Moisture, clouds, and precipitation

BELL RINGER:1.What are the physical properties of a fluid? 2.Do you think air is fluid? Why or why not?

3.How could you possibly demonstrate that air is fluid?

HOMEWORK: Mind Map on Cloud formation DUE 12/5-6

Children’s Book

Weather

Driven by the power of the SUN, weather is a series of CYCLES.

Water evaporates, rises, cools, and falls as rain, only to evaporate once again.

The sun rises and sets every day, with the air warming and cooling in response, and the cycle endlessly repeating.

Weather Continued

Weather, in all its cycles and clashes, arises from a simple fact: the sun heats some parts of the Earth more than others.

Earth is a globe with a 23.5 degree tilt, therefore the sun’s rays heat the Earth unevenly.

Energy moves from HIGH concentrations to LOW concentrations.

The sun heats the equator and it begins to flow out towards the poles seeking to equalize the difference in TEMPERATURE. This is what fuels weather!

Water in the atmosphere

• When it comes to understanding atmospheric processes, water vapor is the most important gas in the atmosphere.

Water and its Changes

A change in state requires an energy transfer, usually in the form of heat.

Solid to Liquid Melting is and example. Latent heat is the energy absorbed or released during

a change in state, but does not produce a temperature change. Ice = 0 degrees and Ice water = 0 degrees and (hidden heat-important in weather).

Water and its Changes

Liquid to Gas Evaporation is the process of changing a liquid to a gas. Condensation is the process where a gas, like water

vapor, changes to a liquid, like water.

Solid to Gas Sublimation is the conversion of a solid directly to a gas

without passing through the liquid state. Deposition is the conversion of a vapor directly to a solid.

Frost is an example of deposition.

Humidity

Humidity is a general term for the amount of water vapor in air. Meteorologist use RELATIVE HUMIDITY and DEW POINT

to measure the amount of water vapor in the air.

Saturated Air is saturated when it contains the maximum

quantity of water vapor that it can hold at any given temperature and pressure.

When saturated, warm air contains more water vapor than cold saturated air.

Relative Humidity

Relative humidity is a ratio of the airs actual water-vapor content compared with the amount of water vapor air can hold at that temperature and pressure.

To summarize, when the water-vapor content of air remains constant, lowering air temperature causes an increase in relative humidity, and raising air temperature causes a decrease in relative humidity.

Dew Point

Dew point is the temperature to which a parcel of air would need to be cooled to reach saturation.

Dew forms at night because temperatures near the ground cool below the dew point causing condensation.

A high dew point (above 65) makes it feel sticky. High dew point indicates high humidity and a low dew point indicates low humidity.

Measuring Humidity

A hygrometer is an instrument to measure relative humidity.

A psychrometer is a hygrometer with dry- and wet-bulb thermometers. Evaporation of water from the wet bulb makes air temperature appear lower than the dry bulb’s measurement. The two temperatures are compared to determine the relative humidity. Remember the drier the air, the faster the evaporation so the cooler the temperature of the wet bulb.

Cloud Formation

Condensation may form dew, fog, or clouds.

Air compression and expansion When air is compressed, the motion of the gas

molecules increase and the air temperature rises. The opposite is true when it expands.

Adiabatic Temperature Changes

Temperature changes that occur even though heat energy isn’t added or subtracted is adiabatic temperature changes.

When air is allowed to expand, it cools, and when it is compressed, it warms.

Expansion and Cooling Dry adiabatic rate is the rate of cooling or heating

that applies only to unsaturated air. Wet adiabatic rate is the rate of adiabatic

temperature change in saturated air.

Adiabatic Temperature Changes

Ascending air encounters lower pressure, rises, expands and cools.

Descending air encounters higher pressure, compresses, and heats

This rate in unsaturated air is the dry adiabatic rate.

Adiabatic Temperature Changes

When a parcel of air rises high enough it will reach its dew point. This is when condensation occurs.

The latent heat is released. This works against the cooling process and causes a slower rate of cooling called the wet adiabatic rate.

Dry adiabatic rate is from the surface to the condensation level. The wet adiabatic rate begins at the condensation level.

Cloud Formation

Clouds form when air is cooled to its dew point or the temperature, if the air is cooled, it reaches saturation.

Air can reach saturation in a number of ways. The most common way is through lifting.

As a bubble or parcel of air rises it moves into an area of lower pressure. As this occurs the parcel expands.

This requires energy, or work, which takes heat away from the parcel. So as air rises it cools. This is called an adiabatic process.

Cloud Formation

Since cold air can hold less water vapor than warm air, some of the vapor will condense onto tiny clay and salt particles called condensation nuclei.

The reverse is also true. As a parcel of air sinks it encounters increasing pressure so it is squeezed inward. This adds heat to the parcel so it warms as it sinks.

Warm air can hold more water vapor than cold air, so clouds tend to evaporate as air sinks.

Lifting of Air

Four mechanisms that can cause air to rise are orographic lifting, frontal wedging, convergence, and localized convective lifting.

Orographic Lifting Orographic lifting occurs when mountains act as

barriers to the flow of air, forcing the air to ascend. The air cools adiabatically; clouds and precipitation

may result.

Frontal Wedging A front is the boundary between two adjoining air

masses having contrasting characteristics.

Convergence Convergence is when air flows together and rises.

Localized Convective Lifting Localized convective lifting occurs where unequal

surface heating causes pockets of air to rise because of their buoyancy.

Stability

Stable air tends to remain in its original position, while unstable air tends to rise.

Stability Measurements Air stability is determined by measuring the

temperature of the atmosphere at various heights. The rate of change of air temperature with height is

called the environmental lapse rate.

The density difference in air causes stable air to stay in its same position and unstable air tends to rise.

The rate at which the air cools with increasing elevation is called the environmental lapse rate.

Air is stable when the temperature decreases gradually with increasing altitude.

The most stable air conditions occur when the air temperature increases with height-a temperature inversion

Temperature Inversion

This frequently occurs on clear nights as the result of radiation cooling.

It is created when the ground and the air immediately above the ground cool more rapidly than air higher above the ground.

There is very little vertical air movement.

Air is unstable when the air close to the surface of Earth is a lot warmer than the air above the surface. The air turns over with the warm air below rises and is displaced by the colder air from above.

Condensation

Condensation of air occurs when it is saturated. This occurs when air is cooled to its dew point or when water vapor is added to the air.

There generally needs to be a surface for water to condense upon. These are called condensation nuclei.

These are important because without them the relative humidity would need to be above 100% for clouds to form.

Cloud Types

Clouds are classified on the basis of their form and height. Here are some of the forms…

Cirrus (cirrus = curl of hair) are clouds that are high, white, and thin.

Cumulus (cumulus = a pile) are clouds that consist of rounded individual cloud masses.

Stratus (stratus = a layer) are clouds best described as sheets or layers that cover much or all of the sky.

Cloud Types-height

High Clouds Cirrus clouds are high, white, and thin. Cirrostratus clouds are flat layers of clouds. Cirrocumulus clouds consist of fluffy masses.

Middle Clouds Altocumulus clouds are composed of rounded masses that

differ from cirrocumulus clouds in that altocumulus clouds are larger and denser.

Altostratus clouds create a uniform white to gray sheet covering the sky with the sun or moon visible as a bright spot.

Cloud Types

Low Clouds Stratus clouds are best described as sheets or layers that

cover much or all of the sky. Stratocumulus clouds have a scalloped bottom that appears

as long parallel rolls or broken rounded patches. Nimbostratus clouds are the main precipitation makers.

Fog

There is physically no difference between a fog and cloud.

Fog is defined as a cloud with its base at or very near the ground.

Fog Caused by Cooling As the air cools, it becomes denser and drains into low

areas such as river valleys, where thick fog accumulations may occur.

Fog Caused by Evaporation When cool air moves over warm water, enough moisture

may evaporate from the water surface to produce saturation.

Precipitation Formation

In order for precipitation to form cloud droplets must grow in volume by one million times.

First there must be a source of moisture. The primary moisture sources in the U.S. are the Atlantic and Pacific Oceans as well as the Gulf of Mexico. Winds around high and low pressure systems (a subject of another lesson) transport this moisture inland.

Once the moisture is in place, clouds still need to form. The most effective way to do this is by lifting the air. This can be accomplished by forcing the air up and over mountains or, more commonly, by forcing air to rise near fronts and low pressure areas.

Cloud droplets and/or ice crystals are too small and too light to fall to the ground as precipitation. So there must be a process(es) for the cloud water, or ice, to grow large enough to fall as precipitation. One process is called the collision and coalescence or warm rain process. In this process, collisions occur between cloud droplets of varying size, with their different fall speeds, sticking together or coalescing, forming larger drops.

Finally the drops become too large to be suspended in the air and they fall to the ground as rain. The other process is the ice crystal process. This occurs in colder clouds when both ice crystals and water droplets are present. In this situation it is "easier" for water vapor to deposit directly onto the ice crystals so the ice crystals grow at the expense of the water droplets. The crystals eventually become heavy enough to fall. If it is cold near the surface it may snow, otherwise the snowflakes may melt to rain.

The Bergeron process is a theory that relates the formation of precipitation to super cooled clouds, freezing nuclei, and the different saturation levels of ice and liquid water.

Forms of Precipitation

Rain and Snow Sleet is the fall of clear-to-translucent ice. Hail is produced in cumulonimbus clouds. Hailstones begin as small ice pellets that grow by collecting

super cooled water droplets as they fall through a cloud.

Cloud in a Bottle

Take your bottle and rinse it and remove the label.

Cover the bottom with warm water.

Put the pressure cap on and start to pump it. Observe and record what happens to the temperature.

I will light a match and put it in your bottle. Pump until the bottle has a lot of pressure. Release the valve. Observe.

Explain what happened.

Air as a Fluid

Fold the file folder in half. I will give you a candle.

Put a little baking soda in the bottom of the beaker.

Pour about a ¼ c vinegar into the beaker.

When the fizzing stops, hold the cardboard funnel so that one end is near the flame of the candle and the other end slightly higher.

Pour the gas in the beaker down the funnel. What happens?