Water in the Atmosphere Unit 6 Chapter 20 Water in the Atmosphere

Section 1 Atmospheric Moisture Water is always in motion, even though we can’t see the molecules moving. States of mater in which water can exist Solid- ice at temperatures below 0o Liquid-water at temperatures between 0o and 100o C Gas- water vapor at temperatures at 100o C or above

Changing Forms of Water Phase change Gas Liquid Solid Loses heat energy SolidLiquid Gas Gains heat energy

Latent Heat Latent Heat is the energy that is absorbed or released during a phase change. Condensation is the process that changes water vapor to liquid water. This process slows down the rate that air cools. Dew – in the morning you find it on the ground Fog – a cloud on the ground Frost – deposition when water vapor condenses as a solid.

Evaporation Sublimation Evaporation- process of water liquid changing into water gas. This process takes energy out of the atmosphere therefore evaporation is a COOLING process. The higher the temperature the faster the evaporation. Air can hold some water vapor Sublimation Snow banks – become smaller due to sublimation- changing a solid to a gas (no liquid phase).

Humidity Humidity is the amount of water vapor present in the air. The amount always varies. Dew point is the equilibrium between condensation and evaporation. Vapor pressure is the part of the total atmospheric pressure that is caused by water vapor. Absolute Humidity It is the mass of the water vapor contained in a given amount of air. It is expressed as the number of grams of water vapor per kilograms of air. The amount changes with temperature. The warmer the air, the more water vapor it can hold. The capacity roughly doubles for every rise in temperature of about 11o C. When the air is holding as much water as it can hold it is said to be saturated.

Relative Humidity Relative Humidity is the how close to the dew point the temperature of the air is. It is the ratio of the actual amount of water vapor in the air to the airs capacity to hold water at a certain temperature and pressure. It is the percent(%) of saturation. Saturated air has a relative humidity of 100%. Changes to RH Increase: If temp does not change and more moisture enters air If moisture remains constant but temperature goes down Decrease: If temp increases but moisture remains constant

Dew Point The Dew Point is the temperature at which air becomes saturated with water and condensation occurs. It is a measure of the amount of water vapor in the air. The more water vapor the air contains, the less the air has to cool in order for it to condense. As the air temperature drops during the night the airs ability to hold water decreases. When the air temperature reaches the dew point temperature the air is saturated and dew forms (frost if the temperatures are below freezing). Dew Point and Relative Humidity changes are related. When the air is saturated 100% (relative humidity), the Dew Point is usually high. As the air warms its ability to hold water rises therefore the dew point temperature raises, Relative Humidity drops. Relative Humidity is usually lowest in the afternoon

Measuring Humidity Meteorologists use a variety of instruments to measure the humidity in the air. Hygrometer This device measures relative humidity by using the expansion and contraction of substance due to changes in the water vapor content of the air. Leonardo da Vinci built the first crude hygrometer in the 1400s. Francesco Folli invented a more practical hygrometer in 1664.

Psychrometer It is an instrument that allows for the determination of relative humidity. There are two thermometers are mounted on a stick One thermometer is normal-the dry bulb The other has a cotton sock on it- the wet bulb The sock gets wet, the psychrometer spins evaporation occurs on the wet bulb cooling it. The drop in temperature is dependent on the amount of moisture in the air. Using the dry bulb temperature and the difference between the wet and dry bulb a chart can be used to determine the relative humidity and the dew point.

Other Methods for Measuring Humidity Dew Cell method. It is a ceramic cylinder with electrodes attached to it and treated with lithium chloride (LiCl). As water is absorbed, the cell creates electricity. Hair Hygrometer As a piece of hair curls it indicates a decrease in relative humidity. As the hair becomes longer, it indicates that it has an increase in relative humidity. Measuring Humidity at High Altitudes Meteorologists use weather balloons to send the instruments into the atmosphere to record numerous weather variables.

Section 2 Clouds and Fog A cloud is a collection of water droplets or ice crystals in the air. Dew-condensation caused by the drop in air temperature Fog- condensation that creates a cloud @ ground level

Cloud Formation Cooling & Condensation Two conditions must be met for water vapor to condense. Air must be cooled below dew point and it needs something to condense on to. Condensation nuclei are tiny particles suspended in the air. Very small like dust, ash etc. (one puff of smoke contains millions of particles). For ice to form you need ice nuclei, tiny bacteria or clay particles work best. Air must cool in order to have condensation. It can accomplish this by hitting a cooler surface, mixing with cooler air, or expanding as the air rises. Air must be saturated to condense Dew forms when air is cooled to the dew point temperature as water vapor hits any colder object for temperatures above 0o. Frost forms when temperatures fall below 0o. A killing frost will occur when ground temperatures go below -2o for several hours.

Adiabatic Cooling Adiabatic Lapse Rate Condensation Level This is the decrease in air mass temperature as it rises and cools. Adiabatic Lapse Rate The adiabatic lapse rate is the rate at which air cools as it rises. Dry air cools as it rises; -1o for every 100 meters up Wet air cools as it rises; between .5o and .9o for every 100 meters up Condensation Level The condensation level is the altitude at which condensation may occur. It is marked by the base of a cloud. Knowing the condensation levels is important for forecasting the severity of a storm. Condensation Levels Dew point decreases 2o/KM When the air temperature and the dew temperature are the same condensation occurs and clouds form.

This occurs when two bodies of moist air mix Lifting continued Mixing This occurs when two bodies of moist air mix Lifting This is when air is lifted upward due to the terrain. The air is forced to cool and condense. This can also occur when a cold air mass comes into an area with a warmer one. It will push the warmer one up and condensation occurs. Advective Cooling This occurs when the temperature of an air mass cools as it moves over a colder surface like land or the ocean. The cold surface will absorb the heat from the air and if it cools below the dew point, condensation will occur.

Classification of clouds

Clouds are classified by shape and altitude. 3Types of clouds are: Clouds continued Clouds are classified by shape and altitude. 3Types of clouds are: Stratus, Cirrus and Cumulus 3 Altitudes are Low – 0 to 2,000 Meters Middle – 2,000 – 6,000 Meters High – Above 6,000 Meters

Stratus clouds The have flat, uniform bases that begins to form at low altitudes. They are sheet like. They form when a layer of warm, moist air lies above a layer of cool air. They cover large areas, and often black out the sun. They contain very little rain. Nimbo Stratus – Rain clouds, dark and heavy Alto Stratus – Middle Altitude, little rain Cumulus Clouds They are low altitude billowy clouds that often look like cotton balls with a dark base. Hot humid days form the highest clouds. Cumulonimbus – high dark storm clouds. Thunderheads. Stratocumulus – a combination of stratus and cumulus formed at low levels. Cirrus Clouds They form in the highest level and are feathery clouds composed of ice crystals. Cirrocumulus – high altitude billowy clouds usually appearing before a snowfall or rain storm. Cirrostratus – Long thin clouds that look like transparent veils. A halo can look like it appears around the sun or moon due to the light bending through the water vapor.

Fog Fog forms when a cold surface cools the warmer moist air above it. As the water vapor condenses, tiny droplets of water form around dust (condensation nuclei) – only light winds are needed to keep these tiny particles in the air (if it is very cold, you can get ice crystals).

Radiation Fog Radiation Fog- When the night sky is clear and the ground loses heat rapidly through radiation. A stirring of the cool ground and warm air occur. The fog at ground level is colder than the air above it. Common in humid valleys- it can be very thick in the mornings.

Other Types of Fog Advection Fog- Warm moist air blows over a cool surface. Northern USA and Canada are prime areas. Forms when warm, moist southerly winds blow over snow-covered ground. Upslope fog Caused by lifting and cooling air by a slop Steam fog This is a shallow layer of fog that forms when cool air moves over an inland body of water.

Section 3 Precipitation Precipitation is water falling from a cloud. This will occur at different areas for different reasons and at different temperatures.

Forms of Precipitation Rain – liquid water Snow – crystallized water Sleet – frozen rain droplets Freezing Rain – super cooled water droplets that don’t freeze until it touches something. Hail – large frozen water droplets formed inside thunder clouds, it moves up and down inside the clouds and grows until it gets too heavy and falls. How ice crystals form: Most clouds contain temperatures usually below freezing (except shallow clouds in warm tropics). When super cooled water evaporates, it becomes deposited on ice crystals

Causes of Precipitation In order for water to fall, it needs to be big enough for gravity to pull it down. It needs to become bigger. This occurs by two processes. Coalescence This process is when droplets formed by condensation bump into another droplet capturing it and combining it together to form a bigger droplet. The more time in a cloud, the bigger it grows. Mixing of air from different parts of a cloud also form variable sizes of water droplets. Supercooling This is a condition in which a substance is cooled below its freezing point, condensation point or sublimation point without going through a change of state. In other words, the water does not have enough freezing nuclei to become a solid. Most of the water will usually evaporate, however some water will condense on the ice crystals that were formed and falls as snow. In summer it falls as rain drops.

Measuring Precipitation Meteorologists use various methods and instruments to measure precipitation. Amount of Precipitation Meteorologists report rain fall in 100ths of an inch. A rain gauge is used, but the rain has to be measured before it can be evaporated, run off or soaked into the ground. Snow is measured with a stick. Rain equivalent is done after the snow is melted. Dry snow is usually deeper than wet snow. Most times the wet snow equivalent is approximately 10:1, and dry snow can be approximately 20:1.

Doppler Radar Doppler radar works by bouncing radio waves of rain or snow droplets and timing how long it takes the signal to return. Meteorologists can determine location, direction of movement and intensity

Weather Modifications Cloud seeding is one way to improve chances of getting rain to areas of the world that need the precipitation. Methods of Cloud Seeding Some scientists try to create conditions for rain by dropping frozen pellets of CO2 or silver-iodide ice crystals. You still need a cloud to do this. Improving Cloud Seeding Because there are so many variables to consider, cloud seeding does not always produce the conditions that are needed (rain). Some scientists are trying to prevent hail and eliminate fog at air ports by seeding extra large clouds but without any real success. Research is still pending.