Remote Sensing Tutorial Page 14-1
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PRECIPITATION, STORMS AND OTHER WEATHER PHENOMENA; CLIMATE

The principal features of weather systems that affect both land and sea and the humans, animals, and vegetation thereon are winds and precipitation. The latter comes in a variety of forms as discussed below. Most weather of consequence to people occurs in storms. These may be local in origin but more commonly are carried to locations in wide areas along pathways followed by active air masses consisting of Highs and Lows. This chart shows a meteorological classification of weather systems at various scales; the two categories of most interest to us are the Mesoscale and the Synoptic Scale (sometimes called Macroscale).

Scales of weather phenomena.

The key ingredient in storms is water, either as a liquid or as a vapor. The vapor acts 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:

Vapor as a constituent of the atmosphere; its amount is suggested by the pressure it contributes to the total air pressure.

Maps of the vapor pressure alone indicate its variability over a wide (subcontinental) region.

Map of vapor pressure variations (as isobars of Pvap) on a continental scale).

Water vapor in the air will vary in amount depending on sources quantities, processes involved, and air temperature. Heat, mainly as solar irradiation but with some contributed by the Earth and human activity and some from change of state processes, will cause some water molecules either in water bodies (oceans, lakes, rivers) or in soils to be excited thermally and escape from their sources. This is called evaporation; if water is released from trees and other vegetation the process is known as evapotranspiration. The evaporated water, or moistue, that enters the air is responsible for a state called humidity. Absolute humidity is the weight of water vapor contained in a given volume of air. The Mixing Ratio is 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 said to be saturated for that temperature. If less than the maximum amount is present, 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 at the given tempeature; this is usually expressed as a percentage. This diagram helps to explain RH, the most common way to indicated the moisture content of air.

Relative Humidity diagram.

The red curve indicates the saturation condition for any given condition; its corresponding water vapor content is specified by its vapor pressure "e". At 30°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 and water in some state other than vapor will begin to form as some type of precipitation. One familiar form is dew, which occurs when the saturation temperature for some quantity of moisture reaches a temperature low at the surface at which condensation sets in, leaving the moisture to coat the ground (especially obvious on lawns). This diagram set the condition for dew formation:

Formation of dew in the cool of early morning.

The term dew point has a more general use, being that temperature at which an air parcel must be cooled to become saturated.

The other types of precipitation are listed in the following table along with descriptive characteristics related to each type:

Precipitation types.

Precipitation requires development of some non-vapor form of water. A droplet of water does not necessarily begin its existence at precisely the saturation temperature, i.e., may require some overcooling, also called supercooling; the air is then referred to as supersaturated. Commonly, the first to form are tiny nuclei of ice, which itself may start to grow on foreign particles such as dust. This ice crystal process is the most common starting point. But under some conditions only small droplets of liquid water are the first product of condensation. As the process proceeds, individual water-coated ice particles or tiny droplets are moved around the condensing air mass (now a cloud) and collide repeatedly; this leds to growth by coalescence. The next figure shows the relative differences in size of the water bodies that form in this way

Relative sizes of water bodies that make rain drops.

This diagram suggests a possible history of water drops in a cloud. When the size of a drop reaches a critical value, it may actually fall towards or to the ground/sea surface as rain. But updrafts may carry the particle upward for more growth or change. Or, the drop may evaporate before it reaches the ground.

A typical history of a water droplet.

Clouds near the surface are called warm clouds and produce almost always liquid precipitation. Clouds that are higher or form in much colder air are called cold clouds and can produce ice nuclei that grow as more cold water precipitates on them (contact icing), yielding sleet (small ice droplets) or hail (ice bodies that can be centimeters in diameter. This is illustrated thusly:

Cold and Warm Clouds, and precipitation types they produce.

The three general conditions in an atmospheric system that led to local to widespread participation are 1) Convectional; 2) Orographic; and 3) Cyclonic.

Convectional precipitation is usually associated with thunderstorms. Warm, moist air rises as an unstable air mass and cools adiabatically. The rising parcel is commonly called a "thermal". As it reaches cooler air, and lower pressures, condensation begins and often yields numerous raindrops. These are eventually too heavy for the growing cloud (cumulonimbus) and fall to Earth in torrents. The updrafts of wind recirculate and windflow on the ground may be turbulent and violent.

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Formation of a Thunderstorm

These thunderstorm clouds are also described as supercells. Here is a sketch of this type of storm, and below it is an actual photograph of a supercell storm that displays a prominent anvil cloud at its top. which occurs when air of different properties is encountered..

A sketch of a supercell thunderstorm.

Photograph of a large thunderstorm, with a spreading anvil cloud extension near its top where the supercell has been blocked by air conditions that force spreading rather than rising

Convective thunderstorms are the most common type of atmospheric instability that produces lightning followed by thunder. As seen in this image, lightning is one of the most spectacular phenomena witnessed in storms:

Multibolts of lightning.

A typical lightning bolt can attain a pressure up to 30 million volts and a current as much as 10000 amperes. It can cause air temperatures to reach 10000°C. But a bolt's duration is extremely short (fractions of a second). Although it can kill people a bolt reaches, some can survive. A lightning bolt is the discharge of electrons (negative charges) that build up in a cloud. Both negative and positive charges accumulate from processes that derive them from the ground or by ionization of the air. With both charges present in a thundercloud (the thunder itself occur as superheated air rushes back into the bolt's path), much lightning is discharged in and remains within the cloud. But if the Earth's surface is induced to have a surplus of + (positive charges), as when - (negative) charges are drawn off and carried upwards, the bolt may strike some spot on the ground if the potential difference is great enough. This diagram shows the sequence involved in forming lightning:

Sequential steps in forming lightning
From Lutgens and Tarbuck, 1998

Orographic precipitation is a straightforward process, as depicted below. Moist air moves toward higher terrain - usually large mountain chains but smaller block mountains can also induce the effect. This wind-driven air is forced up the slopes to elevations where both P and T are reduced. At the lower temperatures, the moist air mass becomes saturated and precipitation ensues - usually as thunderstorms in the summer or as widespread snow storms in the winter. This air then becomes "dried out" - most of its moisture has precipitated in the pass over. The air moves down the opposite slope as dry and then 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, the air mass may gradually pick up moisture.

Schematic showing how orographic precipitation takes place.

Clouds on the windward side of a mountain belt (before uplift) are distinctive, as these cirrus types demonstrate.

Cirrus clouds form as an air mass approaches a mountain range.

An example from space of cloud formation over most of a large, high mountain system - in this case, the European Alps - confirms this tendency for alpine topography to build up cloud cover.

Clouds produced orographically over Europe's Alps.

On a much larger scale are the Mid-Latitude Cyclones which develop when polar air moves into latitudes largely within the 30 to 60° range. Lows develop and compete with highs as both types of systems are moved around by Jet Streams and other contributors. These are the typical massive storms that affect Europe and Asia, North America, and to a lesser extent the southern continents throughout 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 by the University of Arizona.

Before learning how these cyclonic systems form, lets look at one that shows up dramatically over the British Isles as seen from space:

A large cyclonic weather system approaching Europe via the British Isles.

The general characteristics of Mid-Latitude Cyclonic systems are summarized in this chart

The salient features of a Mid-Latitude Cyclone

The development of the cyclone follows a general sequence of stages beginning with the advance of an Arctic Cold Front into cooler air (to the south in the northern hemisphere) and often ending with an occluded phase. A 5 step sequence appears here:

Stage 1:Cold and Warm air masses meet in a front and move parallel to it Stage 2: a wave forms and warm air starts to move poleward; cold air equatorward.

Stage 3: Cyclonic (ccw) circulation develops, with general surface convergence and upliftingStage 4: Cold Front moves faster than Warm Front and starts to overtake it (end of Mature Stage.

Stage 5: Full development of and Occluded Front, with maximum intensity of the wave cyclone.

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 the following: The beginning of a large-scale cyclonic development occurs as a northern 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 is moving. At this first stage, the boundary between air masses becomes stationary and air above it is in a pressure trough as air diverges horizontally. Air from the surface replaces the upper air and this leads to a pressure drop or a low along the front. At the surface, winds move towards to lower pressure centers and begin to circulate as a counterclockwise inspiral. The process is aided by imbalances in the jet stream where air is forced into uplift. The two fronts - cold and warm - are connected by an extratropical cyclone. This strengthens aloft and the process of cyclogenesis begins to produce stormy conditions. With continued pressure drop, the cold front advances into the warm sector and the angle between air masses lessens. During this mature stage, prominent wave shapes are developed in each front (wave cyclones). If the storm tracks to the north of an observation point, that area will receive much rain if temperatures are warm or snow if the near surface conditions are cold. The faster moving cold front eventually overtakes the warm front, developing an occluded state, driving the warm air overhead. In time, the cyclone weakens as the storm moves more to the east. The horizontal pressure gradient diminishes, dissipating the front (frontolysis) and the dissolving stage is reached. The steering of this frontal system is controlled by the Westerlies. (However, some storms tend to move notheastward along (for the U.S.) the coast and produce east and northeast winds causing a severe storm known as a "Noreaster" that can wreak havoc on the Atlantic coastline. After the passage of a mid-latitude low, a high usually follows: If the pressure gradient between air masses is high (steep), a period of strong winds usually results.

During the Mature stage, this diagram indicates the relative conditions of air movement at the surface and aloft.

Surface and aloft movements of air during the Mid-Latitude Cyclone.

This diagram shows the late stage of maturity and beginning of occlusion when rainfall may be maximum.

Conditions favoring heavier rain or snow fall.

There are other conditions leading to precipitation, e.g., hurricanes, that are described below. For now, we will show first a general cross-section following a longitude line from pole to pole that indicates the most characteristic states of precipitation in the various zones previously named:

Precipitation characterics in named zones bounded by different latitudes.

This is a good time in this page to indicate the highly generalized degrees of precipitation at a global scale. The deviations from ideal patterns are due mostly to the locations of continents and to the influence of oceanic currents and streams. These in turn are factors in differing climates (see below)

Precipitation variations (average values regionally) worldwide.

We turn now to some specific types of wind (and sometimes accompanying storm) action. Most of these are mesoscale types of wind flow. We start with sea breezes. During the day these form because the ocean pressures near the surface over cold waters cause wind to flow landward towards the lower pressures of the air warmed by the land. At night, the land ground is cooler than the ocean surface, reversing the relative pressures and causing air flow to be seaward.

Direction of surface flow as breezes from a cooler ocean to a warmer land during the day.

On a larger scale, long period gravity waves can form in the upper atmosphere. This satellite image shows these regularly spaced linear clouds that represent the condensed moisture in a gravity wave train.

Gravity wave clouds in the northeastern U.S.

One mechanism of gravity wave formation is suggested in this diagram

Formation of wave clouds; passage of air over a mountain range can trigger a cyclic response at high altitudes.

Air can move up mountain slopes during the day when high pressure winds move against the mountain range that may have cooler air from the previous night's drop in temperatures, and down slopes, as cold air sinks, into valleys at night

Upslope mountain air.

Cold air from cooling upper reaches of a mountain that flows (drains) into valley as denser air.

The orographic effect can leave lifted air to lose moisture and become drier and warmer. This gives rise to the Chinook Wind effect (a U.S. term; called foehn winds elsewhere) involving hot winds coming off the mountains. When developed these can often be strong and may cause problems if fires occur in timber and brush on the mountains.

Chinook Winds.

On a grander scale, warm air that forms beyond a mountain range cools as it rises to make a high and can then be driven over those mountains, cool more and descend to low lands beyond. In Southern California, these are called Santa Ana winds, which involve heating in the Mojave Desert to the north, passage of that warm air over the Transverse Ranges, and rapid descent (high winds) into the Los Angeles Basin. This was the main factor in the disastrous fires during October 2003.

Pressure field involving production of Santa Ana winds in California.

In large, high land masses in colder regions of the world, e.g., Greenland, the air risen 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.

katabatic wind conditions.

Air can be made to swirl at microscales as eddies (similar to gyres). This circularlike behavior is often produced by small obstructions such as buildings or hills, and can be turbulent (remember the experience in walking through a city's downtown on a windy day, or watching leaves in Fall become lifted in a spiral updraft.

Eddies formed by obstructions.

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 have encountered during air flights. This is often due to wind shear, an airflow condition that develops between two distinct air layers differing in velocity and/or direction of movement. This is shown diagrammatically here, and beneath that diagram is a panel which further indicates how airplanes are affected.

Eddies caused by contrasts in two atmospheric layers, causing shear

Further example of eddy formation responsible for turbulence.

Wind shear condition often occur near ground surface and can affect a landing or taking off of an airplane with sufficient turbulence and downdraft to cause the plane 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) have been described as eddy-like. Here is an example of the coast of Norway:

Eddy cloud formation off the southwest coast of Norway.

We come now to two types of very severe storms - much feared and uncontrollable - that accompany extensive precipitation associated with very high winds.

The first is a mesoscale phenomenon associated with thunderstorms or strong Mid-Latitude Cyclones: in America it is called a tornado (sometimes referred to as a "cyclone", as did Dorothy note in "The Wizard of Oz). When over water, the resulting funnel cloud is known as a "waterspout"; if developed over a desert, it is said to be a "dust devil" (usually small and non-destructive. Here is a ground photo of an advancing tornado:

The dark funnel cloud of a tornado extending surfaceward from a cumulonimbus cloud.

The image below is a radar image of a tornado embedded in its host cloud, not necessarily destined to reach the ground.

Ground radar image of a tornado.

Tornadoes are columns of rotating air that may be as thin as a few tens of feet or as wide as a mile. A tornado that forms a distinct cloud produces a vortex. Its interior pressure may be as much as 20% lower than external air. As air is drawn in, it swirls and spirals upwards - it is thus a very pronounced local low pressure center.. Wind speeds in the swirl can exceed 250 mph (400 km/hr) and those strong winds together with the lower interior pressures can exert great forces on objects encountered, such as building. Air is sucked from structures hit by tornadic winds and in a sense produces an implosion. The tornado cloud is usually a mix of condensed water and dust and debris picked up from surfaces it traverses

The next diagrams are variants designed to show how tornadoes form. They commonly develop along the squall line that marks the Frontal Boundary of an advancing Mid-Latitude Cyclone. Favored conditions are very cold maritime polar air and very warm, moist maritime tropical air moved into temperate zones. In the upper diagram, a typical severe thunderstorm is depicted. Within it rain moves downward pulling air with it. But at some point(s) within it air is carried upward in a counterclockwise swirl to establish the tornado. In the lower diagram, a cutaway sketch of a thunderstorm cloud, the tornado results where warm moist air from outside the cloud circulating counterclockwise is joined by cold air within the cloud that also becomes entrapped and itself circulates ccw.

Mode of formation of a tornado.

Another schematic showing wind circulation in a tornado.

This plan view indicates the wind circulation in a thundercloud capable of developing a tornado.

Vortex development of tornadic winds in a thunderstorm cloud.

Most tornadoes in the U.S. develop east of the Rocky Mountains. In typical years 500 to 1000 tornadoes are either observed or detected by radar. Specialized radar is used to pick up a forming tornado and sounding alarms or alerting via radio and TV. Many tornados do not touch down. Most that do have paths of destruction of only a few miles length (and normally less than a mile in width) but some create paths up to 100 miles long. This is a map that shows the frequency of occurrence of tornadoes in the U.S.

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The Great Plains states are most susceptible to tornadoes, followed by the Upper Midwest and Florida. They are rare, but not unknown, further west or in New England. A system has been devised known as the Fujita scale (named after a meteorologist at the University of Chicago) presented in this Table

The Fujita Tornado Intensity Scale

Most tornadoes are lower intensity events; a Category V tornado is rare and unbelievably destructive. The frequency of occurrence of each category is shown in this pie chart:

Frequency of Occurrence of the five categories of tornadoes.

Now we will consider another potentially violent storm that affects very large areas over which its path crosses. This storm is developed almost exclusively in the Trade Wind zone 30° north and south of the Equator. Called a Hurricane when formed in low latitudes of the Atlantic, the same phenomenon is known as a Cyclone in much of the Pacific. A more general term is "Tropical Cyclone". Here is a satellite view of Hurrican Isadore in the Atlantic.

Hurricane Isadore

Hurricanes are more extreme members of tropical disturbances that, in the Atlantic, tend to begin of the coast of equatorial Africa. In the next two charts, the properties of Hurricanes are listed in the first figure using the Saffir-Simpson scale and the amount of damage expected from each category is given in the second table.

The Saffir-Simpson scale of tropical storms.

Damage inflicted by Hurricanes of different strengths (categories).

In the next four sketches of a developing hurricane, the size and wind patterns are depicted.

The first stage of a hurricane, starting as a tropical depression.

Life Cycle of a Hurricane as it develops to its fullest extent before moving over land or water and blowing itself out.

This diagram shows a side view of the temperature, moisture and pressure conditions that lead to a hurricane.

Cross-section of hurricane build-up as it develops several concentric shells (cells) that participate in the counterclockwise movement of the moisture-saturated warm air in the arms of the 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 air is spiraling downward in counterclockwise motion.

Another view of a hurricane, showing the eyewall and other features.

Hurricanes in the Atlantic tend to average from about 5 to 15 events per year. Most are Categories 1 to 3. Nevertheless, if they make landfall on the North American continent or pass over Atlantic or Carribean islands, the damage they wreak can be huge - hurricanes in the 1990s caused billions of dollars in homes, businesses and infrastructure wiped out or damaged beyond practical repair. In recent decades, Early Warning Systems - built on hurricane tracking programs - have kept loss of life low. But in 1906 a Carribean hurricane struck Texas around the Galveston coastline, killing more than 9000 there. Monsoonal cyclones in the Indian Ocean often take thousands of lives, especially in low-lying parts of the Bangladesh coastal zones where the Ganges River Delta has built up. Here is a global map showing general pathways of hurricanes developed in tropical climes of the different oceans

World map of the principal pathways of hurricanes and cyclones.

Driven by westward flowing upper level trade winds, and powered by their extreme internal energy derived from hot air condensation, hurricanes move across the Atlantic toward North and South America. Depending on interactions with air masses on the continents or the open ocean, hurricanes will frequently be blocked or deflected northward and may or may not make landfall. This diagram, applied to the September 2004 Hurricane Jeanne, shows how the circulation of a mid-continent low and Atlantic offshore high determined how Jeanne was deflected north after leaving Florida. The low generates a counterclockwise wind, the high clockwise, together leading to convergence.

Reasons for northward deflection of Hurricane Jeanne.

Here is a map of the Atlantic Hurricane season of 1995; note that the hurricanes were referred to then by popular human first names (male one year; female the next; now in 2004 the male-female appellations are alternated during a single hurricane season).

Pathways of Hurricanes crossing westward over the Atlantic and then moving anywhere from due west to northeast.

We close this mini-tutorial covering the rudiments of Meteorology with a brief discussion of Climate. Climate can be defined in two ways: 1) the general characteristics of temperature ranges, amounts of precipitation, frequency of cloud cover, and seasonal variations of these conditions that are typical of regions on land and sea at various latitudes and longitudes; and 2) the prevailing weather conditions locally where one lives or moves about. Those who have experienced the year-round climatic conditions in San Francisco and in Pittsburgh, PA will readily recognize the differences seasonally between the two places. In the United States, we often speak of a New England climate, an Atlantic Coastal climate, the Mid-Atlantic climate, Florida's climate, climate around the Mississippi, the Great Plains (north notably different than the south), a Rocky Mountain climate, an Arizona climate, Seattle's climate, and the West Coast climate. People often choose a particular climate as favorable to their health and life style. Within any climate, there will be day-to-day weather changes, but these are just the variants that intrude on a region at different times of the year.

The three factors that most influence what a locality's or region's climate will be are shown in this chart:

Factors influence Climate.

In addition to latitude, terrain and ocean currents, one could also cite the types and entry locations of air masses. For example, the next figure shows that in the summer months two offshore air mass Highs have a big role to play.

The Bermuda and Pacific Highs off the North American coast.

Climates have been categorized and described by various classifications. The one most often used is the Koppen Classification. Two charts show the principal categories, listed by combinations of one to three letters (defined in the uppe chart) which describe temperature and rainfall condition and other relevant factors.

One version of the Koppen Climate Classification.

A variant of the Koppen Classification, that provides some additional details.

Using this classification, a general worldwide map of the major climate types is shown as the final figure.

Climates of the World using the Koppen system of Classification.

So there you have it - the basic principles, ideas, and applications of the Science of Meteorology - Weather and Climate - reduced to four (longish) pages. We hope that those unfamiliar with these concepts will learn enough by perusing (best repetitively) these pages before exploring the rest of Section 14.

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Primary Author: Nicholas M. Short, Sr. email: nmshort@ptd.net