Water in the Atmosphere Earth Science Chapter 23

Atmospheric Water: States Water in the atmosphere can exist as: solid (ice), liquid (rain), or gas (water vapor). Types of changes: Solid to liquid: melting Liquid to gas: evaporation Gas to liquid: condensation Latent Heat: heat required to convert solid into a liquid or vapor, or a liquid into a vapor, without change of temperature. Water may change from one state to another by adding or taking away energy. Most water vapor comes from lakes, oceans, marshes and glaciers. Water vapor is spread throughout the troposphere by convection currents & wind.

Evaporation The molecules of liquid water are always in motion Molecules evaporate when they have sufficient energy to escape the water’s surface. As temperature increases, the molecules gain in energy and evaporation increases. Water and other liquids absorb heat energy from their surroundings when they evaporate. Then the molecules left behind have less energy so that makes evaporation a cooling process.

Condensation and Dew Point Water vapor that condenses on surfaces is called dew. Water vapor that condenses in the air forms clouds or fog. Saturation: occurs when the medium (air) will not allow more of the solute (water vapor) to be added. As temperature decreases, air cannot hold as much water vapor and it begins to condense on surfaces. The temperature at which saturation occurs is called the dew point, and dew begins to form. Condensing water vapor molecules release heat energy to their surroundings and slow down the rate at which the air cools.

Condensation Requires Nuclei Condensation can not occur unless water vapor has something to condense upon. The tiny particles in the air on which water vapor condense are called condensation nuclei. Condensation nuclei are usually substances such as salt, sulfate particles or nitrate particles.

Condensation Requires Cooling For water vapor to condense, air must be cooled below its dew point. Air can lose heat in the following ways: Contacting a cooler surface Radiating heat Mixing with colder air Expanding when it rises

Sublimation Change of a solid directly into vapor. Requires dry air & below freezing temps.

Light and Water Interactions, pg 576

https://www.eoas.ubc.ca/courses/atsc113/sailing/met_concepts/10-met-local-conditions/10f-optical-phenomena/ https://en.wikipedia.org/wiki/Mirage

Absolute Humidity & Capacity The amount of water vapor actually present in the air is called the absolute humidity. Determined by dividing the mass of water vapor by a volume of air (grams of water/ cm3 of air). The capacity of air for “holding” water vapor depends on the temperature of the air. The warmer the air, the more water vapor it can hold. The air’s capacity for holding water vapor roughly doubles for every rise of 11°C.

Specific Humidity (not in book) is also known as moisture content or humidity ratio Is the ratio of water vapor mass to the total air mass (wet & dry) Saturation: occurs when the solvent or medium will not allow more of the solute to be added. Saturated air occurs when the absolute humidity equals the air’s capacity. Saturated air has a humidity of 100%.

Relative Humidity Relative humidity compares the actual amt of water vapor in air to the maximum amt of water vapor the air can hold at that temperature. Relative humidity is calculated using this equation: (Specific humidity/ Capacity) x 100% = Relative humidity Demo: RH vs Absolute Humidity

Finding Relative Humidity Instruments used to measure relative humidity are called hygrometers. They measure the affect of the water vapor on instruments. Experimentation is combined to form a data table that is then used to calculate relative humidity. Psychrometers: use wet & dry bulbs. Diff tween the temps is used to find RH. No difference = 100% RH or saturated air. Making a psychrometer

Dew and Frost from Contact Condensation usually happens when air is cooled below its dew point. If the temperature is above 0°C, dew usually forms on surfaces that cool quickly. Clear nights have greater cooling & heavier dew. If the temperature is below 0°C, water vapor condenses into a solid called frost.

Radiation Fogs When a surface layer of air a few hundred meters thick is cooled below the dew point, water vapor condenses in the entire layer & forms fog. Radiation or ground fogs form under conditions similar to those that form dew.

Advection Advection fogs result when warm, moist air blows over cool surfaces.

A cloud is a large collection of very tiny droplets of water or ice crystals that are so small and light they float in the air. Clouds form when air above the surface cools below its dew point. If air movement is horizontal clouds form in layers and are called stratiform clouds. If air movement is vertical, cumuliform clouds grow in piles. Origin of Clouds

Temperature of Clouds Clouds that form at temperatures above freezing are made of water drops. Below freezing, clouds are mixtures of snow crystals & super cooled water. Super cooled water is water that has cooled below 0°C without freezing. Below -18°C, clouds are entirely snow and ice crystals.

Temperature Changes in Clouds; Dry adiabatic lapse rate Air moves upward because of buoyancy Air is buoyant because it is warmer than surrounding air. Dry-adiabatic lapse rate: Cooling is caused because air is expanding due to less pressure, altho no heat energy is gained or lost. Rising dry air cools at a rate of 1°C per 100 m

Moist-Adiabatic Lapse Rate Air rising in a cloud does not cools as fast as rising dry air does. The condensing water in the cloud releases heat to the air which makes it cool more slowly. The rate of temperature change of a rising or sinking saturated parcel of air (a cloud) is called the moist-adiabatic lapse rate. In a cloud, air cools at 0.6°C per 100 m.

Clouds with Vertical Development Clouds with vertical development form when rising air currents are buoyant or lighter than the surrounding air. Rising air in the cloud is warmer than the surrounding air because saturated air cools slower than dry air even tho it gets cooler as it rises. If a shallow layer of air is unstable, cumulus clouds form. If a deep layer of air is unstable, cumulonimbus clouds may form which produce severe thunderstorms.

Cloud Names & Meanings Three names represent the three main cloud types. Cirrus – thin, feathery or tufted clouds that form so high they are made entirely of ice crystals Stratus – low sheets or layers of clouds Cumulus – formed by vertically rising air currents, they are piled high in thick, puffy masses

More Cloud Types All other cloud names are combinations of the original three types. Cirrostratus – high, thin, smooth or fibrous sheets of ice-crystal clouds, indicate rain or snow Stratocumulus – layers of round puffs, often cover the entire sky in the winter Cirrocumulus – small globular patches of cloud made of ice crystals

Cumulus Clouds Rising buoyant air forms cumulus clouds. The clouds have flat bases and billowy tops. They form over heated ground that cause the raising air to remain buoyant. The flat cloud base is the area where the water vapor begins to condense. The height of the cloud is called the condensation level where the temperature is equal to the dew point.

Lifting Condensation Level The lifting condensation level is the level the air must reach for condensation to begin. It is calculated by dividing the difference in ground air temperature and the dew point by 0.8°C and multiplying by 100. Meteorologists use this to calculate the highest possible cloud tops.

Layer Clouds Layer clouds form in stable air where motions are mainly horizontal. Stable air has a lapse rate that is smaller than the moist-adiabatic lapse rate. Clouds can form in stable air in two ways: The air can be forced slowly upward to its condensation level or, Layer clouds form if radiation or mixing cools a layer of air to its dew point

Even More Cloud Names and Meanings The prefix “alto” (high) and the word nimbus (rain cloud) are also used to describe clouds. Altocumulus – look like stratocumulus clouds, but the puffs appear smaller because they are higher up Altostratus – stratus clouds that appear at a higher level, they appear gray or bluish and produce no halo around the sun or moon. Nimbostratus – dark, gray layers of cloud that produce steady rain

Precipitation Precipitation is the falling of any form of water from the air to Earth’s surface. Precipitation occurs when cloud droplets grow heavy enough to fall to Earth. Raindrops can form in two ways: Warm-cloud processes Ice processes

Warm-cloud Process In the warm cloud process: Tiny droplets form by condensation The drops grow by bumping into other droplets The bigger droplets fall faster and capture smaller droplets. Mixing of air from different parts of the cloud results in droplets of different sizes

Ice Processes Droplets can also form by ice processes. Temperatures in the upper layers of clouds are below freezing. Super-cooled water evaporates faster than ice and the water vapor condense on the ice crystals. When ice crystals get heavy enough they begin to fall and capture smaller crystals on the way down.

Forms of Precipitation Precipitation comes in many forms including: Drizzle – small drops close together & fall slowly Raindrops – larger drops that are farther apart and fall faster Snow –six-sided ice crystal clumps, may melt as they fall Sleet – raindrops falling thru freezing air & form into pellets of ice Hail – a frozen raindrop that forms in cumulonimbus clouds and grows larger the longer it falls due to up & down motion

Measuring Precipitation Rainfall is measured by an instrument called a rain gauge. Snowfall is measured in inches and tenths of an inch and a measuring stick is typically used. The rain equivalent is determined by melting a definite depth of snow. On average 10 inches of snow = 1 inch of rain and varies depending on the type of snow. Check out the NRCS (usda.gov) SNOTEL site https://www.nrcs.usda.gov/wps/portal/nrcs/mt/snow/

Location of Precipitation Precipitation occurs in every part of the world. The differences in rainfall between two locations depends upon how warm air is and how far it rises, & proximity to large bodies of water or large forest areas. Rainy areas on Earth occur where air rises in large quantities.

Rainy Areas Areas that typically get a lot of rain include: Windward side of mountain ranges. Storm areas of all kinds Areas favored by global wind belts

Dry Areas of Earth In areas of sinking air, the air is warmed by compression and thus precipitation does not occur. Sinking air occurs: On the leeward sides of mountains High-pressure areas Areas where global wind belts diverge or blow away from an area.

Rainmaking Two methods of rainmaking: Method #1: Cold, solid C02 pellets are dropped into a super-cooled cloud. The pellets form ice crystals until they are heavy enough to fall. Method #2: Artificial ice nuclei are put into the cloud. Precipitation again forms through the ice processes method.

Acid Clouds and Acid Rain Acid rain is low pH rain that forms from sulfate & nitrate oxides released into the air. Cloud droplets that form on these nuclei are very acidic and can cause damage to soils and plant life. Acid rain also destroys microorganisms & plankton in lakes & streams because the water becomes to acidic, destroying the food web. Acid rain also damages rock, concrete, metals, paints, and plastics used for buildings.