Download Relative Humidity

Weather Instruments
•
Used to measure the 5 major parts of
weather observation
1. Atmospheric pressure
2. Humidity
3. Precipitation
4. Wind
5. Temperature
Tools: Atmospheric Pressure
• Barometer:
• Measures air pressure, millibars or inches of mercury
Tools: Humidity
• Hair Hygrometer:
• Uses hair to measure humidity,
hair expands and contracts in
response to water vapor content in
the air.
• (Sling) Psychrometer:
• Difference between wet bulb and
dry bulb used to determine relative
humidity.
Tools: Precipitation
• Rain Gauge:
• Simple funnel with marks
Tools: Wind
• Anemometer – Wind Speed
• km per hour
• mi per hour
• Wind Vane – Wind Direction
• Cardinal units or numerical
Measuring
Upper-Atmospheric
Conditions
• Radiosonde:
• Carried aloft by a
helium balloon.
• Measures upper-atmospheric. conditions:
Relative humidity, air pressure, air
temperature.
• Information sent back to scientists via radio…
waves.
Satellites
• Carry cameras
• Upper level Cloud speed, direct.
• Hurricane observation
• Infrared reading of cloud tops
Not weather satellites, but still cool
• 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
• To predict weather, you must make observations
of the clouds, wind, temperature, humidity, air
pressure and precipitation
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)
• is an average of all of the kinetic energies- hence
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 dense air and brings high pressure
• Hot air is less dense air and brings 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
• Air pressure drops with increasing
altitude (less air)
• Weather maps show sea level air
pressure
• It ranges from 960mb – 1050mb
Air in the Atmosphere Relationships
Air
Temperature
Density
Air Pressure
Evaporation
Warm
Less Dense
Low Pressure
Cold
More Dense
High Pressure Low
evaporation
High
evaporation
Molecular
reaction
Loose
molecule
particles
Moving fast
Tightly
packed
molecules
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)
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
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
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