Meteorology Part 2:
Weather Variables
Earth Science
Golodolinski/Black
2009
Part II: Weather Variables
1. Temperature
2. Air Pressure
3. Relative Humidity
Weather Variables
 1. Temperatures
– The measure of the
average kinetic
energy
– How fast the
molecules move
– Instrument used to
measure temperature:
Measured
in
°F
Fahrenheit
°C
Celsius
°K
Kelvin
 Thermometer
– ESRT: Temperature
conversion chart on p.
13
1. Temperatures
 Shown on a weather map with:
– Isotherms- Lines that connect places of equal
temperature
ESRT: Temperature conversion chart
on p. 13
& Chart Blank p. 183
ESRT: Temperature Conversion
Chart on p. 13
Chart Answers p. 183
ESRT: Temperature Conversion
Chart on p. 13
Temperature Chart Blank p. 183
ESRT: Temperature Conversion
Chart on p. 13
Temperature Chart Answers p. 183
Weather Variables
2. Air Pressure
 The weight of Earth’s Atmosphere
 Changes depending on the temperature
 Instrument used to measure pressure:
– Barometer
 Measured in inches and milibars
2. Air Pressure
 Shown on a weather
map with:
– Isobars- Lines that
connect places of
equal barometric
pressures
2. Air Pressure
 Mercury barometer
– As the air pressure pushes on
the surface of the mercury in
the dish, the mercury travels up
the tube.
– As pressure increases, the
mercury rises in the tube.
 Cool air sinks- causes higher
pressure
– As the pressure decreases, the
mercury sinks out of the tube.
 Warm air rises- causes lower
pressure
ESRT:
Pressure
Conversion
Chart p. 13
ESRT: Pressure Conversion Chart
p. 13
Blank Chart p. 184
ESRT: Pressure Conversion Chart
p. 13
Answer Chart p. 184
2. Air Pressure
 State the relationship
between altitude and air
pressure.
– As the altitude increases,
the pressure decreases
 Draw the relationship on
the graph.
Weather Variables
3. Relative Humidity
– A ratio between the amount of moisture is in the
atmosphere and how much moisture the
atmosphere can hold
– Measured in %
– When the air is holding as much water vapor as it can,
the air is saturated. When saturated, warm air contains
more water vapor than cold saturated air.
– When the air is saturated, the relative humidity is 100%
To summarize, when the water-vapor content of air remains
constant, lowering air temperature causes an increase in
relative humidity, and raising air temperature causes a
decrease in relative humidity.
3. Relative Humidity
 Temperature & Relative
Humidity
– The warmer the temperature
is, the more moisture it can
hold
 State the relationship between
temperature and relative
humidity:
– As the temperature increases,
relative humidity decreases
 Draw the relationship on the
graph.
3. Relative HumidityInsturements
3. Relative Humidity
 Dew Point Temperature
– The temperature in which the air is saturated
– 100% relative humidity
Determining Relative Humidity and
Dew Point Temperatures
 Dry bulb
– Air temperature
 Wet bulb
– Temperature an air parcel cooled by
evaporation of water (wet cloth)
 When given the wet bulb and dry bulb
temperatures, you can determine the dew
point temperature and relative by follow
directions provided and using ESRT p. 12
ESRT: Dew Point Temp. p. 12
ESRT: Relative Humidity p. 12
Example 1 p. 186
Example 2 p. 186
p. 187 chart and #1-5
Practice Dew Point & Relative Humidity p. 187
Practice Dew Point & Relative Humidity p.
187 Answers
Practice Dew Point & Relative Humidity p.
187
#1-5
 Condensation
– Gas to liquid
– Change of phase from
water vapor (gas) to
liquid vapor (water)
– Examples:
 Water on cold glass of
water, water on mirror
after a shower, dew on
grass, fog, clouds
– Evaporation
 Liquid to gas
3 Things Needed for Condensation
to Occur
1. Water vapor must be present
2. Air must be separated (relative humidity
100%)
3. Condensation nuclei (ex. dust particles)
Density of Air
 Warm air rises because it is less dense
 Cold air sinks because it is more dense
Formation of Clouds
 Warm moist air rises
 Air expands and cools due to the dew
point
 Air becomes saturated
 Water droplets form on dust particles
 Clouds consist of water droplets and ice
crystals
Cloud Classification
Adiabatic Cooling
 The cooling of a parcel of air as it rises
through the atmosphere
 Dry adiabatic lapse rate
– rate of cooling or heating that applies only to
unsaturated air.
– dry air cools faster
 Wet adiabatic lapse rate
– rate of adiabatic temperature change in
saturated air.
– moist air cools slower
Cloud Formation by
Adiabatic Cooling
Precipitation
 Cloud particles (any form of water) too
heavy to remain suspended in the air fall
to Earth from a cloud
– Examples: rain, hail, sleet, snow, freezing
rain
– What does precipitation do for the environment?
 Cleans the air
– ESRT p. 13 Present Weather
Forms of Precipitation
The type of precipitation that reaches Earth’s
surface depends on the temperature profile in the
lower few kilometers of the atmosphere.
Rain & Snow
In meteorology, the term rain means drops of water that
fall from a cloud and have a diameter of at least 0.5 mm.
At very low temperatures (when the moisture content of
air is low) light fluffy snow made up of individual sixsided ice crystals forms.
Sleet is the fall of clear-to-translucent ice.
Hail is produced in cumulonimbus clouds.
ESRT p. 13 Present Weather
Wind
 The horizontal movement of air
 Caused by the uneven heating of Earth’s
surface
 Differences in air temperature cause
differences in air pressure
 The greater the difference in air pressure,
the faster the wind
Wind
 Named by the direction in which they
come from
– Ex. North wind comes from the North
 Isobars
– Connect places of equal barometric
pressure on a weather map
Wind Instruments
 Anemometer
– Measures wind speed
 Wind vane
– Determines wind
direction
Sea Breeze
 Water heats up slower
than land:
–
–
–
–
High specific heat
Cooler temperatures
Air sinks
*HIGH pressure*
 Land heats up faster
than water:
–
–
–
–
Low specific heat
Warmer temperatures
Air rises
*LOW pressure*
Land Breeze
 Water cools down
slower than land (stays
warmer)
– High specific heat
– Warmer temperatures
at night
– Air rises
 Land cools down faster
than water
– Low specific heat
– Cooler temperatures at
night
– Air sinks
Sea and Land Breezes
Coriolis Effect
 The deflection of winds and ocean currents
caused by the rotation of Earth
 Deflection is to the right in the Northern
Hemisphere and to the left in the Southern
Hemisphere
Weather Factors Associated with
Different Pressure Areas
 High Pressure
–
–
–
–
Cool/ cold air
Air sinks/ goes down
Air moves outward
Clockwise
– No clouds
– No precipitation
 Low Pressure
–
–
–
–
Warm air
Air rises
Air moves inward
Counter clockwise
– Clouds
– Precipitation likely
ESRT: Planetary Winds p. 14
Planetary Winds
p. 191 Weather Changesadobe
Weather Changes p. 191
Weather Changes
Air Masses
 Large region of the atmosphere with
uniform temperature and humidity
 ESRT p. 13: Air Masses
Air Masses Table
Air Masses Map
Fronts
 The boundary between 2 air masses
 ESRT p. 13 Front Symbols
Cold Front
 Cold air pushes the
warm, moist air upward
 Cold air is located
behind the front
 The greater the
differences in
temperatures, the more
likely there will be a
major storm
 Usually pass quickly
 Brings colder but clear
weather conditions
Warm Front
 Warm air gently rolls
over the colder air
 Warm air is located
behind the front
 Conditions are
usually cloudy and
rainy for several
hours
 Usually pass slowly
 Brings warmer but
rainy weather
conditions
Occluded Front
 Occurs when a cold
air mass overtakes a
warm mass and
overtakes another
cold air mass
 Precipitation is
possible but not
definite
 Very slight
temperature change
Stationary Front
 Notice no arrows to
show direction in the
weather map symbols
 Stationary means the
front is not moving
 Final direction of the
movement is difficult to
predict
 Winds are blowing in
opposite directions on
each side of the front
 Clouds can last for days
Fronts are usually associated
with…




Clouds
Precipitation
Change in temperature
Change in wind direction
p.195 Station ModelsAdobe
Station Models
 On a station model,
barometric pressure is
ALWAYS written in a
three-digit format
 Converting from
millibars:
– Drop the 9 or 10 in the
front of the number and
loose the decimal point
Millibars
Station
Model
1009.3 mb
093
984.2 mb
1024.2 mb
Station Models
 Converting from the station
model format to milibars
 First:
– If the first number on the
station model is 0-4, place a
10 in front of the number
– If the first number on the
station model is 5-9, place a
9 in front of the number
 Second:
– Place a decimal point
between the last 2 numbers
p. 195
Station
Model
Millibars
146
1014.6 mb
457
986
ESRT: Weather Map Symbols
Weather Stations
Draw a weather station using the
following information:
Need to know:
Dew Point & Air Temperature
 The closer the air temperature is to the dew
point temperature, the greater the chance
for precipitation.
Determining Weather Station Values
6 stations on p. 197; Weather Map practice p. 199-203 #1-31; More
questions p. 204-207 #1-22