Chapter 7Water and Atmospheric

Moisture

Geosystems 6eAn Introduction to Physical Geography

Robert W. ChristophersonCharles E. Thomsen

Frontal LiftingFronts: named after attacking air mass

Remember: cold air is denser, heavier

Cold FrontsCold air forces warm air aloft

400 km wide (250 mi)

Warm FrontsWarm air moves up and over cold air

1000 km wide (600 mi)

Frontal LiftingBoth cold and warm fronts cause air to be uplifted.

This causes adiabatic cooling, low pressure, cloud development, rainfall.

Keep in mind the properties of each type of air mass and their changes as the front passes.

Frontal LiftingThe BIG FIVE! These change depending on front:

HumidityTemperaturePressureWind direction/speedCloud types

Cold front: blue trianglesWarm front: red/orange half circles

Cold Front

Figure 8.11

Warm Front

Figure 8.13

Midlatitude Cyclone

Figure 8.14

Violent Weather Ice Thunderstorms  Tornadoes  Tropical Cyclones

= hurricanes

All of these are examples of ___ pressureAll operate on same principles, but occur on different spatial scalesAll can bring devastating weather

Freezing RainSupercooled water that falls to the surface as rain but upon impact freezes

The weight of accumulated ice breaks power lines and tree limbs

HailHail consists of ice pellets formed in roughly concentric layersFormed when water is frozen in the atmosphere. The ice pellet falls and encounters water, which freezes to the ice pellet forming a second layerThe size of hail is determined by the strength of the updraftHail has more water volume, snowflakes have more air volume

Hailstones

Air mass thunderstorms (ordinary thunderstorms): self-

extinguishing; localized short lived phenomena; limited vertical

wind shear.

Severe Thunderstorms: self-propagating

Supercell

Where: moist air + upliftingmoist air + uplifting Occurs frequently in the tropics, nearly daily in some

locations The U.S.’s most frequent regionThe U.S.’s most frequent region: the Gulf South; absolute

peak in Florida (land protrusion into warm waters)

Thunderstorms

Figure 8.20

Mesoscale Convective ComplexesMesoscale Convective Complexes

A mesoscale convective complex over eastern South Dakota

Squall Line ThunderstormsSquall Line Thunderstorms

Tornadoes

Figure 8.23 Figure 8.24

Tornadoes A tornado is defined as a small, very rapid wind vortex with extremely low air pressure in its center, formed beneath dense cumulonimbus clouds in proximity to a passing cold front.

Known as waterspouts when they form over oceans. If the circulation remains aloft, it is known as a funnel cloud.

Tornadoes – Characteristics & DimensionsTornadoes – Characteristics & Dimensions

A wide variety of shapes and sizes.

DiametersDiameters: typical ~100 yards, some 15 times as larger

DurationDuration: usually a short lived phenomena, only a few

minutes; some have lasted for hours

MovementMovement: is generally about 50km/hr (30 mph) over an

areas about 3-4 km (2-2.5 mi) long

Wind speeds: from 40mph to 280 mph

The most damaging aspect of a tornado is wind speed.

Tornadoes are spawned by frontal boundaries, squall lines, MCCs, and tropical cyclones.

The most severe tornadoes are spawned by severe thunderstorm cells known as supercells.

Tornadoes

Mesocyclone andTornado

Figure 8.23

Nonsupercell Tornado FormationNonsupercell Tornado Formation

Related to strong convection along a convergence zone

Tornadoes predominate in North America, especially in south-central states of Texas, Oklahoma, Kansas, Nebraska, and Iowa, and throughout the Midwest.

This region is commonly called tornado alley. A secondary concentration occurs in the southeastern U.S., especially Florida and Mississippi.

Tornadoes

Figure 8.25

In the U.S. alone, we can expect to have on average about 800 tornadoes, so this is a common phenomenon.

Most occur between 10 AM and 6 PM, and tornado season runs from March to July, with May seeing the heaviest occurrence.

Hurricanes Around the Globe: Atlantic and eastern Pacific: hurricanes.Atlantic and eastern Pacific: hurricanes. Western Pacific: typhoons. Western Pacific: typhoons. Indian Ocean and Australia: cyclonesIndian Ocean and Australia: cyclones

Hurricane CharacteristicsHurricane Characteristics Hurricanes: winds > 120 km/hr (> 74 mph)Hurricanes: winds > 120 km/hr (> 74 mph)

Compared with tornadoes: lesser in intensity, much larger in Compared with tornadoes: lesser in intensity, much larger in

size and longer life span size and longer life span much more devastating much more devastating

Size: Average diameters ~ 600 km (350 mi) and central Size: Average diameters ~ 600 km (350 mi) and central

pressure averages ~ 950 mb; may be as low as 870 mbpressure averages ~ 950 mb; may be as low as 870 mb

Fuel: from latent heat release in the cloud formation processFuel: from latent heat release in the cloud formation process

When & Where:When & Where:

Where warm waters abound and during the times of Where warm waters abound and during the times of

highest SSTs highest SSTs

When: August and September in the Northern When: August and September in the Northern

Hemisphere; January to March in the Southern HemisphereHemisphere; January to March in the Southern Hemisphere

Hurricane CharacteristicsHurricane Characteristics

Hurricanes consist ofHurricanes consist of:

A central eye surrounded by large cumulonimbus A central eye surrounded by large cumulonimbus

thunderstorms occupying the adjacent eye wall thunderstorms occupying the adjacent eye wall

Pressure differences into the center of the storm are about Pressure differences into the center of the storm are about

twice as great as the average mid-latitude cyclone, resulting twice as great as the average mid-latitude cyclone, resulting

in strong sustained windsin strong sustained winds

Eye and Eye WallEye and Eye Wall

EyeEye: an area of descending air and light winds; about 25 km : an area of descending air and light winds; about 25 km

(15 mi) in diameter on average; air in the eye to be warmer (15 mi) in diameter on average; air in the eye to be warmer

than elsewhere than elsewhere

A shrinking eye indicates storm intensificationA shrinking eye indicates storm intensification

Eye wallEye wall: comprised of the strongest winds, the largest : comprised of the strongest winds, the largest

clouds, and the heaviest precipitation with rainfall rates as high clouds, and the heaviest precipitation with rainfall rates as high

as 2500 mm/day (100 in.)as 2500 mm/day (100 in.)

Hurricane FormationHurricane Formation

StartStart: Tropical disturbances : Tropical disturbances

Often begin in the eastern ocean basins as disorganized Often begin in the eastern ocean basins as disorganized

clusters of thunderstormsclusters of thunderstorms

Some form in association with mid-latitude troughs Some form in association with mid-latitude troughs

migrating toward lower latitudes; migrating toward lower latitudes;

Some from ITCZ-related convection.Some from ITCZ-related convection.

Most associated with easterly wave.Most associated with easterly wave.

Hurricane FormationHurricane Formation

Tropical depression: below 37 mphTropical depression: below 37 mph

Tropical storm: further intensification to wind speeds of Tropical storm: further intensification to wind speeds of

60 km/hr (37 mph)60 km/hr (37 mph)

Hurricane: when winds reach or exceed 120 km/hr (74 Hurricane: when winds reach or exceed 120 km/hr (74

mph)mph)

A high percentage of depressions become tropical A high percentage of depressions become tropical

storms and an even higher percentage reach hurricane storms and an even higher percentage reach hurricane

status status

Conditions Necessary for Hurricane FormationConditions Necessary for Hurricane Formation

Hurricanes form only over deep water layers with surface Hurricanes form only over deep water layers with surface

temperatures in excess of 27 temperatures in excess of 27 ooC (81 C (81 ooF)F)

Coriolis force is an important contributor, and as such, Coriolis force is an important contributor, and as such,

hurricanes do not form equatorward of 5hurricanes do not form equatorward of 5oo

Strong vertical shear must be absent Strong vertical shear must be absent

Destruction by HurricanesDestruction by Hurricanes WindsWinds

Heavy rainfall Heavy rainfall

Storm surge is responsible for a large percentage of damage Storm surge is responsible for a large percentage of damage

along coastal regions (e.g., hurricane Camille caused a storm along coastal regions (e.g., hurricane Camille caused a storm

surge of 7 m (23 ft) along the Mississippi coast)surge of 7 m (23 ft) along the Mississippi coast)

High surf occurs atop the surge, increasing damageHigh surf occurs atop the surge, increasing damage

Destruction by HurricanesDestruction by Hurricanes

Winds and surge are typically most intense in the right front Winds and surge are typically most intense in the right front

quadrant of the storm. Why? Combination of wind speeds and quadrant of the storm. Why? Combination of wind speeds and

the speed of the storm’s movement.the speed of the storm’s movement.

Destruction by HurricanesDestruction by Hurricanes

The right front quadrant also produces the greatest The right front quadrant also produces the greatest

frequency of tornadoes within the hurricane. frequency of tornadoes within the hurricane.