Download Weather

• The study of weather = METEROLOGY
• Weather is the condition of the atmosphere at
any given time or place
• Much of weather forecasting and prediction can
be accomplished by direct observation – LOOK
OUT THE WINDOW!!
• To predict weather, you must make observations
of the clouds, wind, temperature, humidity, air
pressure and precipitation
Atmospheric Composition
• The Earth’s atmosphere is made up of
many gases, but it is mostly nitrogen
and oxygen (Reference Tables)
• It also contains water vapor, dust, and
ozone – O3 (a form of oxygen which
absorbs UV rays from the sun)
• As you go higher in the atmosphere,
the amount of air and air pressure
(weight of air) decreases
Structure of the Atmosphere
• made up of four layers, each with different
characteristics (Reference Tables)
• Troposphere- lowest layer (we live in this layer).
All of Earth’s weather occurs in the troposphere.
Temperature decreases as you go higher.
• Stratosphere- above the troposphere, has strong
winds. Temperature increases as you go higher
because of the absorption of UV rays by ozone.
• Mesosphere- above the stratosphere.
Temperature decreases as you go higher.
• Thermosphere- above the mesosphere.
Temperature increases as you go higher due to
oxygen and nitrogen absorbing solar energy
ESRT PAGE 14
ATMOSPHERIC VARIABLES
• things that can be measured and
change from moment to moment
– Temperature
– Air Pressure
– Wind Speed and Direction
– Water Content & Humidity
– Cloud Cover
– Precipitation
– Others (dust, transparency, pollen, etc)
TEMPERATURE
• Measures the average kinetic energy of
molecules.
• Heat enters the atmosphere from the
sun as solar radiation (Insolation)
• Average Kinetic Energy- kinetic energy
is the energy of motion
• The temperature of any material is an
average of all of the kinetic energieshence Average Kinetic Energy
3 ways to Measure temperature:
• Fahrenheit
– Water freezes at 32°
– Water boils at 212°
• Celsius
–
–
–
–
AKA Centigrade (100 levels)
Water freezes at 0°
Water boils at 100°
Makes more sense and is easier to make a
thermometer
• Kelvin
– Same scale as Celsius but 0 means zero energy
– No degrees mark for Kelvin
– 0K means that all atomic vibrations stop.
Air Pressure
• the weight of the air in a certain area
• Sinking air pushes down more- cold air is usually
high pressure
• Rising air pushes down less- hot air is usually low
pressure
• Air pressure is measured using an instrument
called a barometer
• It is measured in inches (height of the mercury
column in the barometer), or with the metric
units called millibars
• Use your reference tables for converting
inches to millibars
• Air pressure drops with increasing
altitude (less air)
• Weather maps show sea level air
pressure
• It ranges from 960mb – 1050mb
• Isobars are lines on the weather
maps which connect areas of equal
air pressure
Changes in Air Pressure
• Temperature- warm air is lighter than
cold air!
• Humidity- humid (moist) air is lighter
than dry air!! Water vapor (H2O) weighs
less than the N and O gases that it
pushed out
• Winds are caused by the unequal
heating of the atmosphere (remember
that temperature causes changes in air
pressure)
Water Vapor in the Air
• The amount of water vapor in the air
depends on the air temperature
• The warmer the air, the more water vapor
it can hold (think of those hot, humid
summer days)
• The actual amount of water vapor in the
air is called specific humidity
Relative Humidity
• the amount of water vapor in the air,
compared to how much water vapor it
could hold at that temperature
• Relative humidity is a percent
measurement
• Air with 100% humidity is said to be
saturated
– Precipitation will occur
Finding Relative Humidity
• use an instrument called a hygrometer
• One particular type of hygrometer is called a sling
psychrometer
– This instrument is made of two thermometers attached two
a handle
– One thermometer has a piece of gauze which is the soaked
with water - This is called the WET BULB
• The other thermometer is called the DRY BULB
– These thermometers are swung in the air for a few minutes,
and then the temperatures are recorded (take the wet bulb
temperature first)
• The wet bulb thermometer should have a lower
temperature, since it removes heat from the
thermometer as water evaporates from it
• The two temperatures are then used to look up the
relative humidity on a chart (Reference Tables)
Interactive Site http://www.eram.k12.ny.us/education/sctemp/12345/1269020
014/rhdp.swf
Condensation and Dew
• remember that air’s ability to hold moisture
decreases as the temperature decreases
• As the air cools, its relative humidity increases
• When the relative humidity reaches 100%
(saturated), the water comes out of the air as
liquid droplets
• The temperature at which this will occur is called
the dew point
• **The dew point can be found using the same
method for relative humidity, except use the dew
point chart (Reference Tables)
Condensation and Dew
• Condensation can only occur if the water has
something to condense on
• Tiny particles in the air on which water condenses is
called condensation nuclei
• This can be dust, salts, chemicals from burning fuel,
etc.
• * Water vapor that condenses on the ground is
called dew (think of walking across wet grass in the
morning)
• If the dew freezes on the ground, it is called frost
• Water vapor condensing in the air forms clouds
• If the cloud forms close to the ground, it is called fog
Interactive Site http://www.eram.k12.ny.us/education/sctemp/12345/126902
0469/rhdp.swf
Try These…
Dry Bulb
Wet Bulb
R Humidity DPT
26°C
20 °C
57%
17 °C
6 °C
5 °C
86%
4 °C
12 °C
8 °C
57%
4 °C
18 °C
17 °C
14 °C
14 °C
0 °C
-3 °C
91%
100%
45%
16 °C
14 °C
-9 °C