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.