Moisture, Clouds, and Precipitation

• Lesson 17 discusses moisture, cloud formation, and precipitation.
• The lesson explains how water changes into the atmosphere and how it is measured.
• The lesson describes how clouds are formed.
• The lesson identifies different forms of precipitation.

Recall Task 1

• Carbon dioxide, methane, nitrous oxide, water vapor, ozone, and fluorinated gas are examples of greenhouse gasses.
• Greenhouse effect keeps the planet warm enough for life to exist.
• Without the greenhouse effect, Earth would be frozen and uninhabitable.
• Human activities have increased the amount of greenhouse gasses strengthening the greenhouse effect.

Task 2

• 4 Pic 1 Word tasks
• Humidity is the amount of water vapor in the air.
• Dew point is the temperature at which air becomes saturated with water vapor.
• Moisture is water in liquid form or trace amounts of water in a substance.
• Moisture content refers to the amount of water in a substance.

Different States of Water

• Water in the atmosphere exists in three phases: solid, liquid, gas.
• Water changes its phase when energy is absorbed or released by it in terms of heat.
• Evaporation: Molecules of liquid absorb energy causing their bonds to break and turn into gas.
• Condensation: Molecules of water vapor release energy and turns to liquid state.
• Melting: Absorption of energy transforming from solid to liquid.
• Freezing: Releasing of energy transforming from liquid to solid.
• Sublimation: Solid to Gas, absorbs energy, dry ice is used as smoke during plays.
• Deposition: Gas to solid, releases energy, frost in freezer compartments.

Humidity

• Humidity is the amount of moisture (water vapor) in the air.
• Hygrometer measures moisture, metron measures.
• Psychrometer is a type of hygrometer.
• When water evaporates, cooling occurs at the water below the surface and vapor pressure increases.
• Rate of evaporation is determined by the temperature of air.
• When vapor pressure increases, the rate of condensation increases
• When the rate of evaporation and condensation are equal, air is said to be saturated.
• The temperature at which the rate of evaporation and condensation is equal, is the dew point.
• Absolute humidity measures how humid is a parcel of air.
• Since volume of air changes frequently, this is hard to measure.
• Absolute humidity = mass of water vapor (grams) / volume of air (cubic meters)
• Mixing ratio = mass of water vapor / mass of dry air
• Relative humidity is the ratio of the actual water vapor content compared to the amount required for saturation.
• Relative Humidity = amount of water in air / amount of water vapor needed to reach saturation * 100

Cloud Formation

• Clouds are formed through adiabatic cooling of water vapor.
• Cooling process when air is cooled or warmed because of expansion or compression.
• Expansion and compression are not due to thermal energy transfer.
• As air rises, atmospheric pressure decreases, thus expansion can take place.
• Expansion causes the air to lower its temperature (10°C/1000 meters).
• Dry adiabatic rate: rate of cooling of unsaturated air.
• Wet adiabatic rate: rate of cooling of air during condensation of moisture, this is slower than dry adiabatic.

Types of Clouds

• Clouds are classified according to form and height.
• High clouds: cirrus, cirrostratus, cirrocumulus.
• Cirrus clouds are a curl of hair, high, white and thin.
• Middle clouds: altostratus, altocumulus
• Sun/moon is visible as a bright spot with mid-level clouds.
• Low clouds: stratus, stratocumulus, nimbostratus
• Stratus clouds form sheets or layers that cover the sky.
• Cumulus clouds consist of globular individual cloud masses (cauliflower structure).
• Nimbus clouds are dark, grey clouds associated with precipitation.
• Fogs are clouds near the ground, mostly due to cooling of water vapor near the surface.

Precipitation

• Precipitation is any form of moisture that falls from air to earth.
• Coalescence is a process where large droplets of water combine with smaller droplets to precipitate and reach the ground.
• Supercooling is how ice crystals form due to freezing nuclei.
• Rain is liquid precipitation
• Snow is solid precipitation made of ice crystals.
• Sleet is rain that falls into freezing air (clear ice pellets).
• Sleet forms when snow melts slightly and then refreezes as it falls through a colder layer of the atmosphere.
• Hail is large ice pellets with irregular shape.
• Hail forms when water freezes onto a hailstone's surface.