UNIT 6 & 7: Meteorology
Unit 6 & 7: Meteorology
Exam Study Guide KEY
UNIT 6 & 7: Meteorology
Weather
1. Is the present condition of the atmosphere at any location
2. Is described by measuring temperature, humidity, pressure and winds
3. Oceans are the primary source of moisture for the atmosphere
Ozone layer of the atmosphere
1. The ozone layer protects life from ultraviolet radiation
2. Chlorofluorocarbons (CFC's) threaten to reduce the amount of ozone
3. Ultraviolet radiation can cause skin cancer and plant damage
Heat Transfer
1. Heat is transferred from an area of high
concentration to an area of low concentration
2. Methods of heat transfer
a. Conduction
i. Heat energy is transferred through
matter by the direct contact of
molecules
ii. Is most effective in solids
b. Convection
i. The transfer of heat by movement
in fluids (gases and liquids)
ii. Caused by differences in density
within the fluids
iii. Cooler fluid sinks, warmer fluid
rises
iv. Convection currents transfer heat
in the earth's atmosphere [RT14],
hydrosphere and part of the
lithosphere [RT10]
c. Radiation
i. The emission and transfer of heat energy by means of electromagnetic waves
ii. The only way that energy can travel through empty space
UNIT 6 & 7: Meteorology
Temperature in general
1. Temperature often varies in a daily cycle.
2. Less cloud cover, more sunlight, and the arrival of a warm air mass will result in warmer
temperatures.
3. More cloud cover, less sunlight, and the arrival of a cold air mass will result in cooler
temperatures.
4. Cloudy nights are usually warmer than clear nights.
To force air to rise
1. Heating, convection currents, or air moving over a mountain, or frontal boundary can cause air to
rise.
2. Rising air will cool by expansion.
3. Temperature vs. capacity to hold water vapor:
4. Warmer air can hold more water vapor than cooler air.
5. Cooler air can hold less water vapor than warmer air.
Relative humidity in general
1. Relative humidity compares the amount of moisture in the air with the amount of moisture the air
could hold at its present temperature.
2. Relative humidity is how much moisture the air has divided by how much it could hold at a
certain temp.
3. Relative humidity is usually expressed as a percent.
4. A relative humidity of 25% means the air is holding only 25% of what it could hold at a given
temp.
5. Relative humidity is 100% at the dew point.
To change relative humidity
1. If the amount of moisture remains the same, cooled air will have a higher relative humidity.
2. If temp decreases, relative humidity will increase because capacity will decrease.
UNIT 6 & 7: Meteorology
Dew point in general
1. Condensation: change in phase from gas to liquid.
2. Dew point is the temperature at which condensation occurs.
3. Dew point is the temperature at which water vapor changes into water droplets (relative humidity
is 100%).
4. At the dew point, air is holding all the moisture that it can hold.
Air temperature, dew point, and humidity
1. As the air temperature cools to the dew point,
the relative humidity approaches 100%
(saturation).
2. The smaller the difference between dew
point and temp, the higher the relative
humidity.
3. As the air temperature approaches the dew
point, precipitation becomes more likely.
To change dew point
1. If the amount of water vapor in the air
increases, dew point temp will increase (it
will be saturated at a higher temp).
2. Dew point can change if moisture is added to
or removed from air.
3. A change in temp alone will NOT change
dew point.
Sling psychrometer
1. To determine the relative humidity, use a psychrometer and RT12.
2. To determine the dew point, use a psychrometer and RT12.
3. Evaporation is a cooling process.
4. The drier the air, the more water that evaporates from the wet-bulb, the lower the wet-bulb
reading will be.
UNIT 6 & 7: Meteorology
Condensation in general
1. Condensation is a phase change of gas to liquid.
2. Cooling saturated air causes condensation.
Clouds may form if
1. Moist air rises, expands (because of less pressure), cools to the dew point, and
2. Water vapor condenses on small particles in the air (condensation nuclei).
Precipitation (PPT)
1. Precipitation depends on water being released from the atmosphere.
2. Precipitation occurs when cloud droplets or ice crystals joint together and become heavy enough
to fall.
3. Precipitation requires saturated sir, presence of condensation nuclei, colliding water droplets to
make larger drops that can fall.
4. There is a greater chance of PPT if dew point temp is close to air temp.
5. Clouds can exist without any precipitation.
Precipitation and cleaning the atmosphere
1. Precipitation cleans the atmosphere by bringing down condensation nuclei and other suspended
material.
2. Notice how clear the air may be after a storm.
UNIT 6 & 7: Meteorology
Air Pressure Review
1. Air pressure is a result of the _______________ of the atmosphere above us.
2. _____________________ measure air pressure.
3. The pressure of the atmosphere is exerted in all directions.
Weather and pressure
1. _____________________ pressure generally
indicates bad weather (warm, moist).
2. Rising pressure generally indicates __________
weather (cool, dry).
Altitude vs. pressure [ESRT 14]
1. Air pressure ________________ with an increase in
altitude.
2. Reason: At the surface there is a taller column of air
pushing down than at the upper altitudes.
Temperature vs. pressure
1. As temperature increases, pressure decreases.
2. As temperature _____________, pressure increases.
Reasons why pressure changes with a change in temp
1. As air cools, it contracts making it ________dense.
2. Cooler air is heavier and exerts _______ air pressure.
3. Heated air expands.
4. Heated air is ________ and exerts ____ air pressure.
UNIT 6 & 7: Meteorology
Humidity vs. pressure
1. As water vapor increases, pressure decreases.
2. As water vapor decreases, pressure increases.
Reasons why a change in humidity causes a change in pressure
1. Because moist air is less dense (lighter) than dry air, an increase in humidity generally results in
_______________________ air pressure.
2. Water vapor is less dense than the air (mostly oxygen and nitrogen) that it displaces (takes the
place of).
UNIT 6 & 7: Meteorology
Wind in general
1. The primary cause of winds is unequal heating of earth's atmosphere.
2. Winds generally blow from regions of high pressure to regions of low air pressure.
Land and sea breezes
1. Land and sea breezes result from the different heating rates of land and water.
2. Land heats and cools faster than water.
3. Different temperatures cause different pressures.
UNIT 6 & 7: Meteorology
Winds, Earth's rotation and the Coriolis effect
1. Winds in the northern hemisphere are deflected to their right because the Earth is rotating.
2. Therefore, winds don't blow directly from high to low pressure.
3. Satellite photos of cloud patterns show the deflection to the right.
UNIT 6 & 7: Meteorology
Wind speed and direction
1. Wind is described in both speed and direction (north wind at 5 mph).
2. Winds are named according to the direction from which they blow.
3. A north wind blows from the north.
4. Wind vanes measure wind direction.
5. An anemometer measures wind speed.
6. Wind direction is very important in weather prediction.
Wind and weather maps
1. Close isobars indicate steep pressure gradient and strong winds.
2. On a station model, more feather mean faster wind. [RT13]
3. On a station model, read wind shaft like a compass direction. [RT13]
UNIT 6 & 7: Meteorology
Movement of weather systems in the U.S.
1. Weather systems usually move from west to east.
2. Reason: the U.S. is in the belt of the prevailing southwesterlies. [RT15]
3. Given a weather map, be able to predict the movements of weather systems and predict the
weather at selected localities.
Air masses in general
1. Air masses: large bodies of air with uniform
properties of temperature and humidity.
2. At a given altitude, humidity and temp are nearly
uniform.
3. The characteristics of an air mass depend on its
geographic origin. [RT13]
4. Air masses are large bodies of air that takes on the
characteristics of the place of their origin. e.g. cT
mT cP mP [RT13]
5. Be able to use and interpret air mass symbols on
RT13.
UNIT 6 & 7: Meteorology
Fronts
1. Fronts are boundaries between
two different air masses.
2. Where two different air masses
meet they form boundaries called
fronts (cold, warm, occluded,
stationary).
3. The leading edges of air masses
(fronts) are the most likely areas
for precipitation to occur.
4. Unsettled and rainy weather often
occurs where different air masses
come into contact (at the front).
Indications of the passage of a front
1. Pressure drops when the front
passes, then rises.
2. Winds shift clockwise in N.
Hemisphere when a front passes.
3. Temperature will change with the passage of a front.
4. Isobars bend at the front (as contour lines do at stream crossings).
Forecasts
1. Most weather forecasts are based on the movement of air masses.
2. Be able to use a weather map to make predictions.
UNIT 6 & 7: Meteorology
Forecasts (cont.)
SYMBOL
DIRECTION
OF
MOVEMENT
ADVANCING
AIR
SLOPE
CLOUDS
WEATHER
COLD FRONT
Triangles
WARM FRONT
Semi-circles
Triangles points to
direction
Semi-circles point to
direction
Cold
Steep
Cumulus (puffy)
PPT just before and
after where the front
reaches the surface…
Heavy PPT for a short
time
Warm
Gentle
Stratus (layered)
Broad band of PPT
before where the front
reaches the surface…
Gentle PPT for a longer
time
UNIT 6 & 7: Meteorology
Low pressure systems (cyclones)
1. Low pressure systems (cyclones) are areas of air circulating counter-clockwise and rising at the
center in the Northern Hemisphere.
2. Be able to draw a diagram showing horizontal and vertical air movements in a cyclone.
3. Decreasing air pressure often brings warm and unsettled or rainy weather.
4. Be able to associate the movement of low pressure into an area with overcast skies and possible
precipitation.
High-pressure systems
1. Areas of high air pressure (anticyclones) show a clockwise rotation away from the center in the
Northern Hemisphere due to sinking air at the center.
2. Be able to use a diagram to interpret air movement within an anticyclone (high).
LOW PRESSURE HIGH PRESSURE
SYSTEM
SYSTEM
NAME
Cyclone
Anticyclone
CIRCULATION
Counter-clockwise
Clockwise
WINDS
Inward
Bad
Warm & Moist Air
Outward
Fair
Cool & Dry Air
WEATHER
UNIT 6 & 7: Meteorology
Tornadoes
1. Tornadoes are small, brief, but intense disturbances that usually develop over land when very
different air masses collide.
2. Tornadoes are most common in the spring in the central part of the U.S.
Hurricanes
1. Warm tropical oceans provide the energy for the development and maintenance of hurricanes.
2. Hurricanes form over tropical oceans.
3. Hurricanes develop most often in the late summer or fall when ocean waters have reached 27º C
or higher.
4. The tropics supply the key ingredients needed: wide expanses of warm ocean water, air that is
both warm and humid.
5. Hurricanes lose strength as they move over land or cool water.
6. Hurricanes usually cause the most damage when they first move over land.
7. Hurricanes often last for many days and they can be many miles in diameter.
8. Hurricanes are our most destructive storms.
Predictions
1. Records of past weather allow meteorologists to predict future weather as a probability of
occurrence.
2. Computer analysis and weather data from satellites have become crucial in making forecasts.
3. Short-term (1-3 days) weather forecasts are usually far more accurate than long-term forecasts (a
week or more).