2 Moisture, Clouds, and Precipitation Chapter 18Moisture, Clouds, and PrecipitationWho is Stan Hatfield and Ken Pinzke
3 18.1 Water in the Atmosphere Water’s Changes of State18.1 Water in the Atmosphere Precipitation is any form of water that falls from a cloud. When it comes to understanding atmospheric processes, water vapor is the most important gas in the atmosphere.
4 Water’s Changes of State Solid to Liquid• The process of changing state, such as melting ice, requires that energy be transferred in the form of heat. Energy is absorbed.• Latent heat is the energy absorbed or released during a change in state. Temperature can NOT change during a change of state!
5 Water’s Changes of State Liquid to GasEvaporation is the process of changing a liquid to a gas. Energy is absorbed, and the surroundings cool. So when you step out of the shower you feel cool because the water on your skin is evaporating.Condensation is the process where a gas, like water vapor, releases heat and changes to a liquid, like water.
6 Dew on a Spider WebMakes no sense without caption in book
7 18.1 Water in the Atmosphere Water’s Changes of State18.1 Water in the Atmosphere Solid to Gas• Sublimation is the conversion of a solid directly to a gas without passing through the liquid state.Dry ice = solid carbon dioxide sublimes without becoming a liquid first.• Deposition is the conversion of a vapor directly to a solid.
9 Changes of StateMakes no sense without caption in book
10 18.1 Water in the Atmosphere Humidity18.1 Water in the Atmosphere Humidity is a general term for the amount of water vapor in air. Saturation• 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.
11 18.1 Water in the Atmosphere Humidity18.1 Water in the Atmosphere Relative Humidity• Relative humidity is a ratio of the air’s 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. Warm air holds more water vapor than cold air – so winter air is dry.
12 Check your understanding So, on a cool summer morning, the relative humidity is _______ than in the warmer afternoon (if the amount of water vapor stays constant).
13 Relative Humidity Varies with Temperature Makes no sense without caption in book
14 18.1 Water in the Atmosphere Humidity18.1 Water in the Atmosphere Dew Point• Dew point is the temperature to which a parcel of air would need to be cooled to reach saturation.High dew point = moist airLow dew point = dry air** Dew point can NEVER be greater than the temperature! Why not?(only slightly in the atmosphere in the case of the updraft of a thunderstorm when hail forms)If the temperature dips below the dew point, moisture will leave the air, forming dew. Moisture will condense on anything colder than the air.thunderstorm updraft can be a supersaturated condition.
15 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.
17 Sling PsychrometerAir must continually pass over bulbs of each thermometer– one wet & one dry. Water evaporates from wet bulb and it cools. Dryer air will let wet bulb cool more. Compare temperatures to a table to get the relative humidity.Makes no sense without caption in book
19 Why is Winter Air Dry? Water vapor content of air stays constant Warm air holds more water vapor than cold airIn winter outside air is cold so it doesn’t hold a lot of moistureWhen you pump that air into your house and warm it up the relative humidity drops to less than 10%Causes static electricity, dry skin, sinus headaches, etc.
20 18.1 ReviewWhat is the most important gas for understanding atmospheric processes?What happens to heat during a change of state? Temperature?How does the temperature of air influence its ability to hold water?List two ways relative humidity can be changed?What does relative humidity describe about air?If the dew point is low, is the air moist or dry?
21 18.2 Cloud Formation- Clouds form when air is cooled to its dew point - Air must be saturated to be at the dew pointBut these clouds form during warmest part of day – so how is that possible?- Air Compression and ExpansionThink of pumping up bicycle tire - When you put air in the hose gets warm because compressing air increases motion of gas molecules & this causes temperature to riseTire losing air - When air is released it expands and coolsThese are Adiabatic Temperature Changes =Changes that happen even though heat isn’t added or subtracted
22 18.2 Cloud Formation Air Compression and Expansion Adiabatic Temperature Changes(adiabatic = without loss or gain of heat)• 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.
23 Cloud Formation by Adiabatic Cooling Makes no sense without caption in book
24 18.2 Cloud Formation Processes That Lift Air Four mechanisms that can cause air to rise are orographic lifting, frontal wedging, convergence, and localized convective lifting. Orographic Lifting (remember orogeny?)• 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.
25 Orographic Lifting Elevated terrain acts as barrier Causes air to rise and reach dew pointClouds form and it rains on windward sideOther side of barrier is dry because all of precipitation fell outAir descends and warmsup making it even drierRain Shadow Desert forms
26 18.2 Cloud Formation Processes That Lift Air Frontal Wedging • A front is the boundary between two adjoining air masses having contrasting characteristics (warm/cold, etc.)
28 Orographic Lifting and Frontal Wedging Frontal wedging - Warm air meets cold air and a front formsMakes no sense without caption in bookWarm air is less dense so it rises over cold air
29 18.2 Cloud Formation Processes That Lift Air Convergence • Convergence is when air flows together and rises.Leads to adiabatic cooling and possible cloud formationThis is why it rains many afternoons in Florida
30 Localized Convective Lifting Localized convective lifting occurs where unequal surface heating causes pockets of air to rise because of their buoyancy.Rising parcels of air are called thermalsBirds use this to soar high in skyPeople use it when they want to go hang glidingIf parcel rises above condensation level then clouds form and may produce rain
31 Convergence and Localized Convective Lifting Makes no sense without caption in book
32 18.2 Cloud Formation Stability Density Differences • 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.
33 18.2 Cloud Formation Stability Degrees of Stability • A temperature inversion occurs in a layer of limited depth in the atmosphere where the temperature increases rather than decreases with height. (warmer air over cooler air)
34 Stability and Daily Weather When stable air is forced above the Earth’s surface, the clouds that form are widespread and have little vertical thickness compared to their horizontal dimension. (spread out wide but thin from top to bottom)
35 18.2 Cloud Formation Condensation For any form of condensation to occur, the air must be saturated. Types of Surfaces• Generally, there must be a surface for water vapor to condense on.• Condensation nuclei are tiny bits of particulate matter that serve as surfaces on which water vapor condenses when condensation occurs in the air. (Like when they ‘seed’ clouds’)
36 18.2 ReviewDescribe what happens to air temperature when work is done on the air to compress it.What does stability mean in terms of air movement?What are the four mechanisms that cause air to rise?Describe conditions that cause condensation of liquid water in air.What is a temperature inversion?Hypothesize about other regions on Earth, other than the Florida peninsula, where convergence might cause cloud development and precipitation.
37 18.3 Cloud Types and Precipitation Types of Clouds18.3 Cloud Types and Precipitation Clouds are classified on the basis of their form and height.• 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.
38 Cirrus CloudsMakes no sense without caption in book
39 Cloud Form #2 - Cumulus Rounded individual cloud mass Usually have flat base and rise as domes or towersFluffy tops described as cauliflower head
40 Cloud Form #3 - Stratus Sheets or layers that cover the sky No distinct unitsUsually bring precipitation
41 18.3 Cloud Types and Precipitation Types of Clouds18.3 Cloud Types and Precipitation High Clouds – Base above 6000 meters• Cirrus clouds are high, white, and thin.• Cirrostratus clouds are flat layers of clouds.• Cirrocumulus clouds consist of fluffy masses. Middle Clouds – 6000 meters• 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.
42 18.3 Cloud Types and Precipitation Types of Clouds18.3 Cloud Types and Precipitation Low Clouds - Form below 2000 meters• 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.
43 Cloud ClassificationMakes no sense without caption in book
44 18.3 Cloud Types and Precipitation Clouds of Vertical Development• Some clouds do not fit into any of the other categories. They may startin one layer and extend intoanother.Form in unstable airCumulus are usually fairweather but grow out of controlunder right circumstances– become cumulonimbusProduce rain orthunderstorms
45 18.3 Cloud Types and Precipitation Fog18.3 Cloud Types and Precipitation 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.
46 Fog caused by coolingWarm moist air over Pacific moves over cold California Current then carried onshore by prevailing windsCool, clear, calm nights air near surface of Earth cools when Earth loses heat gained during day and reaches its dew pointAs the air coolsit becomes denseand sits in low areas
48 18.3 Cloud Types and Precipitation How Precipitation Forms18.3 Cloud Types and Precipitation For precipitation to form, cloud droplets must grow in volume by roughly one million times. Cold Cloud Precipitation• The Bergeron process is a theory that relates the formation of precipitation to supercooled clouds, freezing nuclei, and the different saturation levels of ice and liquid water.
49 The Bergeron ProcessMakes no sense without caption in book
50 18.3 Cloud Types and Precipitation How Precipitation Forms18.3 Cloud Types and Precipitation Cold Cloud Precipitation• Supercooled water is the condition of water droplets that remain in the liquid state at temperatures well below 0oC.• Supersaturated air is the condition of air that is more concentrated than is normally possible under given temperature and pressure conditions.
51 18.3 Cloud Types and Precipitation How Precipitation Forms18.3 Cloud Types and Precipitation Warm Cloud Precipitation• The collision-coalescence process is a theory of raindrop formation in warm clouds (above 0oC) in which large cloud droplets collide and join together with smaller droplets to form a raindrop.
52 18.3 Cloud Types and Precipitation Forms of Precipitation18.3 Cloud Types and Precipitation The type of precipitation that reaches Earth’s surface depends on the temperature profile in the lower few kilometers of the atmosphere. Rain and Snow• In meteorology, the term rain means drops of water that fall from a cloud and have a diameter of at least 0.5 mm.• At very low temperatures (when the moisture content of air is low) light fluffy snow made up of individual six-sided ice crystals forms.
53 18.3 Cloud Types and Precipitation Forms of Precipitation18.3 Cloud Types and 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 supercooled water droplets as they fall through a cloud.
54 Liquid Forms of Precipitation RainSize: 0.5-5mm diameterDrizzleSize: <0.5mm diameterFalls very slow
55 Solid Forms of Precipitation SnowIce particlesUsually falls as flakessmall flakes at low tempslarger flakes closer tofreezingdepends on moisturecontent in air
56 Solid Forms of Precipitation Sleetice pelletsrain falls throughlayer of freezing air
57 Solid Forms of Precipitation Freezing RainGlaze IceRain doesn’t freezeuntil it strikes surfacenear ground
58 Largest Recorded Hailstone Makes no sense without caption in book