Earth Science Name:
Mrs. Valenti Period:
WEATHER & THE ATMOSPHERE
A. Weather - the conditions of the variables of the atmosphere for any short period of
time
Meteorology - the study of weather and weather related variables - the variables:
1. Air Temperature
a. depends on the intensity and duration of insolation received by an area
b. measured with a thermometer
2. Air/Atmospheric/Barometric Pressure - the force exerted by the column of air overhead
a. measured with a barometer
3. Atmospheric Moisture/Humidity
a. measured with a hygrometer
4. Wind Speed and Direction
a. wind speed measured with an anemometer
b. wind direction measured with a wind vane
5. Atmospheric Transparency - a measure of how clean and clear the air is
a. depends on the number of aerosols, pollutants (natural and man-made)
and amount of cloud cover at the time the measurement is made - its value
varies as time passes
Facts about weather related variables:
1. all these variables interact in the troposphere - the lowest portion of the atmosphere
2. the relationship between these variables is very complex and poorly understood
3. the relationships are expressed as a probability of occurrence - very few occur with
100% certainty
4. accurate weather predictions are difficult to make - especially those made more than 36
hours in advance
5. changes in the weather are caused by the uneven distribution of insolation/heat
energy within the atmosphere - it varies indirectly with latitude
B. The atmosphere gains energy four ways:
1. The Greenhouse Effect - carbon dioxide and water vapor trap outgoing infrared rays
released by the Earth’s surface
2. Direct absorption of insolation - 19% mentioned in previous topic
3. Change of Phase reactions - condensation and sublimation release stored latent heat energy
into the atmosphere
4. Conduction from the lithosphere - the warm ground releases heat into the air
NOTE : all the energy gained by the atmosphere is distributed throughout the atmosphere by
convection cells
C. Adiabatic Temperature Changes - a change in the temperature of the air due to expansion or
compression with no loss or gain of heat energy
1. as air rises, it expands, its volume increases, so there is now more area to be heated by
the same amount of heat energy - temperature decreases
2. as air sinks, its compressed, its volume decreases, so now there is less area to be
heated by the same amount of heat energy - temperature increases
3. there is a 10C change in temperature for every kilometer dry air changes its
elevation
4. there is a 6C change in temperature for every kilometer moist air changes its
elevation
a. there is a 4C difference because in moist air condensation or sublimation
are releasing stored latent heat energy into the air, slowing the cooling process (-
10+4= -6)
D. Air/Atmospheric/Barometric Pressure - the force exerted by the column of air overhead
1. it averages 14.7 lbs/in2, 1013 millibars, or 29.92 inches of mercury
2. air pressure values are influenced by:
a. air temperature - as air temperature increases, air density decreases, so air pressure
decreases
b. elevation above sea level - as elevation increases there is less air overhead so air
pressure decreases
c. moisture content of the air - as air becomes more moist, heavy nitrogen molecules are
being replaced with lighter water vapor molecules so air density and air pressure
decrease
E. Wind Speed and Direction
Wind - the movement of air parallel to the Earth’s surface - the horizontal movement of air
Facts about wind:
1. winds are caused by differences in air pressure between two locations
2. winds are caused by the unequal heating of the Earth’s atmosphere - this results in
differences in air pressure
3. winds always move from areas of higher pressure towards areas of lower pressure
4. winds always move from areas of divergence towards areas of convergence
5. winds are named for where they come from, NOT for where they are going
6. surface ocean currents are caused by energy transfer from winds to water
7. wind direction in influenced by the coriolis force
a. winds are deflected to their right in the northern hemisphere
b. winds are deflected to their left in the southern hemisphere
8. wind speed depends on the pressure gradient - the numerical difference in air
pressure between two locations divided by the map distance between these
two locations
a. pressure gradients are expressed by the spacing of isobars on a weather map
1. an isobar is a line of equal air pressure
b. as the distance between isobars increases (as they get further apart) the
pressure gradient decreases and the wind speed decreases
9. Special winds
a. seabreeze/onshore wind - during the day, land heats faster than the sea -
air pressure over the warmer land falls and cooler, more dense air from
over the sea rushes in - air moves from the sea towards the land
b. landbreeze/offshore wind - during the night, land cools faster than the sea -
air pressure over the cooler land rises and this cooler, more dense air
from the land rushes out to sea - air moves from the land towards the sea
F. Planetary Wind and Pressure Belts (ESRT p. _____ )
1. at 0&60 latitude, there is generally low air pressure, low air density, so air rises
2. at 30&90 latitude, there is generally high air pressure, high air density, so air sinks
3. at 0&60 latitude, at the Earth’s surface, winds blow towards one another, these are called
areas of convergence
4. at 30&90 latitude, at the Earth’s surface, winds blow away from one another, these are
called areas of divergence
5. Prevailing winds - geographic zones on the Earth’s surface where winds tend to move in the
same direction as measured over long periods of time
a. from 30 North to 60 North (USA) the prevailing wind direction is from the
southwest - called the prevailing southwesterlies
b. weather systems in the United States tend to move from southwest to
northeast under the influence of the prevailing southwesterlies
6. Jet Streams - a narrow zone of very strong winds in the upper troposphere
a. usually located over 40 North latitude although they migrate north in summer
and south in winter
b. they tend to “steer” weather systems across the country from west to east
G. Atmospheric Moisture
Water vapor enters the atmosphere two ways:
1. evaporation - a change of phase from a liquid to a vapor at moderate temperatures
2. transpiration - plants release water vapor directly into the air
3. evapotranspiration - a combination of terms - the process whereby water vapor enters
the atmosphere
a. this requires the addition of heat energy to occur - 540 cal/gm
b. this water vapor carries a lot of potential energy (latent heat) into
the atmosphere
There are 4 factors that influence the rate at which water vapor enters the air:
1. air temperature - as air temperature increases, evapotranspiration increases
2. surface area - (how “spread out” the water is) - as surface area increases,
evapotranspiration increases
3. air circulation/wind - as air circulation increases, evapotranspiration increases
4. moisture content of the air/humidity - as humidity increases, evapotranspiration
decreases (the air is already holding a lot of water vapor and doesn’t have room for
much more)
H. Atmospheric Moisture Terminology
1. Saturation - when the air is holding all the water vapor it can hold
a. this occurs when the air temperature is cooled to the dewpoint temperature
b. at saturation, dynamic equilibrium exists and the amount of water vapor
entering the air through evaporation is equal to the amount of water vapor
leaving the air through condensation
2. Capacity - the maximum amount of water vapor the air can hold
a. it depends on air temperature, directly
3. Absolute/Specific Humidity - a measure of the actual amount of water vapor in a given
volume of air
4. Relative Humidity - the ratio between actual humidity and capacity
how much water vapor is in the air compared to the maximum
amount the air can hold
a. it’s expressed as a percent (%)
b. if the relative humidity is 65% - the air is holding 65% of what it could hold there’s
room to hold 35% more than it is holding
There are 3 factors that influence relative humidity values:
a. air temperature - as air temperature increases, capacity increases. If no new
water vapor enters the air but the air can now hold more,
relative humidity decreases
b. time of day - relative humidity is lowest in the mid-afternoon when temperatures are
highest; flip side…relative humidity is highest in the early morning when
temperatures are lowest
c. absolute humidity - if more water vapor enters the air while temperature remains the
same, relative humidity increases
5. Dewpoint Temperature - the temperature to which the air must be cooled before:
a. it becomes saturated
b. it reaches its maximum capacity
c. it reaches 100% relative humidity
d. condensation or sublimation begins
Facts about dewpoint temperature: (ESRT p. ___ )
a. it’s measured in C
b. it’s measured with a psychrometer - a combination of a dry bulb and wet bulb thermometer
c. it depends on the actual amount of water vapor in the air - the higher the dewpoint
temperature, the greater the amount of water vapor in the air - values are high in
summer and low in the winter
d. if the dewpoint temperature is above freezing and saturation occurs, condensation
will begin and liquid water droplets will begin to form in the air or dew on the ground
e. if the dewpoint temperature is below freezing and saturation occurs, sublimation
will begin and snow flakes/ice crystals will begin to form in the air or frost on the
ground
6. Dry Bulb Temperature - the same as air temperature
7. Wet Bulb Temperature - the temperature recorded by a thermometer whose alcohol reservoir
is wrapped in a piece of saturated cloth
a. it’s always lower than or equal to the dry bulb temperature
b. evaporation of water from the cloth draws heat away from the alcohol and the
level falls
c. if the air is saturated (100%RH) NO evaporation can take place and the wet
bulb temperature will be the same as the dry bulb temperature
8. Wet Bulb Depression - the difference between the dry and wet bulb temperatures
a. it’s used to calculate dewpoint temperature and relative humidity
I. Clouds and Precipitation
Cloud - a collection of tiny liquid water droplets and/or ice crystals which remain suspended in
the atmosphere
a. clouds are NOT made of water vapor
b. condensation and sublimation cause clouds to form
Condensation nuclei - tiny particles in the atmosphere around which liquid water droplets or ice
crystals collect
a. rising, moist air with condensation nuclei present is the “recipe” for clouds
Precipitation - the falling of liquid water or solid water from clouds towards the Earth’s surface
a. there are 4 types of precipitation : rain, snow, sleet, hail - fog, dew, and frost
are NOT precipitation
b. precipitation cleans the air - it enhances atmospheric transparency - it brings
condensation nuclei from the air to the Earth’s surface
c. precipitation cannot occur unless the water droplets/ice crystals grow large
enough to fall under their own weight - not all clouds bring precipitation even
though all clouds are made of tiny water droplets or ice crystals
d. without condensation nuclei there can be no precipitation - there’s no surface
for the droplets/ice crystals to collect and grow on
1) cloud seeding is an attempt to introduce artificial condensation nuclei
into clouds to make the droplets/crystals grow in size
e. precipitation is associated with the passage of a front through an area
J. Air Masses - a large body of air in the troposphere with similar temperature, pressure, and relative
humidity values throughout
a. wind speed and direction vary within an air mass
b. weather predictions are based on air mass movements
There are 2 basic types of air masses:
1. Cyclone - Low Pressure Center - Convergent Air Mass
a. winds circulate counterclockwise and towards the center of the air mass
b. air tends to be warm, moist, and cloudy
c. air rises in the centers of cyclones - warm, moist air has a low density
d. these can form at a Polar Front - where cold air from higher latitudes meets
warmer air from lower latitudes - mid-latitudes like NYS
2. Anticyclone - High Pressure Center - Divergent Air Mass
a. winds circulate clockwise and away from the center of the air mass
b. air tends to be cold, dry, and clear
c. air sinks in the centers of anticyclones - cold, dry air has a high density
K. Source Regions - those areas on the Earth’s surface where air masses form and acquire their
characteristics - there are 5 source regions that produce air masses that influence weather in NYS
(ESRT p.___ )
1. Arctic Continental - cA - extremely cold and dry - anticyclones - limited to winter in
NYS - form up near the North Pole
2. Polar Continental - cP - cold and dry - anticyclones - form in central Canada and move into
NYS
3. Polar Maritime - mP - cold and moist - form over the North Atlantic Ocean
4. Tropical Continental - cT - warm and dry - form over the SW United States and Mexico
5. Tropical Maritime - mT - warm and moist - cyclones - form over warm bodies of water like
the Gulf of Mexico
a. they can develop into hurricanes
b. hurricanes get their energy from the condensation of water vapor
c. they die out over land having lost their source of moisture
d. they follow a path called a track - can impact weather in NYS
Shortcut : A=extremely cold, P=cold, T=warm, m=moist, c=dry
Facts about source regions/air masses:
a. an air mass must stagnate (stop moving) over a source region to acquire
the characteristics of that source region
b. in the United States, air masses track from southwest to northeast under the
influence of the prevailing southwesterlies and the jet stream
c. polar and arctic air is from higher latitudes while tropical air is from lower
latitudes
L. Fronts - the boundary or interface between 2 different air masses - where 2 or more air masses meet
NOTE : whenever warm air and cold air meet, the warm air is forced to rise over the frontal interface
There are 4 basic types of fronts:
1. Cold Front - an anticyclone overtaking a cyclone - they call it a cold front because it’s the
front of the advancing cold air
a. as the front passes over you:
1) short, severe precipitation from cumulonimbus clouds
2) air temperatures fall - it IS a cold front
3) winds shift to the north from the south
b. after the front passes over you:
1) skies clear
2) the air pressure rises
3) relative humidity falls
NOTE : on a weather map, the precipitation is occurring right along the frontal line
2. Warm Front - a cyclone overrunning an anticyclone - they call it a warm front
because it’s the front of the advancing warm air
a. as the front passes over you:
1) slow, steady precipitation from stratus or nimbostratus clouds
2) air temperatures rise - it IS a warm front
3) winds shift to the south from the north
b. after the front passes over you:
1) skies remain cloudy
2) the air pressure continues to fall
3) relative humidity remains high
NOTE : on a weather map, the precipitation precedes the frontal line itself
3. Occluded Front - warm air is trapped above/between 2 cooler air masses
a. it’s associated with lots of precipitation where the air masses meet
4. Stationary Front - 2 different air masses are in contact, but neither is moving
a. the air masses are said to “stall”
b. steady precipitation right at the frontal line