that has the
properties of transverse waves.
Parts of waves:
Crest = top
Trough = bottom
Wavelength = distance crest to crest
Frequency = # of wavelengths per time
Electromagnetic spectrum is the entire range of
electromagnetic wavelengths.
The Electromagnetic Spectrum
Solar Energy
Stars, our Sun, produce electromagnetic
energy or solar energy.
Doppler effect is an apparent change in
wave frequency.
Red Shift = a phenomenon observed as a
source of light moves away from an
observer.
Blue Shift = a phenomenon observed as a
source of light moves away from an
observer.
Matter and Electromagnetic Energy
When electromagnetic energy comes in
contact with a material it can interact in
several ways. It can be:
bent or refracted;
reflected in a different direction;
scattered (refracted and reflected)
in several directions
absorbed or taken in
Energy Transfer in the Atmosphere
• Convection is the transfer of heat energy
by movement of fluids.
• Conduction is the transfer of heat energy
by contact (molecule to molecule)
• Radiation is the transfer of
electromagnetic energy through space in
the form of waves.
Energy Transformation
Conservation of Energy
A closed energy system is one in which no
energy from outside the system can get in
and no energy from inside can get out.
Total amount of energy does not change.
The total energy given off by an energy
source is equal to the amount absorbed
by an energy sink.
Heat vs Temperature
• Heat is the total kinetic energy of the
particles in a sample of matter.
• Temperature is a measure of the
average kinetic energy of the particles
in a sample of matter.
Heat Terms
One calorie is defined as the quantity of heat
needed to raise one gram of water by one
degree Celsius.
Specific heat is the quantity of heat needed to
raise one gram of a substance one degree
Celsius.
Latent heat is the term for energy added to a
substance that does not raise the temperature of
the substance. This is heat energy being used
in a phase change.
What Variables Affect the State
of the Atmosphere?
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Temperature
Humidity
Atmospheric pressure
Wind speed and direction
Clouds and precipitation
• Note: weather is greatly affected by
water.
Temperature Variations
The “weather machine” of the Earth is
driven by uneven heating of the Earth’s
surface.
Temperature is greatly affected by the
intensity and duration of insolation.
Insolation is incoming solar radiation.
Factors Affecting Local
Temperature
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Latitude
Altitude
Closeness to large bodies of water
Amount of moisture in the air.
What is Humidity?
• Humidity is the concept of the air
holding water.
– Think of the air as a sponge with water in
the holes between the molecules of air.
What is Humidity?
• The amount of moisture in the air is
constantly changing.
• The amount of water vapor the air can
hold is called its capacity.
• When the air contains all the moisture it
can hold, at a particular temperature, it
is filled to capacity or saturated.
What is Saturation?
• Saturation = the point where the air
contains the maximum amount of water
vapor possible.
What is Relative Humidity?
• Relative humidity is the measure of the
amount of water vapor in the air at a
particular temperature compared with
the total amount of water vapor that the
air can hold at that temperature.
Calculating Relative Humidity
Relative Humidity = absolute humidity x 100
capacity
Relative Humidity (cont.)
• Warm air holds more water vapor
because the water molecules are moving
more rapidly and will not condense.
• Cold air holds less water vapor because
the water molecules are moving less and
are more likely to condense.
Relative Humidity (cont.)
• When relative humidity = 100% the air
contains all the water vapor it can possibly
hold and it said to be saturated.
• Relative humidity is measured with a sling
psychrometer, a wet versus dry bulb
instrument.
What is Dew Point?
• Dew point = the temperature at which
the air is saturated and condensation
takes place.
Moisture and Air Pressure
Air pressure, or atmospheric pressure is the
weight of the air pushing down on a
surface area.
Air pressure is measured with a barometer.
Low pressure = greater moisture capacity.
High pressure = less moisture capacity.
Altitude and Air Pressure
As altitude increases air pressure
decreases (less air above you).
Moisture and Energy Input
Evaporation is the process of changing liquid
water into water vapor in the atmosphere.
Transpiration is the process by which plants
release water vapor into the atmosphere.
Evapotranspiration refers to all the water vapor
released into the atmosphere.
Both processes require energy to change
liquid water into water vapor.
Evapotranspiration constitutes an energy
input to the atmosphere (~540 calories/gram
of water).
Factors Affecting the Rate of Evaporation
Vapor pressure is the pressure exerted by only
the water vapor in the air.
More vapor pressure = less evaporation
Amount of energy available.
More energy = more evaporation
Surface area of water.
More surface = more evaporation
Amount of moisture in the air.
More moisture = less evaporation
Dynamic Equilibrium
Dynamic equilibrium occurs when the number of
molecules changing from liquid to vapor equals
the number changing from vapor to liquid.
When the air is saturated, the amount of water
vapor evaporating and the amount condensing
are the same. This is called the saturation
vapor pressure.
Remember warm air holds more moisture so
temperature affects saturation vapor pressure.