Meteorology Exam 2
When the rising parcel of air is UNsaturated, the atmosphere is ___________.
Stable (inverse relationship).
True or False: Increasing temperature will also influence evaporation.
True (because warm water will evaporate faster than cold water).
True or False: Increasing the wind will increase evaporation.
True (because we are adding more energy to the atmosphere). *Mixing (wind) will cause the air to become UNsaturated*
When the parcel of air is saturated, the atmosphere is ______________ .
UNstable (inverse relationship).
Frost Point
The temperature at which the air becomes saturated with respect to ice when cooled at constant pressure and constant water vapor content.
Unusual Clouds: Pileus
"Cap Cloud" A smooth cloud in the form of a cap. - Resembles a silken scarf capping the top of a sprouting cumulus cloud. - Form when moist winds are deflected up and over the top of a building cumulus congestus or cumulonimbus
An air parcel in an UNstable atmosphere has an ________-___________ force acting on it.
"Upward-directed" (called *buoyant force*) The warmer the air parcel compared to its surroundings, the greater the buoyant force, and the more rapidly the air parcel rises.
Unusual Clouds: Lenticular
A cloud in the shape of a lens. - Form one above the other like a stack of pancakes; - At a distance, they may resemble a hovering spacecraft
Water Vapor (Saturation): In a covered glass, the number of evaporation molecules will eventually __________ the number of molecules that are condensing.
Balance. Molecules escaping = evaporating Molecules returning = condensing *E + C = Saturation*
True or False: Evaporation and condensation are equal.
False. E does not equal C, but, E is greater than C.
Upslope Fog
Fog formed as moist, stable air flows upward over a topographic barrier.
Does a volume of hot, humid air weigh more than a similarly sized volume of hot, dry air?
No. At the same temperature and at the same level, humid air weighs less than dry air. (Keep in mind that we are referring strictly to water vapor—a gas—and not suspended liquid droplets.)
Advection Fog
Occurs when warm, moist air moves over a cold surface and the air cools to below its dew point. *Usually along a coastal region*.
Humidity
One of a number of ways of specifying the amount of water vapor in the air.
Satellites: Purpose/Importance
Provides extremely valuable images of clouds over areas where there are no ground-based observations. - Water covers over 70 percent of Earth's surface, there are vast regions where few (if any) surface cloud observations are made. Satellites spot storms while they are still far out in the ocean and track them accurately.
Convection and Cloud Development:
Sun warms the ground... Ground heats the air (conduction)... Bubbles of warm air rise (thermals/parcels)... Thermals cool, mix out and slow down... If it reaches saturation, CUMULUS CLOUDS form.
Time of day the atmosphere is MOST stable:
The atmosphere is usually most stable in the *early morning around sunrise,* when the lowest surface air temperature is recorded. If the surface air becomes saturated in a stable atmosphere, a persistent layer of haze or fog may form.
Mixing Ratio
The ratio of the mass of water vapor in a given volume of air to the mass of dry air. MR = mass of water vapor / mass of dry air Expressed as grams of water vapor per kilogram of air (g/kg). *Remain constant as long as water vapor is not added to or removed from the parcel.* - This happens because the total number of molecules (and, hence, the mass of the parcel) remains constant, even as the parcel expands or contracts (since changes in parcel size do not affect specific humidity and mixing ratio).
Heat Stroke
A physical condition induced by a person's overexposure to high air temperatures, especially when accompanied by high humidity.
Deposition
A process that occurs in subfreezing air when water vapor changes directly to ice without becoming a liquid first.
Relative Humidity
*Does NOT indicate the actual amount of water vapor in the air, instead, it tells us how close the air is to being saturated.* The ratio of the amount of water vapor in the air compared to the amount required for saturation (at a particular temperature and pressure). Ratio of the air's water vapor content to its capacity: *RH = WV content / WV capacity* GIVEN AS A PERCENTAGE ex. Air with a 50 percent relative humidity actually contains one-half the amount of water vapor required for saturation. ex. Air with a 100 percent relative humidity is said to be saturated because it is filled to capacity with water vapor. ex. Air with over 100 percent is said to be "supersaturated". Cool air in the MORNING = high RH Warm air in the AFTERNOON = low RH
Condensation Nuclei: Hygroscopic v. Hydrophobic
*Hygroscopic* ("water-seeking") - The ability to accelerate the condensation of water vapor. - Water vapor condenses upon these surfaces when the relative humidity is considerably lower than 100 percent. Ex. Ocean salt, common table salt, sulfuric and nitric acid particles. - In humid weather, it is difficult to pour salt from a shaker because water vapor condenses onto the salt crystals, sticking them together. *Hydrophobic* ("water-repelling*) - The ability to resist the condensation of water vapor. Ex. Oils, gasoline, and paraffin waxes—resist condensation even when the relative humidity is above 100 percent.
Formation of Dew:
*Radiational cooling of the surface* - On clear, calm nights objects near Earth's surface cool rapidly by emitting longwave, infrared radiation. - The ground/objects on it become much colder than the surrounding air. Air that comes in contact with these cold surfaces *cools by conduction*. - Air cools to the dew point—the temperature at which saturation occurs. - Surfaces such as twigs, leaves, and blades of grass cool below this temperature, water vapor begins to condense upon them, forming tiny visible specks of water called DEW. *More likely to form on nights that are clear and calm.*
Wall Cloud
- A *low hanging cloud* that usually rotates and *spins in a counterclockwise* direction from a cumulonimbus cloud - *Tornados will sometimes drop down* and form from a wall cloud
Shelf Cloud
- Cool, sinking air from a t-storm downdraft spreads out across the and surface - This outflow cuts underwater air being drawn into the storm's updraft - As the lower, cooler air lifts the warm moist air, the water condenses, creating a cloud which often *rolls with the different winds above and below*
Formation of Frozen Dew:
- If the air temperature should drop to freezing or below, the dew will freeze, becoming tiny beads of ice called frozen dew.
Determining Sky Conditions
- In meteorology, descriptions of sky conditions are *defined by the fraction of sky covered by clouds*. - Reported in eighths of sky coverage (ex. "one eight") - Automated Surface Observing System (ASOS) only sees clouds below 12,000ft
Undulatus Asperatus
- New cloud - Described as "visible" air motions rolling through the sky
Conditional Instability depends on...
... whether or not the rising air is *saturated*.
Adiabatic Process
A process that takes place without a transfer of heat between the system (such as an air parcel) and its surroundings. If a parcel of air expands and cools, or compresses and warms, with no interchange of heat with its surroundings, this situation is called an AP. - Compression always results in warming - Expansion always results in cooling As long as the *air in the parcel is unsaturated (the relative humidity is less than 100 percent), the rate of adiabatic cooling or warming remains constant*.
Supercooled Droplets
A cloud composed of liquid droplets at temperatures below 0C (32F). When the cloud is on the ground it is called "supercooled fog" or "cold fog".
Unusual Clouds: Banner
A cloud extending downwind from an isolated mountain peak, often on an otherwise cloud-free day.
Fog
A cloud with its base at Earth's surface. - RH increase; - Visibility decreases; - Landscape becomes masked with a grayish tint. - RH gradually approaches 100 percent - Haze particles grow larger, and condensation begins on the less-active nuclei; - A large fraction of the available nuclei has water condensing onto them, causing the droplets to grow even bigger, until eventually they become visible to the naked eye. - Increasing size and concentration of droplets further restrict visibility. - When the visibility lowers to less than 1 km (0.62 mi), and the air is wet with countless millions of tiny floating water droplets, the wet haze becomes a cloud resting near the ground (aka FOG)
Unusual Clouds: Contrail
A cloudlike streamer frequently seen forming behind aircraft flying in clear, cold, humid air. - A condensation trail. Evaporate rapidly when the relative humidity of the surrounding air is low. Form directly from the *water vapor added to the air from engine exhaust*. - Must be sufficient mixing of the hot exhaust gases with the cold air to produce saturation Cooling may supersaturate the air, producing an "aerodynamic contrail" - Disappears quickly
Supersaturated (Air)
A condition whereby the atmosphere contains more water vapor than is needed to produce saturation with respect to a flat surface of pure water or ice, and the relative humidity is greater than 100 percent. *Does not tend to occur often or last long.*
Hydraulic (water) Cycle
A model that illustrates the movement and exchange of water among Earth, atmosphere, and oceans. Water molecules travel from ocean to atmosphere to land and then back to the ocean: *Ocean > Atmosphere > Land > Ocean (repeat)* - Rainwater evaporates into the atmosphere - Precipitation intercepted by vegetation (evaporates or drips; "transpiration") - Water soaks into the ground ("groundwater") - Puddles collect water and it then runs off into streams and back into the ocean
Satellites: Geostationary
A satellite that *orbits Earth at the same rate that Earth rotates* and thus remains over a fixed place above the equator. Altitude of nearly 36,000 km (22,300 mi). "Real Time" data system.
Satellites: Polar-Orbiting
A satellite whose orbit closely parallels Earth's meridian lines and thus *crosses the polar regions on each orbit*. Lower altitude (about 850 km). ADVANTAGE of scanning clouds directly beneath them as they move. - Gather *sharp images* from polar regions, where images from a geostationary satellite are distorted because of the low angle.
Subsidence Inversions
A temperature inversion produced by compressional warming—the adiabatic warming of a layer of sinking air.
Rotor Cloud
A turbulent cumuliform type of cloud that forms on the leeward side of large mountain ranges. The air in the cloud rotates about an axis parallel to the range.
When does Absolute Instability of the atmosphere usually occur?
Absolute instability is usually limited to a *very shallow layer near the ground on hot, sunny days*. Here the environmental lapse rate can exceed the dry adiabatic rate, and the lapse rate is called "superadiabatic".
Air Rising UP and Over a Mountain
Air descending a mountain warms by compressional heating and, upon reaching the surface, can be much warmer than the air at the same level on the windward side, especially when condensation occurs and latent heat is released on the windward side. Air on the leeward side of a mountain is normally drier (has a lower dew point) than the air on the windward side because water in the form of clouds and precipitation often remains on the windward side. The lower dew point and higher air temperature on the leeward side produce a lower relative humidity, a greater potential for evaporation of water, and a rain-shadow desert.
Condensation Nuclei:
Also called "cloud condensation nuclei". Tiny particles upon whose surfaces condensation of water vapor begins in the atmosphere. - Small nuclei less than 0.2 μm in radius are called Aitken nuclei; - Those with radii between 0.2 and 1 μm are large nuclei; - While giant nuclei have radii larger than 1 μm. Enter the atmosphere in a variety of ways: - Dust, volcanoes, factory smoke, forest fires, salt from ocean spray, and even sulfate particles emitted by phytoplankton in the oceans. *Most abundant over industrial cities, with highly polluted air.* *Decrease in cleaner "country" air and over the oceans.*
Middle Clouds: (2)
Altostratus (As) - Gray or blue-gray clouds - Composed of ice crystals and water droplets - Often cover the entire sky - Sun is dimly visible; "watery sun" - *No shadows on ground* - Often form ahead of mid-latitude cyclonic storms Altocumulus (Ac) - Composed mostly of water droplets - Appear as gray, puffy masses - *Individual puffs of the altocumulus appear larger than those of the cirrocumulus* - "Little castles" (castellanus) - Portends thunderstorms if apparent on humid summer mornings
Neutral Stability
An atmospheric condition that exists in dry air when the environmental lapse rate equals the dry adiabatic rate. *dry air = ELR = DAR* In saturated air the environmental lapse rate equals the moist adiabatic rate. *saturated air= ELR = MAR*
Absolutely Unstable
An atmospheric condition that exists when the environmental lapse rate is greater than the dry adiabatic rate. *ELR > dry AR* This results in a lifted parcel of air being warmer than the air around it.
Conditionally Unstable Atmosphere
An atmospheric condition that exists when the environmental lapse rate is less than the dry adiabatic rate but greater than the moist adiabatic rate. Also called "conditional instability". *moist AR < ELR < dry AR* When a parcel of unsaturated air rises, it cools dry adiabatically and is colder at each level than the air around it. It will, therefore, tend to sink back to its original level because it is in a STABLE atmosphere... Suppose the rising parcel is saturated... The *rising air is warmer* than its environment at each level. Once the parcel is given a *push upward*, it will *tend to move in that direction*; the atmosphere is UNstable for the saturated parcel.
Saturated (Air)
An atmospheric condition whereby the level of water vapor is the maximum possible at the existing temperature and pressure.
Heat Index (HI)
An index that combines air temperature and relative humidity to determine an apparent temperature—*how hot it actually feels*.
Hygrometer
An instrument designed to measure the air's water vapor content. The sensing part of the instrument can be hair (hair hygrometer), a plate coated with carbon (electrical hygrometer), or an infrared sensor (infrared hygrometer).
Psychrometer
An instrument used to measure the water vapor content of the air. It consists of two thermometers (dry bulb and wet bulb). After whirling the instrument, the dew point and relative humidity can be obtained with the aid of tables.
Terms to Describe Clouds (2)
Anvil: flat anvil top of a tall cumulonimbus cloud - Shows movement of the thunderstorm by the direction the anvil is pointing Castellanus: "castle" or tower-like vertical extensions - Found on cumulus clouds
Precipitation
Any form of water particles—liquid or solid—that falls from the atmosphere and reaches the ground.
Relationship between WV, RH, and temperature.
As the air temperature increases (with no change in water vapor content), the relative humidity decreases. *^T = WV constant = vRH* This decrease in relative humidity occurs because in the warmer air the water vapor molecules are zipping about at such high speeds they are unlikely to join together and condense. *T and RH are inversely related* The higher the temperature, the faster the molecular speed, the less likely saturation will occur, and the lower the relative humidity. As the air temperature lowers, the water vapor molecules move more slowly, condensation becomes more likely as the air approaches saturation and the relative humidity increases. *vT = ^RH = approaching saturation* In summary, with no change in water vapor content, an increase in air temperature lowers the relative humidity, while a decrease in air temperature raises the relative humidity.
Condensation Nuclei:
Bits of dust, smoke, and salt that water easily adheres to. *In warm air, molecules move quickly and bounce away.* *In cool air, molecules move slow and are more apt to stick and condense to the nuclei (more moisture)*
Billow Clouds
Broad, nearly parallel lines of wavelike clouds oriented at right angles to the wind. Also called Kelvin-Helmholtz wave clouds.
2 Ways to Change Relative Humidity
Change vapor content - Adding water vapor INCREASES the RH - (^WV = ^RH) Change air temperature - Warming the air DECREASES RH - (^T = vRH)
High Clouds: (3)
Cirrus (Ci) - Most common high cloud - *Thin, wispy clouds* blown by high winds into long streamers called "mares' tail" - Move from WEST to EAST - Occur during periods of fair, pleasant weather Cirrostratus (Cs) - *Thin, sheetlike*, that often cover the entire sky - Ice crystals in these clouds bend the light passing through them and will often produce a *halo—a ring of light that encircles the sun or moon* - Can be used to predict rain or snow within 12 to 24 hours Cirrocumulus (Cc) - Appear as small, rounded, *white puffs (rippling)* - Rarely cover more than a small portion of sky - Resembles *scales of a fish*; hence, the expression "mackerel sky"
Determining Sky Conditions: Chart
Clear = No clouds present Few = 1/8 to 2/8 (few clouds visible) Scattered= 3/8 to 4/8 (partly cloudy) Broken = 5/8 to 7/8 (mostly cloudy) Overcast = Total sky covered
Review^
Cloud droplets are very small, much too small to fall as rain. The smaller the cloud droplet, the greater its curvature relative to water molecules, and the more likely it will evaporate. Cloud droplets form on cloud condensation nuclei. Hygroscopic nuclei, such as salt, allow condensation to begin when the relative humidity is less than 100 percent. Cloud droplets, in above-freezing air, can grow larger as faster-falling, bigger droplets collide and coalesce with smaller droplets in their path. In the ice-crystal (Bergeron) process of rain formation, both ice crystals and liquid cloud droplets must coexist at below-freezing temperatures. The difference in saturation vapor pressure between liquid and ice causes water vapor to diffuse from the liquid droplets (which shrink) toward the ice crystals (which grow). Most of the rain that falls over middle latitudes results from melted snow that formed from the ice-crystal (Bergeron) process. Cloud seeding with silver iodide can only be effective in coaxing precipitation from a cloud if the cloud is supercooled and the proper ratio of cloud droplets to ice crystals exists.
Nacreous or Polar Stratospheric Clouds
Clouds of unknown composition that have a soft, pearly luster and that form in the *stratosphere* at altitudes about *25 to 30 km above Earth's surface*. They are also called "mother-of-pearl clouds". Best viewed in *polar latitudes* during the *winter months* when the sun, being just below the horizon, is able to illuminate them because of their high altitude. Composed of water in either solid or liquid (supercooled) form.
Unusual Clouds: Mammatus
Clouds that look like pouches hanging from the underside of a cloud. Most frequently form on the underside of: - Cumulonimbus; they may develop beneath: - Cirrocumulus, Altostratus, Altocumulus, and Stratocumulus For mammatus to form, the *sinking air must be cooler than the air around it* and have a *high liquid water/ice content*. - Sinking air can extend below the cloud base, appearing as rounded masses *Mammatus clouds forming BENEATH a THUNDERSTORM ANVIL!!*
Clouds with Vertical Development:
Cumulus (Cu) - Puffy cloud takes on a variety of shapes, but most often it looks like a *piece of floating cotton with a flat base* - Base appears white to light gray - Usually with a great deal of blue sky between each cloud - Only slight vertical growth are called "cumulus humilis" (associated with fair weather); "fair-weather cumulus." - Ragged-edge, *smaller than cumulus humilis* and scattered across the sky are called "cumulus fractus". - Resemble head of cauliflower, it becomes a "cumulus congestus", or "towering cumulus" (Tcu). Cumulonimbus (Cb) - If a *cumulus congestus continues to grow vertically*, it develops into a giant cumulonimbus - "Thunderstorm" cloud - Strong upper-level *winds blowing from right to left produce a well-defined anvil* - Tremendous amounts of energy released by the condensation of water vapor within a cumulonimbus result in the development of *violent updrafts and downdrafts*
Warming of the surface air may be due to: (3)
Daytime solar heating of the surface. An influx of warm air brought in by the wind (warm advection). air moving over a warm surface.
Review^
Dew, frost, and frozen dew generally form on clear nights when the temperature of objects on the surface cools below the air's dew-point temperature. Visible white frost forms in saturated air when the air temperature is at or below freezing. Under these conditions, water vapor can change directly to ice, in a process called deposition. Condensation nuclei act as surfaces on which water vapor condenses. Those nuclei that have an affinity for water vapor are called hygroscopic. Fog is a cloud resting on the ground. It can be composed of water droplets, ice crystals, or a combination of both. Radiation fog, advection fog, and upslope fog all form as the air cools. The cooling for radiation fog is mainly radiational cooling at Earth's surface; for advection fog, the cooling is mainly warmer air moving over a colder surface; for upslope fog, the cooling occurs as moist air gradually rises and expands along sloping terrain. Evaporation (mixing) fog, such as steam fog and frontal fog, forms as water evaporates and mixes with drier air.
Haze (visibility, particle sizes, dry v. wet)
Fine dry/wet *dust or salt particles* dispersed through a portion of the atmosphere. Individually these are not visible but *cumulatively they will diminish visibility*. Particle Size: - *Dry haze* particles are very small, on the order of 0.1 mm. - *Wet haze* particles are larger. DRY: - Warm afternoon - RH of the air below the point where WV begins to condense - The floating particles remain *small* ("dry") - Scattering effect of dry haze produces a bluish color when viewed against a dark background; a yellowish tint when viewed against a light-colored background. WET: - Cool, nighttime air - RH increases to about 75 percent, condensation may begin on the most active hygroscopic nuclei, producing a "wet haze". - *Restricts visibility more than dry haze* - Appears dull gray or white
Radiation Fog
Fog produced over land when radiational cooling reduces the air temperature to or below its dew point. It is also known as "ground fog" and "valley fog".
Evaporation Mixing Fog
Fog produced when sufficient water vapor is added to the air by evaporation, and the moist air mixes with relatively drier air. 2 Types: - *Steam Fog*; which forms when cold air moves over warm water. - "Frontal Fog"; which forms as warm raindrops evaporate in a cool air mass.
Advection-Radiation Fog
Fog that forms as relatively warm moist air moves over a colder surface that cooled mainly by radiational cooling.
Acid Fog
Fog that forms in polluted air can turn acidic as the tiny liquid droplets combine with gaseous impurities, such as oxides of sulfur and nitrogen.
Formation of Frost:
Forms on cold, clear, calm mornings when the dew-point temperature is *at or below freezing*. - When the air temperature cools to the dew point (now called "frost point") and further cooling occurs, water vapor can change directly to ice without becoming a liquid first— *a process called deposition*. White crystals of ice that form in this manner are called hoarfrost, white frost, or simply FROST. Frost has a *treelike branching pattern* that easily distinguishes it from the nearly spherical beads of frozen dew.
Cloud Heights:
High - Above 20,000-60,000ft (6000m) - Almost exclusively "ice crystals" clouds - Little moisture available (thin clouds) Middle - Above 6,500-26,000ft (2000m) - Water droplets and ice crystals - Thicker clouds that sometimes dim the sun Low - Surface to 6500ft (0-2000m) - Comprised of water droplets (mostly) - Gloomy looking sky--often in winter Clouds with Vertical Development - Cloud Bases: 1,000-5,000 - Cloud Tops: 5,000-80,000 - Comprised of Water and Ice
Graupel
Ice particles between 2 and 5 mm in diameter that form in a cloud often by the process of accretion. Snowflakes that become rounded pellets due to riming are called graupel or snow pellets. As the graupel falls, it may fracture or splinter into tiny ice particles when it collides with cloud droplets. These splinters may grow to become new graupel, which, in turn, may produce more splinters.
Curvature Effect
In cloud physics, as cloud droplets decrease in size, they exhibit a greater surface curvature that causes a more rapid rate of evaporation. *** When air is saturated with respect to a flat surface, it is unsaturated with respect to a curved droplet of pure water, and the droplet evaporates. So, to keep tiny cloud droplets in equilibrium with the surrounding air, the *air must be supersaturated*; that is, the relative humidity must be greater than 100 percent.
Satellite Imagery Types (3)
Information on *cloud thickness and height*. Visible Imagery - Sunlight is reflected from the clouds - Only available when the sun is out - Thick clouds have a higher albedo (reflectivity) than thin clouds, so they appear brighter on a visible satellite image. - Middle and low clouds have just about the same albedo, so it is difficult to distinguish among them simply by viewing them in visible light. -Very clear and concise picture Infrared Imagery - Amount of radiation increases with temperature - Warm clouds are lower in the atmosphere (closer to warm ground) - Cold clouds are higher in the atmosphere (temperature decreases in the troposphere) - Infrared produces a *better image of the actual radiating surface* because it does not show the strong visible reflected light. *Water Vapor Imagery - Shows areas of high moisture content in the atmosphere - Very useful at showing upper-level storm systems - Darker areas-dry air (orange and black) - Lighter areas- moist air (green, gray & white)
Hair Hygrometer
Is constructed on the principle that the length of human hair increases by 2.5 percent as the relative humidity increases from 0 to 100 percent. This instrument uses human or horse hair, or a synthetic fiber, to *measure relative humidity*.
Cloud Streets
Lines or rows of cumuliform clouds.
The most important factor in the production of raindrops is the cloud's...
Liquid Water Content.
Actual Water Vapor
Measurment of the amount of water vapor in a volume of air. The actual vapor pressure is normally a small fraction of the total air pressure. *High actual vapor pressure indicates large numbers of water vapor molecules, whereas low actual vapor pressure indicates comparatively small numbers of vapor molecules.*
Moist AR v. Dry AR
Moist AR is much less than the dry AR when the rising air is warm; *MAR < DAR when air is WARM* However, the two rates are *nearly the same* when the rising air is VERY COLD.
Solid (2)
Molecules arrange themselves into an ordinary pattern. Each molecule is "locked" into a rigid position.
Phases of Water: Liquid (WATER)
Molecules arranged close together, bumping and moving at slightly different speeds. Phase change occurs when water changes energy levels: - Liquid to Vapor = Evaporation (latent heat of vaporization) - Liquid to Solid = Freezing (latent heat of fusion)
Phases of Water: Gas (WATER VAPOR)
Molecules arranged far apart, zipping past each other. Phase change occurs when water changes energy levels: - Vapor to Liquid = Condensation (latent heat of condensation) - Vapor to solid = Deposition (latent heat of sublimation)
Phases of Water: Solid (ICE)
Molecules arranged in a rigid pattern, vibrate against each other. Phase change occurs when water changes energy levels: - Ice to Vapor = Sublimation (latent heat of vaporization) - Ice to Liquid = Melting (latent heat of fusion)
Types of Precipitation: (5)
Rain - Precipitation in the form of *liquid water drops* that have diameters >drizzle. - Must have a *diameter* equal to, or greater than *0.5 mm* Snow - A solid form of precipitation *composed of ice crystals* in complex *hexagonal form*. Sleet - A type of precipitation consisting of transparent pellets of ice 5 mm or less in diameter. Same as ice pellets. Freezing Rain - Rain or drizzle that *falls in liquid form and then freezes upon striking a cold object/ground*. - Both can produce a coating of ice on objects which is called "glaze". Hail - Transparent or partially opaque particles of ice that range in *size from that of a pea to that of golf balls*.
Review ^
Relative humidity tells us how close the air is to being saturated. Relative humidity can change when the air's water-vapor content changes, or when the air temperature changes. With a constant amount of water vapor, cooling the air raises the relative humidity and warming the air lowers it. The dew-point temperature is a good indicator of the air's water-vapor content: High dew points indicate high water-vapor content; and low dew points, low water-vapor content. Where the air temperature and dew point are close together, the relative humidity is high; when they are far apart, the relative humidity is low. Dry air can have a high relative humidity when the air is very cold and the air temperature and dew point are close together.
Howard's Cloud Classification: LATIN (4)
Sheetlike cloud *stratus* (Latin for "layer") Puffy cloud *cumulus* ("heap") Wispy cloud *cirrus* ("curl of hair") Rain cloud *nimbus* ("violent rain")
Low Clouds:
Stratus (St) - Uniform grayish cloud that often covers the entire sky - *Resembles a fog that does not reach the ground* - No precipitation falls from stratus, but sometimes it is accompanied by a light mist or drizzle Stratocumulus (Sc) - Low, lumpy clouds that appear in *rows, in patches*, or as rounded masses - *Blue sky visible in between individual clouds* - Sun will shine through the cloud breaks, producing bands of light (called "crepuscular rays") - Color ranges light to dark grey - *Altocumulus cloud elements will generally be about the size of your thumbnail, whereas stratocumulus will usually be about the size of your fist* - Precipitation RARELY falls from stratocumulus Nimbostratus (Ns) - Dark-gray, "wet"-looking - Continuously falling rain or snow - Precip. is light or moderate - Nimbostratus is easily confused with altostratus. (*Thin nimbostratus is usually darker gray than thick altostratus, and you normally cannot see the sun or moon through a layer of nimbostratus*) - *Sheetlike cloud from which light rain is falling*
Absolute Humidity
Suppose we enclose a volume of air in an imaginary large balloon ("a parcel"). With a chemical drying agent, we can extract the water vapor from the air, weigh it, and obtain its mass. If we then compare the vapor's mass with the volume of air in the parcel, we would have determined the absolute humidity of the air: *Absolute Humidity = mass of WV / volume of air* Absolute humidity represents the water vapor density (mass/volume) in the parcel and is expressed as *grams of water vapor in a cubic meter of air*. For example, if the water vapor in 1 cubic meter of air weighs 25 grams, the absolute humidity of the air is 25 grams per cubic meter.
Mechanisms Primarily Responsible for the Development of the Majority of Clouds: (4)
Surface heating and free *convection* Uplift along *topography* *Widespread ascent* due to a convergence of surface air. *Uplift* along cold/warm weather fronts.
Bergeron Process ("Ice Crystal Process")
The "ice-crystal process", also known as the Bergeron process, is after the Swedish meteorologist *Tor Bergeron*, who proposed that essentially *all raindrops begin as ice crystals*. Extremely IMPORTANT in middle and high latitudes, where clouds extend upward into regions where the air temperature is well below freezing.
Review^
The air temperature in a rising parcel of unsaturated air decreases at the dry adiabatic rate, whereas the air temperature in a rising parcel of saturated air decreases at the moist adiabatic rate. The dry adiabatic rate and moist adiabatic rate of cooling are different due to the fact that latent heat is released in a rising parcel of saturated air. In a stable atmosphere, a lifted parcel of air will be colder (heavier) than the air surrounding it. Because of this fact, the lifted parcel will tend to sink back to its original position. In an unstable atmosphere, a lifted parcel of air will be warmer (lighter) than the air surrounding it, and thus will continue to rise upward, away from its original position. The atmosphere becomes more stable (stabilizes) as the surface air cools, the air aloft warms, or a layer of air sinks (subsides) over a vast area. The atmosphere becomes more unstable (destabilizes) as the surface air warms, the air aloft cools, or a layer of air is either mixed or lifted. A conditionally unstable atmosphere exists when the environmental lapse rate is between the moist adiabatic rate and the dry adiabatic rate. The atmosphere is normally most stable in the early morning and most unstable in the afternoon. Layered clouds tend to form in a stable atmosphere, whereas cumuliform clouds tend to form in a conditionally unstable atmosphere.
Winter Chilling
The amount of time the air temperature during the winter must remain below a certain value so that fruit and nut trees will grow properly during the spring and summer.
What causes an atmosphere to become MORE unstable?
The atmosphere becomes more unstable as the environmental lapse rate steepens; that is, as the air temperature drops rapidly with increasing height. This circumstance may be brought on by either air aloft becoming colder, or the surface air becoming warmer.
What is a stable atmosphere?
The atmosphere is always absolutely stable when the *environmental lapse rate* is LESS than the moist adiabatic rate. AKA: Difference in temperature between the surface air and the air aloft is relatively small. This results in a *lifted parcel of air* being *colder than the air around it*. - Atm stabilizes as the air aloft warms or the surface air cools. *ELR decreases = stabilizing atmosphere* Air in an *absolutely stable* atmosphere strongly RESISTS upward vertical motion, it will, if forced to rise, tend to spread out horizontally. ANOTHER way the atmosphere becomes more stable is when an entire layer of air sinks
Aggregation
The clustering together of ice crystals to form snowflakes.
Terminal Velocity
The constant speed obtained by a falling object when the upward drag on the object balances the downward force of gravity. Because larger drops have a smaller surface-area-to-weight ratio, they must fall faster before reaching their terminal velocity. Thus, larger drops fall faster than smaller drops.
Solute Effect
The dissolving of hygroscopic particles, such as salt, in pure water, thus reducing the relative humidity required for the onset of condensation.
Accretion
The growth of a precipitation particle by the collision of an ice crystal or snowflake with a supercooled liquid droplet that freezes upon impact.
Condensation Level
The level above the surface marking the base of a cumuliform cloud.
Lifting Condensation Level
The level at which a parcel of air, when lifted dry adiabatically, would become saturated.
Orographic Uplift
The lifting of air over a topographic barrier. Clouds that form in this lifting process are called orographic clouds. This lifting produces cooling, and, if the air is humid, clouds form.
Wet-Bulb Temperature
The lowest temperature that can be obtained by evaporating water into the air. *A good measure of how cool the skin can become*. ex. On a hot day when the wet-bulb temperature is LOW, rapid evaporation (cooling) takes place at the skin's surface.
Saturation Water Vapor
The maximum amount of water vapor necessary to keep moist air in equilibrium with a surface of pure water or ice. It represents the maximum amount of water vapor that the air can hold at any given temperature and pressure. Describes how much water vapor is necessary to make the air saturated at any given temperature. Put another way, *saturation vapor pressure is the pressure that the water vapor molecules would exert if the air were saturated with vapor at a given temperature*. DEPENDS PRIMARILY ON AIR TEMP Ex. When both water and ice exist at the same temperature below freezing, the saturation vapor pressure just above the water is greater than the saturation vapor pressure over the ice. - At any temperature below freezing, it takes *more* vapor molecules to saturate air directly above water than it does to saturate air directly above ice.
Coalescence
The merging of cloud droplets into a single larger droplet.
Entrainment
The mixing of environmental air into a pre-existing air current or cloud so that it becomes part of the current or cloud.
Equilibrium Vapor Pressure
The necessary vapor pressure around liquid water that allows the water to remain in equilibrium with its environment. Also called "saturation vapor pressure".
Evaporation
The process by which a liquid changes into a gas.
Contact Freezing
The process by which contact with a nucleus such as an ice crystal causes supercooled liquid droplets to change into ice.
Condensation
The process by which water vapor becomes a liquid.
Collision-Coalescence Process
The process of producing precipitation by liquid particles (cloud droplets and raindrops) colliding and joining (coalescing).
Sublimation
The process whereby ice changes directly into water vapor without melting.
Adiabatic Process: Dry Adiabatic Lapse Rate
The rate of change of temperature in a rising or descending UNsaturated air parcel. The rate of adiabatic cooling or warming is about: *10C per 1000m (5.5F per 1000F)* As long as the air parcel *remains unsaturated*, it expands and cools by 10C per 1000 m; the sinking parcel compresses and warms by 10C per 1000 m.
Adiabatic Process: Moist Adiabatic Lapse Rate
The rate of change of temperature in a rising or descending saturated air parcel. The rate of cooling or warming varies but a common value of: *6C per 1000m (3.3F per 1000ft)* Unlike the dry AR, the moist AR is *not constant*, but varies greatly with temperature and, hence, with moisture content - Warm saturated air produces more liquid water than cold saturated air.
Environmental Lapse Rate
The rate of decrease of air temperature with elevation. It is most often measured with a *radiosonde*.
Specific Humidity
The ratio of the mass of water vapor in a given parcel, to the *total* mass of air in the parcel. AKA: Mass of the water vapor in the air parcel is compared with the mass of all the air in the parcel (including vapor). *use MIXING RATIO to determine* Expressed as grams of water vapor per kilogram of air (g/kg). *Remain constant as long as water vapor is not added to or removed from the parcel.* - This happens because the total number of molecules (and, hence, the mass of the parcel) remains constant, even as the parcel expands or contracts (since changes in parcel size do not affect specific humidity and mixing ratio).
Rain Shadow
The region on the leeside of a mountain where the precipitation is noticeably less than on the windward side.
Dew Point Temperature (DP)
The temperature to which air must be cooled (at constant pressure and constant water vapor content) for saturation to occur. DP is an important measurement used in predicting the formation of dew, frost, fog, and even the minimum temperature. DP can help determine the height of the base of a *cumulus cloud*. The dew point is a good indicator of the air's actual water vapor content. - High dew points indicate high water vapor content; *^DP = ^WV* - Low dew points, low water vapor content; *vDP = vWV* When air cools to the dew point, the relative humidity becomes 100 percent and the air is saturated.
UNUSUAL Types of Precipitation: (7
Virga: - Precipitation that falls from a cloud but *evaporates before reaching the ground*. Drizzle: - Small water drops between *0.2 - 0.5mm* in diameter, fall slowly, reduce visibility. Shower: - Intermittent precipitation from a *cumuliform cloud*, usually of short duration but often heavy. Fallstreaks: - Falling ice crystals that evaporate before reaching the ground. Blizzard: - A severe weather condition characterized by low temperatures and strong winds (*greater than 35 mi/hr*) bearing a great amount of snow either falling or blowing. Rime: - A white or milky granular deposit of ice formed by the rapid freezing of supercooled water drops as they come in contact with an object in below-freezing air. Snow Grains: - Precipitation in the form of very small, opaque grains of ice. The solid equivalent of drizzle.
Liquid (2)
Water molecules are much closer together. Constantly bump into each other.
Gas (2)
Water molecules move about freely. Mix well with neighboring atoms and molecules.
Noctilucent Clouds
Wavy, thin, bluish-white clouds that are best seen at twilight in *polar latitudes*. They form at altitudes about *80 to 90 km above the surface*. Composed of tiny ice crystals.
What is an UNstable atmosphere?
We determine the atmosphere's stability by comparing the environmental lapse rate (11C per 1000m) to the moist and dry adiabatic rates. A rising parcel of unsaturated surface air will cool at the dry adiabatic rate. Because the *dry AR < ELR, the parcel will be warmer than the surrounding air* and will continue to *rise, constantly moving upward*, away from its original position. The atmosphere is UNstable. Of course, a parcel of saturated air cooling at the lower moist adiabatic rate will be even warmer than the air around it. In both cases, the air parcels, once they start upward, will *continue to rise on their own because the rising air parcels are warmer and less dense than the air around them*. The atmosphere in this example is said to be ABSOLUTELY UNSTABLE.
Clouds found in a stable atmosphere: (4)
We might expect to see stratiform clouds—such as cirrostratus, altostratus, nimbostratus, or stratus—forming in a stable atmosphere.
Apparent Temperature
What the air temperature "feels like" for various combinations of air temperature and relative humidity. ex. An air temperature of 100F and a relative humidity of 60 percent produces an apparent temperature of 129F. ^T = ^RH = ^RT
Relationship between T, RH, and DP:
When the air temperature and dew point are far apart, the relative humidity is low; *T----DP = vRH* When they are close to the same value, the relative humidity is high; *T-DP = ^RH* When the air temperature and dew point are equal, the air is saturated and the relative humidity is 100 percent; *T=DP=saturation = RH of 100%*
Cooling of the air aloft may be due to: (2)
Winds bringing in colder air (cold advection). Clouds (or the air) emitting infrared radiation to space (radiational cooling).
