Download Water in the Atmosphere

Water in the Atmosphere
The Water Cycle
The Water Cycle
 The movement of water between the
atmosphere and Earth’s surface is called the
water cycle.
 Three steps of the water cycle:
– Evaporation
– Condensation
– Precipitation
How Does the Water Cycle Work?
 Density makes the water cycle work.
 Water is more dense than air in its liquid form so it collects
together via gravity into bodies of water such as the
oceans and lakes etc.
 As it is heated by the sun (driving energy source) and
evaporates, it becomes a gas and that gas is less dense
than air.
 As it rises to very high altitudes in the troposphere, it cools
due to lower temperatures and becomes more dense than
air again.
 Once enough water vapor has condensed into clouds and
the clouds move away from the source of water, the water
vapor will eventually condense enough that the cloud is
more dense than the air.
 Precipitation will fall in the form of rain, sleet, snow or hail.
Density of Air in the Atmosphere
Phase Change Diagram Review
Evaporation
 Evaporation is the process by which water
molecules in liquid water escape into the air
as water vapor.
 Water molecules.
 Water molecules i a liquid state are more
dense than air.
 When water is heated by the sun, the liquid
moves faster and becomes less dense
causing it to rise into the atmosphere.
Examples of Evaporation
 Humidity-the measure of the amount of
water vapor in the air
 Humidity depends upon temperature.
Examples of Evaporation
 Relative Humidity-the percentage of water vapor
that is actually in the air compared to the
maximum amount of water vapor the air can hold.
 Warm air is less dense and can “hold” more water
than cold air that is more dense.
Warm air has
more space to
fit water vapor
Cold air has
less space to fit
water vapor
Measuring Relative Humidity
 Psychrometer-Instrument used to measure
relative humidity
 Made of 2 thermometers
– Wet-bulb thermometer
– Dry-bulb thermometer
Measuring Relative Humidity
 Wet-bulb thermometer-bulb has a cloth
covering that is moistened with water
 Psychrometer is spun by the handle and air
blows over both thermometers
 Wet-bulb thermometer is cooled by
evaporation
 Compare readings on the two thermometers
How to Determine Relative Humidity
1. Measure the dry bulb temperature.
2. Measure the wet bulb temperature.
3. Calculate the difference between the dry bulb
and wet bulb temperatures.
4. Find the dry bulb temperature on the table.
5. Find the difference between the wet and dry bulb
temperatures on the table.
6. The relative humidity will be at the intersection of
the row and column.
Relative Humidity Table Examples
 Example #1:
 Suppose the relative humidity of the air is
51% and the reading of the dry-bulb
thermometer is 200C. What is the reading of
the wet-bulb thermometer?
 Answer: 14 oC
Relative Humidity Table Examples
 Example #2:
 The readings on a sling psychrometer are
14 0C for the dry-bulb thermometer and
12 0C for the wet-bulb thermometer. What is
the relative humidity?
 Answer: 79%
Example of Evaporation
 Sweat is your body’s natural way of
cooling down.
 When sweat reaches the surface of our
skin, it then evaporates, which cools us
down!
 Heat is transferred from your skin to the
water which makes you feel cooler.
Example of Evaporation
 Outdoor drying: When you hang socks
outside to dry, you're using energy from the
Sun and the wind to evaporate and disperse
the water they contain.
Condensation
Condensation
Condensation
 As warm, moist air rises in the
atmosphere, its temperature begins to
decrease.
 Molecules of water vapor in the air
become liquid water in the process of
condensation.
How Clouds Form
 Clouds form when water vapor in the air
condenses to form liquid water or ice
crystals
 Two conditions required for condensation:
– Cooling of the air
– Presence of particles in the air
The Role of Cooling
 Cold air holds less water vapor than warm
air
 Dew Point-the temperature at which
condensation begins
 Dew point above freezing=water vapor
forms water droplets
 Dew point below freezing=water vapor may
change into ice crystals
Dew Point and Humidity
 The higher the dew point, the more moisture
in the air
– Example: Key West, Florida has high dew
points so it feels “sticky” or humid
 The lower the dew point, the less moisture
in the air
– Example: Death Valley, California has low dew
points so it feels dry
The Role of Particles
 For water vapor to condense, tiny particles must
be present so the water has a surface on which to
condense
 In cloud formation, most of these particles are salt
crystals, dust from soil, and smoke
 Water vapor also condenses onto solid surfaces,
such as blades of grass or window panes
The Role of Particles continued
 Liquid water that condenses from the air
onto a cooler surface is called dew
 Ice that has been deposited on a surface
that is below freezing is called frost
Fog
 Clouds are classified by their shape and their altitude
 Different types of clouds are associated with different types
of weather
 Clouds that form at or near the ground are called fog
 Fog often forms when the ground cools at night after a
warm, humid day
 The ground cools the air just above the ground to the air’s
dew point
 The next day the heat of the morning sun “burns” the fog
off as its water droplets evaporate
Cloud Seeding
 During drought conditions, a method called
cloud seeding is used to produce
precipitation.
 Tiny crystals of silver iodide and dry ice are
sprinkled into clouds from airplanes.
 Water vapor can condense on the particles
of silver iodide and dry ice cools the water
vapor from the air.
 As a result, clouds form.
Precipitation
 Precipitation is any form of water that falls
from clouds and reaches Earth’s surface
 Not all clouds produce precipitation
 For precipitation to occur, cloud droplets or
ice crystals must grown dense enough to fall
through the air
Types of Precipitation
 Common types of precipitation include:
– Rain
– Sleet
– Freezing rain
– Snow
– Hail
Rain
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
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Most common form of precipitation
Drops of water 0.5 mm in diameter
Drizzle-drops of water less than 0.5 mm
Mist- drops of water smaller than drizzle
Sleet
 Raindrops fall through a layer of air that is
below 0oC
 Ice particles smaller than 5 mm in diameter
Freezing Rain
 Raindrops falling through cold air near the
ground do not freeze in the air, instead they
freeze on a cold surface
Snow
 Water vapor in a cloud that is converted
directly to ice crystals is called a snowflake
Hail
 Round pellets of ice larger than 5 mm in
diameter
 Form during thunderstorms
 Difference between sleet and hail is the size
of the ice pellets and where the ice pellets
formed
Measuring Precipitation
 Rain gauges- open-ended can or tube that
collects rainfall
– The amount of rainfall is measured by dipping a
ruler into the water or by reading a marked
scale
 Measuring sticks