Atmospheric Phenomena: Heat Transfer and Precipitation

• Atmospheric Phenomena encompasses heat transfer and precipitation.

Learning Objectives

• Students will understand how the Earth and atmosphere are warmed
• Students will understand incoming solar radiation
• Students will understand latent heat
• Students will understand the precipitation process

Temperature and Kinetic Energy

• Kinetic energy is associated with molecular motion.
• Temperature is a measure of average kinetic energy and average speed of molecules.
• Faster average speeds indicate higher temperature.

Temperature and Density

• Warm air results in molecules moving faster and farther apart. With the same mass occupying more volume, warm air is less dense.
• Cold air results in molecules moving slower and closer together. With the same mass occupying less volume, cold air is more dense.

Temperature Scales: Absolute Zero

• Absolute zero represents the lowest possible temperature.
• Molecular motion ceases entirely at absolute zero.
• Absolute zero equals 0 K, -273°C, or -459°F.

Heat and Temperature

• Temperature measures the average speed of molecules.
• Heat is the energy transferred from one object to another due to a temperature difference.
• Heat moves from higher to lower temperatures.

Heat Transfer Modes

• Three modes of heat transfer are conduction, convection, and radiation.

Heat Transfer by Conduction

• Heat conductivity is a substance's ability to conduct heat as a result of molecular motion.
• Air and wood conduct heat poorly, while iron and silver conduct heat very quickly.

Heat Transfer by Convection

• Convection: Heat transfer by mass movement happens in liquids and gasses because they can flow freely
• Atmospheric convection: Thermally driven vertical movement of air both upward and downward
• Advection: Horizontal motion of air (conservation of mass)

Heat Transfer by Radiation

• Radiation: Heat transfer of energy by waves that release energy when absorbed by an object
• Waves have electrical and magnetic properties; they are called electromagnetic waves (radiation)
• Electromagnetic waves do not need a medium for propagation.
• All matter emits and absorbs radiation.
• Radiation with longer wavelengths carries less energy compared to radiation with shorter wavelengths.

Wien's Law of Radiation

• The wavelength of radiation emitted by a body is inversely proportional to its absolute temperature.
• λmax = C / T, where C = 2897 µm·K and T = K (absolute temperature)
• The lower the body's absolute temperature, the higher its wavelength emitted.

Sun's Radiation

• The sun is hot (6000 K) and emits shortwave radiation, determined by Wien's law.

Earth's Radiation

• The Earth is cool (288 K), its emitted radiation has a longer wavelength per Wien's law.
• Earth's radiation, terrestrial longwave radiation/outgoing longwave radiation.

Greenhouse Effect

• A black body perfectly absorbs and emits radiation. The Earth and Sun are black bodies.
• The atmosphere isn’t a black body. The gases in the atmosphere are selective absorbers, absorbing and re-emitting radiation as certain wavelengths.

Greenhouse Effect Explained

• The sun's shortwave radiation (gamma rays, x-rays, all UV radiation) is absorbed by the atmosphere; the rest pass through it.
• Earth's surface absorbs the visible and all infrared radiation and re-emits it as longwave radiation. That energy is absorbed by the other gases on Earth.

Greenhouse Effect Contributors

• The selective absorbers are the greenhouse gases. Water vapor and carbon dioxide are particularly effective.
• As selective absorbers absorb and radiate infrared radiation, they act as an insulating layer.

Warming from Below

• Solar radiation passes through the atmosphere and strikes Earth's surface (black body), warming the surface.
• Air molecules in the first few centimeters are warmed via conduction.
• As warm air becomes less dense than the air above it, heat is transferred upwards by convection.

Incoming Solar Radiation

• 51% of incoming solar radiation is absorbed by the surface and 19% by the atmosphere/clouds.
• Radiation strikes an object, is deflected in all directions in a process called scattering.
• Reflected radiation is when radiant energy is turned backwards from the object.

Solar Radiation Appearance

• At noon, the sun appears bright white.
• At sunrise and sunset, sunlight passes through a thick portion of the atmosphere.
• Much of the blue light is scattered. This causes the sun to appear more red.

Albedo

• Albedo is the percentage of radiation returned from an object compared to the radiation striking the object.

Changes in Latitude

• High latitudes lose more energy than they gain. Low latitudes receive more energy than they lose.

Change of Seasons

• Seasons are regulated by the amount of solar radiation received by the Earth and determined by the angle at which sunlight strikes the Earth.
• Solar radiation striking the Earth directly is more intense than solar radiation hitting at an angle.
• At an angle, light is spread out over a larger surface area as the solar radiation has to penetrate a thicker layer of the atmosphere.
• The second factor regulating season is the length of time the sun shines everyday (photoperiod).
• Summer days are longer, hence, emitting more solar radiation during this time.

Latent Heat

• Change of state (phase change): transitions between solid, liquid, and gaseous phases of a substance, due to effects of temperature and/or pressure.
• Latent heat: heat required to be released or absorbed by a substance to change its phase without changing temperature.
• Sensible heat: heat absorbed or released with change in temperature.
• Latent heat of condensation: a warming process as the heat from water vapor is released to change from gas to liquid.
• Latent heat of evaporation: a cooling process as the heat is absorbed by liquid water to change from liquid to gas.

Latent Heat Details

• Liquid water must reach 100°C to evaporate with consistently added heat.
• Then, an input of 540 cal/gram of water is required to change from liquid to gas.
• Changing water to gas from ice, absorbs 720 cal/g of water from environment. The environment is then cooled in return.
• Water that condenses and freezes back to ice releases 720 cal/g of water to the atmosphere, and the environment is then warmed in return.

Hydrologic Cycle

• Evaporation
• Transpiration
• Condensation
• Precipitation
• Collection

Evaporation, Condensation, Saturation

• Evaporation is the change from liquid to vapor (cooling process). Condensation is the change from vapor to liquid (heating process).
• Evaporation and condensation happens at the same time. When the number of evaporating molecules equals the number of condensing molecules, saturation occurs and air becomes saturated.

Condensation Nuclei

• Condensation nuclei are tiny particles upon which condensation of water vapor begins.
• Condensation is driven by changes in temp. Due to change in temp, heat from the water vapor molecules flows to the cold air.

Humidity

• Humidity refers to the amount of water vapor in air.
• Absolute humidity is the mass of water per volume of air.
• Specific humidity is the mass of water per mass of total air, including water vapor.
• Vapor pressure is the pressure exerted by water molecules inside an air parcel.

Vapor Pressure

• In an air parcel containing water vapor, all gases exert pressure.
• Vapor pressure is the pressure exerted by molecules of water vapor.

Saturation Vapor Pressure

• Refers to the pressure exerted by water vapor if air is saturated.
• This depends on air temperature.
• At higher temps, it takes more water vapor to saturate the air.
• Higher temperature = higher saturation vapor pressure

Relative Humidity

• The ratio of the amount of water vapor actually in the air to the maximum amount of water vapor required for saturation at a particular temperature.
• Relative humidity says how close the air is to being saturated, NOT how much water vapor is in the air.
• A relative humidity of 20% is far from being saturated. A relative humidity of 90% is close to it.

Temperature, Dew Point, Relative Humidity

• The temperature and relative humidity graph shows the inverse relationship.
• Air is closest to saturation in the morning, so fogging occurs and grass is wet.

Dew Point

• Dew point refers to the temperature to which air would have to be cooled, with no change in air pressure or water content, for saturation to occur.
• Adding water vapor to the air increases the dew point. Removing water vapor from the air lowers the dew point.
• As the air cools, the temperature approaches the dew point and relative humidity increases.
• When air temperature reaches the dew point, the air is saturated and relative humidity is 100%.
• Continued cooling causes water vapor to condense into liquid water. The colder air cannot "hold" as much water vapor.
• In the atmosphere, condensation results in clouds. At the surface, condensation results in dew, frost, haze and fog.

Dew, Frost, Haze and Fog

• Dew: Water has condensed on objects on the ground. The temp of the object's surface has fallen below the dew point of the surface air.
• Frost: Forms when the dew point is at or below freezing. Air cools below the freezing point. Depositing water vapor makes ice.
• Haze: Forms as relative humidity climbs to 75% and water vapor condenses onto condensation nuclei, these particles are big enough to scatter visible light.
• Fog: Forms as relative humidity approaches 100%, causing water droplets to grow large enough to be seen.

Types of Fog

• Radiation Fog: Further radiational cooling on top of fog deepens it, while forming at surfaces and thickening as cooling continues.
• Advection Fog: Warmer, moist air moves over a colder surface. The surface and its temperature drop.
• Upslope Fog: Moist air flows toward slope and cools to condensation temp as air rises with the terrain.
• Valley Fog: Air cools at higher elevations; Cool air drains downslope and enters valley. Cold air drainage reduces air temperature in valley to condensation point
• Precipitation Fog: Fog forms from saturation as precipitation falls through air and evaporative cooling occurs.

Clouds

• Clouds consist of visible aggregates of tiny water droplets or ice crystals suspended in air.
• Continuous rise of air is responsible for the formation of clouds that keep them floating.
• Clouds are classified according to their elevation and visual characteristics.

Cloud Classification Chart

• High Clouds: Cirrus (Ci), Cirrostratus (Cs), Cirrocumulus (Cc)
• Middle Clouds: Altostratus (As), Altocumulus (Ac)
• Low Clouds: Stratus (St), Stratocumulus (Sc), Nimbostratus (Ns)
• Clouds with Vertical Development: Cumulus (Cu), Cumulonimbus (Cb)

Clouds with Vertical Development

• Cumulus (Cu) are puffy clouds with many shapes, a floating cotton, and the base is white to light gray and low.
• Cumulus humilis: Shows limited vertical growth / associated with fair weather, known as fair weather cumulus
• Cumulus congestus: Growing cumulus clouds that resembles the head of a cauliflower.
• Cumulonimbus (Cb): Cumulus congestus that continue to grow vertically. A giant thunderstorm cloud. These often become larger and more vertically developed in the afternoon.
• Swift winds can reshape these storm clouds into huge flattened anvil.
• Water molecules move between water droplets and ice crystals, causing the ice to grow. As the water droplets move away, more water molecules evaporate from the water droplet and it grows smaller.

Cumulonimbus Details

• Tremendous amount of energy released by the condensation of water vapor results in violent up- and downdrafts.

Cloud Development and Stability- Key mechanisms

• Clouds development and stability occurs as air rises, expands and cools. Key mechanisms are:
• Surface heating and free convection caused by differential heating that causes thermal convection effects.
• Uplift over topography. Orographic lift occurs as air is lifted along topographic barriers.
• Surface convergence and increasing air over area of low pressure
• Frontal Lift along weather fronts

Air Shadow Effect

• Air rising along the side of the mountain. Air reaches its dew point forming cloud.
• Air that moves down the slope experiences compression which causes warming. This air creates rain shadow effect where no clouds are formed and no precipitation occurs.

Precipitation

• Cloud droplets (very small) should increase in size by million times,
• Two main processes of rain drops formation: collision-coalescence (warm cloud) and Bergeron ice-crystal (cool cloud).

Collision-Coalescence Process

• Droplets interact by bouncing off each other (collision). Some droplets merge (coalescence) to form larger droplets.
• When droplets are large enough, they fall to Earth as precipitation.
• In warm clouds, water droplets are very small. Collision-coalescence happens slowly.
• Important factors for collision-coalescence are: amount of liquid water content, range of water droplet sizes, cloud thickness and residence time of cloud droplets in updrafts, strength of updrafts, electrical charge separation.
• Warm thin stratus clouds produce few raindrops mostly drizzle, while towering cumulus produce heavy downpours.

Bergeron (Ice-Crystal) Process

• Occurs when both ice crystals and liquid cloud droplets coexist in clouds as 'cold clouds'.
• Liquid water exists at the lowest level just below the freezing level. Liquid and ice exist (mostly supercooled liquid) above the freezing level. Mostly ice crystals are found in the upper part of the cloud.
• At saturation, it is easier for the water molecules to escape the surface of water droplet. They can escape the surface of the ice crystal.
• More water molecules in water droplet means that exerts higher vapor pressure.
• Water molecules move from a region of higher vapor pressure to a lower vapor pressure, from the water droplet to the ice crystal.
• This causes the ice to grow.
• As water droplets move away, more and more water molecules evaporate from droplet, and it grows smaller

Rain and Snow Types

• Rain: greater than or equal to 0.5mm
• Drizzle is defined as uniformed drops smaller than 0.5mm
• Virga is evaporating streaks of precipitation, rain that never reaches ground
• Snowflake shape varies by temp & humidity

Blizzard

• Low temps and winds greater than 30 knots of fine + dry particles make blizzards

Sleet and Freezing Weather Types

• Sleet: ice pellets that become snow then are icy then watery
• Freezing rain is black ice with supercooled water and will quickly freeze on a surface

Rime and Graupel Weather Types

• Rime is supercooled clouds or fog where freezing accumulates as granular ice on objects.
• Graupel: Have heavy coated ice particles, but softer than usual hail

Hailstone Weather Types

• Hailstones: From golf all sized peas

Precipitation and Temperature

• Different temperature profiles (location of cold and warm air with height and the depth and warm air with height) in the atmosphere produce different type of precipitation.

Measuring Precipitation

• Rain gauge: instrument that collects and measure rainfall
• Standard rain gauge: funnel-shape collector attached to long measuring tube
• Tipping bucket rain gauge: each time bucket fills with one-hundredth of an inch of rain, it tips. These actions send electric signal to remote recorder.

Doppler Radar and Precipitation

• Pulse and Return Echo help to indicate level and density of precipitation.

Measuring Precipitation from Space

• Satellites calculate amount of precipitation