PRECIPITATION, STORMS AND OTHER WEATHER PHENOMENA; CLIMATEThe principal actions brought on by weather systems that affect both land andsea and the humans, animals, and vegetation thereon are winds andprecipitation. The latter comes in a variety of forms as discussed below. Mostweather of consequence to people occurs in storms. These may be local in originbut more commonly are carried to locations in wide areas along pathwaysfollowed by active air masses consisting of Highs and Lows. This chart shows ameteorological classification of weather systems at various scales; the twocategories of most interest to us are the Mesoscale and the Synoptic Scale(sometimes called Macroscale).The key ingredient in storms is water, either as a liquid or as a vapor. The vaporacts like a gas and thus contributes to the total pressure of the atmosphere,making up a small but vital fraction of the total, as seen in this diagram:
Maps of the vapor pressure alone indicate its variability over a wide(subcontinental) region.Water vapor in the air will vary in amount depending on sources, quantities,processes involved, and air temperature. Heat, mainly as solar irradiation butwith some contributed by the Earth and human activity and some from change ofstate processes, will cause some water molecules either in water bodies(oceans, lakes, rivers) or in soils to be excited thermally and escape from theirsources. This is called evaporation; if water is released from trees and othervegetation the process is known as evapotranspiration. The evaporated water, ormoisture, that enters the air is responsible for a state called humidity. Absolutehumidity is the weight of water vapor contained in a given volume of air. TheMixing Ratio refers to the mass of the water vapor within a given mass of dry air.At any particular temperature, the maximum amount of water vapor that can be
contained is limited to some amount; when that amount is reached the air is saidto be saturated for that temperature. If less than the maximum amount ispresent, then the property of air that indicates this is its Relative Humidity (RH),defined as the actual water vapor amount compared to the saturation amount atthe given tempeature; this is usually expressed as a percentage. RH alsoindicates how much moisture the air can hold above its stated level, which, afterattaining, could lead to rain. This diagram helps to explain RH, the most commonway to indicated the moisture content of air.The red curve indicates the saturation condition for any given condition; itscorresponding water vapor content is directly related to its vapor pressure "e". At30°C, e is 40 millibars and the RH = 100%. If at that temperature, e was 20 mb,the RH would be 20/40 x 100 = 50%.When a parcel of air attains or exceeds RH = 100% condensation will occur andwater in some state will begin to organize as some type of precipitation. Onefamiliar form is dew, which occurs when the saturation temperature for somequantity of moisture reaches a temperature Td at the surface at whichcondensation sets in, leaving the moisture to coat the ground (especially obviouson lawns). This diagram set the condition for dew formation:
The term dew point has a more general use, being that temperature at which anair parcel must be cooled to become saturated. Dew frequently forms when thecurrent air mass contains excessive moisture after a period of rain but the air isnow clear; the dew precipitates out to coat the surface (noticeable on vegetation).Ground fog is a variant in which lowered temperatures bring on condensationwithin the near surface air as well as the ground.The other types of precipitation are listed in the following table along withdescriptive characteristics related to each type:
Precipitation requires development of some non-vapor form of water. A droplet ofwater does not necessarily begin its existence at precisely the saturationtemperature, i.e., may require some overcooling, also called supercooling; the airis then referred to as supersaturated. Commonly, the first to form are tiny nucleiof ice, which may start to grow around foreign particles such as dust. This icecrystal process is the most common starting point. But under some conditionsonly small droplets of liquid water are the first product of condensation. As theprocess proceeds, individual water-coated ice particles or tiny droplets aremoved around the condensing air mass (now a cloud) and collide repeatedly; thisleds to growth by coalescence. The next figure shows the relative differences insize of the water bodies that form in this way
This diagram suggests a possible history of water drops in a cloud. When thesize of a drop reaches a critical value, it may actually fall towards or to theground/sea surface as rain. But updrafts may carry the particle upward for moregrowth or change. Or, the drop may evaporate before it reaches the ground.Clouds near the surface are called warm clouds and almost always produceliquid precipitation. Clouds that are higher or form in much colder air are calledcold clouds and can produce ice nuclei that grow as more cold water precipitateson them (contact icing), yielding sleet (small ice droplets) or hail (ice bodies thatcan be centimeters in diameter). This is illustrated thusly:
The three general conditions in an atmospheric system that lead to local towidespread precipitation are 1) Convectional; 2) Orographic; and 3) Cyclonic.Convectional precipitation is usually associated with thunderstorms. Warm, moistair rises as an unstable air mass and cools adiabatically. The rising parcel iscommonly called a "thermal". As it reaches cooler air, and lower pressures,condensation begins and often yields numerous raindrops. These are eventuallytoo heavy for the growing cloud (cumulonimbus) and fall to Earth in torrents. Theupdrafts of wind recirculate and windflow on the ground may be turbulent andviolent.
These thunderstorm clouds are also described as supercells. Here is a sketch ofthis type of storm, and below it is an actual photograph of a supercell storm thatdisplays a prominent anvil cloud at its top. which occurs when air of differentproperties is encountered..
Convective thunderstorms are the most common type of atmospheric instabilitythat produces lightning followed by thunder. As seen in this image, lightning isone of the most spectacular phenomena witnessed in storms:
A typical lightning bolt can attain an electric potential up to 30 million volts and acurrent as much as 10000 amperes. It can cause air temperatures to reach10000°C. But a bolts duration is extremely short (fractions of a second).Although a bolt can kill people it hits, most can survive. A lightning bolt is thedischarge of electrons (negative charges) that build up in a cloud. Both negativeand positive charges accumulate from processes that derive them from theground or by ionization of the air. With both charges present in a thundercloud(the thunder itself occurs as a sound wave as superheated air rushes back intothe partial vacuum along the bolts path), much lightning is discharged in andremains within the cloud. But if the Earths surface is induced to have a surplus of+ (positive charges), as when - (negative) charges are drawn off and carriedupwards, the bolt may strike some spot on the ground if the potential difference isgreat enough. This diagram shows the sequence involved in forming lightning: From Lutgens and Tarbuck, 1998Orographic precipitation is a straightforward process, as depicted below. Moistair moves toward higher terrain - usually large mountain chains but smaller blockmountains can also induce the effect. This wind-driven air is forced up the slopesto elevations where both P and T are reduced. At the lower temperatures, themoist air mass becomes saturated and precipitation ensues - usually asthunderstorms in the summer or as widespread snow storms in the winter. Thisair then becomes "dried out" - most of its moisture has precipitated in the passover. The air mass that moves down the opposite slope is now drier and warmer.This side is said to be in a "rain shadow", i.e., general storms are infrequent and
arid conditions, with their characteristic vegetation, prevail. As it moves on, theair mass may gradually pick up moisture.Clouds on the windward side of a mountain belt (before uplift) are distinctive, asthese cirrus types demonstrate.An example from space of cloud formation over most of a large, high mountainsystem - in this case, the European Alps - confirms this tendency for alpinetopography to build up cloud cover.
On a much larger scale are the Mid-Latitude Cyclones which develop when polarair moves into latitudes largely within the 30 to 60° latitude range. Lows developand compete with highs as both types of systems are moved around by JetStreams and other contributors. These are the typical massive storms that affectEurope and Asia, North America, and to a lesser extent the southern continentsthroughout the year but are most noticeable and influential in the winter months.Some of the illustrations shown below are found on a Weather site maintained bythe University of Arizona.Before learning how these cyclonic systems form, lets look at one that shows updramatically over the British Isles as seen from space:
The general characteristics of Mid-Latitude Cyclonic systems are summarized inthis chartThe development of the cyclone follows a general sequence of stages beginningwith the advance of an Arctic Cold Front into cooler air (to the south in thenorthern hemisphere) and often ending with an occluded phase. A 5 stepsequence appears here; read the captions (click on image) for descriptions:
Stage 6 = weakening of the pressure gradient and system dissipation; not shown.
In the above five diagrams, read the captions for information that ties into thefollowing: The beginning of a large-scale cyclonic development occurs as anorthern cold air mass moves south (and often with an eastern component)against an air mass that is cooler, or even describable as warm; each air mass ismoving. At this first stage, the boundary between air masses becomes stationaryand air above it is in a pressure trough as air diverges horizontally. Air from thesurface replaces the upper air and this leads to a pressure drop or a low alongthe front. At the surface, winds move towards to lower pressure centers andbegin to circulate as a counterclockwise inspiral. The process is aided byimbalances in the jet stream where air is forced into uplift. The two fronts - coldand warm - are connected by an extratropical cyclone. This strengthens aloft andthe process of cyclogenesis begins to produce stormy conditions.With continued pressure drop, the cold front advances into the warm sector andthe angle between air masses lessens. During this mature stage, prominentwave shapes are developed in each front (wave cyclones). If the storm tracks tothe north of an observation point, that area will receive much rain if temperaturesare warm or snow if the near surface conditions are cold. The faster moving coldfront eventually overtakes the warm front, developing an occluded state, drivingthe warm air overhead. In time, the cyclone weakens as the storm moves moreto the east. The horizontal pressure gradient diminishes, dissipating the front(frontolysis) and the dissolving stage is reached. After the passage of a mid-latitude low, a high usually follows: If the pressure gradient between air massesis high (steep), a period of strong winds usually results.For the North American continent, the steering of this cold frontal system iscontrolled by the Westerlies (winds from the West) and storms in its path comeusually from the Canadian West. In the U.S some storms tend to movenotheastward along; these often originate in the Southwest, move east, and thentrack near the coast. Maritime polar air masses (see below) in the northwestAtlantic can produce east and northeast winds (along the northwest sector of aLow) causing severe storm conditions known as a "Noreaster" that can wreakhavoc on the Atlantic coastline and inland.During the Mature stage, this diagram indicates the relative conditions of airmovement at the surface and aloft.
This diagram shows the late stage of maturity and beginning of occlusion whenrainfall may be maximum.There are other conditions leading to precipitation, e.g., from hurricanes, that aredescribed below. For now, we will show first a general cross-section following alongitude line from pole to pole that indicates the most characteristic states ofprecipitation in the various zones previously named:
Note that for the United States the wettest region on average is the Southeast.Semi-arid conditions prevail from the western Great Plains over the Rockies andinto the Great Basin. True desert conditions are not widespread.This is a good time in this page to indicate the highly generalized degrees ofprecipitation proceeding from the equator to the poles. The deviations from idealpatterns are due mostly to the locations of continents and to the influence ofoceanic currents and streams. These in turn are factors in differing climates (seebelow)
We turn now to some specific types of wind (and sometimes accompanyingstorm) action. Most of these are mesoscale types of wind flow. We start with seabreezes. During the day these form because the ocean pressures near thesurface over cold waters cause wind to flow landward towards the lowerpressures of the air warmed by the land. At night, the land ground is cooler thanthe ocean surface, reversing the relative pressures and causing air flow to beseaward.On a larger scale, long period gravity waves can form in the upper atmosphere.This satellite image shows these regularly spaced linear clouds that representthe condensed moisture in a gravity wave train.
One mechanism of gravity wave formation is suggested in this diagramAir can move up mountain slopes during the day when high pressure winds moveagainst the mountain range that may have cooler air from the previous nightsdrop in temperatures, and down slopes, as cold air sinks, into valleys at night
The orographic effect can cause lifted air to lose moisture and become drier andwarmer. This gives rise to the Chinook Wind effect (a U.S. term; called foehnwinds elsewhere) involving hot winds coming off the mountains. When developedthese can often be strong and may cause problems if fires occur in timber andbrush on the mountains.
On a grander scale, warm air that forms beyond a mountain range cools as itrises to make a high and can then be driven over those mountains, cool moreand descend to low lands beyond. In Southern California, these are called SantaAna winds, which involve heating in the Mojave Desert to the north, passage ofthat warm air over the Transverse Ranges, and rapid descent (high winds) intothe Los Angeles Basin. This was the main factor in the disastrous fires duringOctober 2003 and since.In large, high land masses in colder regions of the world, e.g., Greenland, the airrisen adiabatically above the topographic highs can become quite cold. It then,
being heavy, moves off the pressure High to lower areas which may also be cool.The descending air is said to create katabatic winds.Air can be made to swirl at microscales as eddies (similar to gyres). Thiscircularlike behavior is often produced by small obstructions such as buildings orhills, and can be turbulent (remember the experience in walking through a citysdowntown on a windy day, or watching leaves in Fall become lifted in a spiralupdraft).Eddies can also have a vertical component as seen in this figure:
Eddies are also one of the factors responsible for air turbulence which most haveencountered during air flights. This is often due to wind shear, an airflowcondition that develops between two distinct air layers differing in velocity and/ordirection of movement. This is shown diagrammatically here, and beneath thatdiagram is a panel which further indicates how airplanes are affected.
Wind shear conditions often occur near ground surface and can affect a landingor taking off of an airplane with sufficient turbulence and downdraft to cause theplane to veer off course or drop too rapidly for the pilot to maintain control.Several major airline disasters in recent years are attributed to wind shear.Swirls of air, and clouds formed therein, at small mesoscales (50 - 100 km) havebeen described as eddy-like. Here is an example of the coast of Norway:We come now to two types of very severe storms - much feared anduncontrollable - that accompany extensive precipitation associated with very highwinds.The first is a mesoscale phenomenon associated with thunderstorms or strongMid-Latitude Cyclones: in America it is called a tornado (sometimes referred toas a "cyclone", as Dorothy called it in "The Wizard of Oz). When over water, theresulting funnel cloud is known as a "waterspout"; if developed over a desert, it issaid to be a "dust devil" (usually small and non-destructive). Here is a groundphoto of an advancing tornado:
Below is a radar image (side-looking) of a tornado embedded in its host cloud,not necessarily destined to reach the ground.Tornadoes are columns of rotating air that may be as thin as a few tens of feet oras wide as a mile. A tornado that forms a distinct cloud produces a vortex. Itsinterior pressure may be as much as 20% lower than external air. As air is drawnin, it swirls and spirals upwards - it is thus a very pronounced local low pressurecenter.. Wind speeds in the swirl can exceed 250 mph (400 km/hr) and thosestrong winds together with the lower interior pressures can exert great forces onobjects encountered, such as buildings. Air is sucked from structures hit bytornadic winds and in a sense this produces an implosion. The tornado cloud isusually a mix of condensed water and dust and debris picked up from surfaces ittraversesThe next diagrams are variants designed to show how tornadoes form. Theycommonly develop along the squall line that marks the Frontal Boundary of anadvancing Mid-Latitude Cyclone. The favored condition is a clash between verycold maritime polar air and very warm, moist maritime tropical air moved into
temperate zones. In the upper diagram, a typical severe thunderstorm isdepicted. Within it rain moves downward pulling air with it. But within the formingvortex rising air is carried upward in a counterclockwise swirl to establish thetornadic winds. In the lower diagram, a cutaway sketch of a thunderstorm cloud,the tornado results where warm moist air from outside the cloud circulatingcounterclockwise is joined by cold air within the cloud that also becomesentrapped and itself circulates ccw.
This plan view indicates the wind circulation in a thundercloud capable ofdeveloping a tornado.Most tornadoes in the U.S. develop east of the Rocky Mountains. The flatness ofthe terrain in the Great Plains eastward means that there are few obstructions tohamper wind circulation. In typical years 500 to 1000 tornadoes are eitherobserved or detected by radar in that region. Specialized radar is used to pick upa forming tornado and, if there is enough time, alarms are sounded and peoplealerted via radio and TV. Many tornados do not touch down. Most that do havepaths of destruction of only a few miles length (and normally less than a mile inwidth) but some create paths up to 100 miles long. This is a map that shows thefrequency of occurrence of tornadoes in the U.S.
The Great Plains states are most susceptible to tornadoes, followed by the UpperMidwest and Florida. They are rare, but not unknown, further west or in NewEngland. A system has been devised known as the Fujita scale (named after ameteorologist at the University of Chicago) to rate tornadic severity and expecteddamage, as presented in this Table
Most tornadoes are lower intensity events; a Category V tornado is rare andunbelievably destructive. The frequency of occurrence of each category is shownin this pie chart:Dust devils form as a swirling updraft under sunny conditions during fair weather.Also called whirlwinds, they are common in desert climates, and are almostalways harmless.
Waterspouts closely resemble continental tornadoes. Many are in fact tornadicwinds and are associated with storm clouds (as in the figure below). Others arenon-tornadic and occur beneath cumulus clouds. A waterspout usually is light-colored owing to the water it has sucked up from an ocean or lake.
Now we will consider another potentially violent storm that affects very largeareas over which its path crosses. This storm is developed almost exclusively inthe Trade Winds zone 30° north and south of the Equator. Called a Hurricanewhen formed in low latitudes of the Atlantic, the same phenomenon is known asa Cyclone in much of the Pacific. A more general term is "Tropical Cyclone".Here is a satellite view of Hurrican Isadore in the Gulf of Mexico.
Hurricanes are impressive storms when seen from the ground - and even moreimpressive when any individual is caught within one. Here are two groundphotos:One of the hallmarks of hurricanes often is the appearance of small, puffycumulus clouds that build up before the hurricane arrives; sometimes this type isa predecessor of a tornado:
Hurricanes are more extreme members of tropical disturbances that, in theAtlantic, tend to begin of the coast of equatorial Africa. In the next two charts, theproperties of Hurricanes are listed in the first figure using the Saffir-Simpsonscale and the amount of damage expected from each category is given in thesecond table.
In the next sketches of a developing hurricane, its growing size and resultantwind patterns are depicted. Note that there are three stages of development:Tropical Disturbance (winds less than 35 kph [22 mph]); Tropical Depression(max. winds < 61 kph [38 mph]); Tropical Storm (max winds < 115 kph [71 mph];Hurricane (when its central winds exceed 115 kph [71 mph]):
This diagram shows a side view of the temperature, moisture and pressureconditions that lead to a hurricane.A cutaway diagram of a hurricane locates the rain cells and the central eye,within which the winds tend to be much lower ("calm of the Eye") where warm airis spiraling downward in counterclockwise motion. Note that outer winds aloftappear to drive the swirls in a clockwise motion.
Atlantic hurricanes usually begin off the African coast in subtropical waters thatare very warm in summer - often in excess of 28° C (however, in the southernAtlantic, wintertime waters prevent hurricanes from forming there). Ocean wateris vaporized by the Suns rays, producing warm air that rises as shown above. Itsmoisture then condenses with upward cooling, releasing great amounts of latentheat that drive the disturbance until, if conditions persist, it graduates into ahurricane. This rapid upward deployment also reduces surface pressures at thebase which further accelerates the rise. Some of the rising air spills outward,some inward to flow down around and in the Eye.Hurricanes in the Atlantic tend to average from about 5 to 15 events per year.Worldwide, between 80 and 100 normally occur. Most are Categories 1 to 3.Nevertheless, if they make landfall on the North American continent or pass overAtlantic or Carribean islands, the damage they wreak can be huge - hurricanes inthe 1990s caused billions of dollars in homes, businesses and infrastructurewiped out or damaged beyond practical repair. In recent decades, Early WarningSystems - built on hurricane tracking programs - have kept loss of life low. But in1906 a Carribean hurricane struck Texas around the Galveston coastline, killingmore than 9000 there. Monsoonal cyclones in the Indian Ocean often takethousands of lives, especially in low-lying parts of the Bangladesh coastal zoneswhere the Ganges River Delta has built up. Here is a global map showinggeneral pathways of hurricanes developed in tropical climes of the differentoceans
Driven by westward flowing upper level trade winds, and powered by theirextreme internal energy derived from hot air condensation, hurricanes moveacross the Atlantic toward North and South America. Depending on interactionswith air masses on the continents or the open ocean, hurricanes will frequentlybe blocked or deflected northward and may or may not make landfall. Thisdiagram, applied to the September 2004 Hurricane Jeanne, shows how thecirculation of a mid-continent low and Atlantic offshore high determined howJeanne was deflected north after leaving Florida. The low generates acounterclockwise wind, the high clockwise, together leading to convergence.An the Atlantic, hurricanes move westward but usually start to move towards theNorth as they meet continental airflow. Many then swerve to the northeast.whether on land by then or if they are deflected into the Atlantic. There they
encounter cooler waters that deprive the waning hurricanes of the energy neededfor sustenance.Here is a map of the Atlantic Hurricane season of 1995; note that the hurricaneswere referred to then by popular human first names (male one year; female thenext; now in 2004 the male-female appellations are alternated during a singlehurricane season).It is instructive to look at the map below which plots the worldwide distribution ofthe paths of major cyclones (hurricanes) during the last 150 years.
There are several distinctive patterns revealed in this map. First, the regionssusceptible to the most frequent and intense storms are the Philippines in thewestern Pacific, off the Mexican coast, and the Caribbean-Gulf of Mexico off thesoutheast U.S. Second, there appears to be a dearth of cyclones around theequatorial latitudes. That is because of the minimal influence of the Corioliseffect, near zero around the equator (maximum at the poles), which thusprevents unstable air that moves to the right north of the equator and to the left inthe southern hemisphere from acquiring the spin needed to organize andmaintain coherence of cyclonic air masses in this zone. Third, note the nearabsence of cyclones in the South Atlantic and Southwest Pacific, where oceancurrents (Peru Current in the Pacific and Benguela Current off Africa) break upthe formation of cyclones because of the cool waters they introduce. Fourth, thetendency of hurricanes/cyclones to start to swing poleward and even intotrajectories towards the east is evident; this results from the action of the westerlywinds in both hemispheres.We close this mini-tutorial covering the rudiments of Meteorology with a briefdiscussion of Climate. Climate can be defined in two ways: 1) the generalcharacteristics of temperature ranges, amounts of precipitation, frequency ofcloud cover, and seasonal variations of these conditions that are typical ofextensive regions on land and sea at various latitudes and longitudes; and 2) theprevailing weather conditions locally where one lives or moves about. Those whohave experienced the year-round climatic conditions in San Francisco and inPittsburgh, PA will readily recognize the differences seasonally between the twoplaces. In the United States, we often speak of a New England climate, anAtlantic Coastal climate, the Mid-Atlantic climate, Floridas climate, climatearound the Mississippi, the Great Plains (north notably different than the south), aRocky Mountain climate, an Arizona climate, Seattles climate, and the WestCoast climate. People often choose a particular climate as favorable to theirhealth and life style. Within any climate, there will be day-to-day weatherchanges, but these are just the variants that intrude on a region at different timesof the year.The three factors that most influence what a localitys or regions climate will beare shown in this chart:
In addition to latitude, terrain and ocean currents, one could also cite the typesand entry locations of air masses. For example, the next figure shows that in thesummer months two offshore air mass Highs have a big role to play.Climates have been categorized and described by various classifications. Theone most often used is the Koppen Classification. Two charts show the principalcategories, listed by combinations of one to three letters (defined in the upperchart) which describe temperature and rainfall condition and other relevantfactors.
Using this classification, a general worldwide map of the major climate types isshown as the final figure; refer to the letters in the previous chart to find thedescription of the climate in a particular region.So there you have it - the basic principles, ideas, and applications of the Scienceof Meteorology - Weather and Climate - reduced to four (longish) pages. Wehope that those unfamiliar with these concepts will learn enough by perusing(best repetitively) these four pages before exploring the rest of Section 14.Primary Author: Nicholas M. Short, Sr.