Relative humidity

•Air Pressure: The weight of the air in the
atmosphere pressing down.
•At sea level the air pressure is
1013.2 mb (millibars) = 1 atmosphere
1 atmosphere = 14 lbs of air pressured per
1 square inch.
How is air pressure Measured?
• Barometer:
Instrument that
measures air
Air pressure units
can be in inches
and millibars.
Characteristics of High and Low
• High Pressure
– Cooler
– Dry
– Associated with clear
skies and cooler
• Low Pressure
– Warm
– Humid/moist
– Associated with
stormy, cloudy, rainy
When air pressure is falling a storm is approaching.
• In a high the winds move out and clockwise
• In a low they move in and counter clockwise.
• Wind move from high pressure to low
Wind Blows from areas of High Pressure to Areas
Of low pressure
Wind speed: The greater the difference in air pressure the faster the wind
speed. Wind moves from areas of high pressure to areas of low
This is shown with closely spaced isobars.
Isobars are lines on a map that connect points of equal air pressure
Sea Breeze:
During the day, the land warms up faster than the water.
The warm air (Low pressure) over the land rises, while the cooler air
(High Pressure air) over the water sinks down and moves in to replace
the warm air.
Land Breeze:
At night, dry land cools faster than water, Since the land cools faster
than the water, the air above the water is now warmer and less dense
than the air above the dry land. The Low pressure air over the water
rises and the high pressure air over the land sinks and blows from the
land to the sea.
How is wind measured?
Anemometer: Measures wind speed
Wind Vane: Shows the direction of the wind
The sun is the main source of energy for Earth.
The transfer of heat and light from the sun is called radiation.
Isotherms are lines on a map that connect points of equal
Air temperature.
• When water absorbs enough heat energy it
will change state and become water vapor.
• Most water is evaporated for the oceans and
large lakes.
Factors that increase the rate of evaporation
• Increased Heat: more heat more evaporation
• Increased Wind: Wind provides a steady supply of dry air
• Surface area: The more surface area exposed to the heat, more water will
• Water will evaporate faster on a warm, dry, windy day
Humidity: Refers to the amount of water vapor in the
Relative humidity (%): compares the actual amount
of water vapor in the air with the amount of water
vapor the air can hold.
Saturated Air: When the air cannot hold any more
water vapor. 100 % humidity.
* Warm air can hold more water vapor than cold air.
• When water vapor rises to cooler
temperatures in the atmosphere and
condenses back into a liquid.
• Condensation droplets stick to tiny dust
particles to form clouds.
When warm humid air
rises, expand, cool and
condense (stick) onto
condensation nuclei,
forming clouds.
Condensation Nuclei: Tiny
particles in the atmosphere that
water vapor sticks to.
As the air rises it will cool to dew
point temperature
This instrument measures Relative humidity and Dew point (see chart)
The wet bulb’s temperature decreases when you spin it because water will
evaporate and evaporation has a cooling effect.
• As more and more water vapor condenses
the water droplets become heavier and fall
back to Earth.
• Precipitation can be in the form of rain,
sleet, ice or snow.
• Precipitation can help remove
pollutants from the atmosphere.
Skills you need to know
1. Read and interpret a station model.
2. Know how to read a wet and dry bulb
thermometer to determine dew point and relative
humidity. Know how to read dew point and
relative humidity chart.
3. Draw isobars/isotherms
4. use the temperature and air pressure conversion
5. Read and interpret the Layers of the atmosphere
The following pages show some of these charts or
you can find them in your workbook.
How to use a psychrometer
Step 1: Read and record the dry bulb
Step 2: Spin psychrometer
Step 3: Read and record the wet bulb.
Step 4: Find the difference.
Step 5: Match the difference and the dry bulb
on the chart to find relative humidity and
dew point.
There are four main layers of the atmosphere. These layers have
been divided by temperature changes.
Facts to know: The troposphere contains most water vapor.
The ozone layer is in the stratosphere. The ozone absorbs ultra
violet radiation from the sun.