Chapter 7 Water and Atmospheric Moisture Geosystems 6e An Introduction to Physical Geography Robert W. Christopherson Charles E. Thomsen

Frontal Lifting Fronts: named after attacking air mass Remember: cold air is denser, heavier Cold Fronts Cold air forces warm air aloft 400 km wide (250 mi) Warm Fronts Warm air moves up and over cold air 1000 km wide (600 mi)

Frontal Lifting Both 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 Lifting The BIG FIVE! These change depending on front: Humidity Temperature Pressure Wind direction/speed Cloud types Cold front: blue triangles Warm 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 ___ pressure All operate on same principles, but occur on different spatial scales All can bring devastating weather

Freezing Rain Supercooled water that falls to the surface as rain but upon impact freezes The weight of accumulated ice breaks power lines and tree limbs

Hail Hail consists of ice pellets formed in roughly concentric layers Formed when water is frozen in the atmosphere. The ice pellet falls and encounters water, which freezes to the ice pellet forming a second layer The size of hail is determined by the strength of the updraft Hail 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 + uplifting  Occurs frequently in the tropics, nearly daily in some locations The U.S.’s most frequent region: the Gulf South; absolute peak in Florida (land protrusion into warm waters)

Thunderstorms Figure 8.20

Mesoscale Convective Complexes A mesoscale convective complex over eastern South Dakota

Squall 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 & Dimensions A wide variety of shapes and sizes. Diameters: typical ~100 yards, some 15 times as larger Duration: usually a short lived phenomena, only a few minutes; some have lasted for hours Movement: 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

Tornadoes 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.

Mesocyclone and Tornado Figure 8.23

Nonsupercell 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. Western Pacific: typhoons. Indian Ocean and Australia: cyclones

Hurricane Characteristics Hurricanes: winds > 120 km/hr (> 74 mph) Compared with tornadoes: lesser in intensity, much larger in size and longer life span  much more devastating Size: Average diameters ~ 600 km (350 mi) and central pressure averages ~ 950 mb; may be as low as 870 mb Fuel: from latent heat release in the cloud formation process When & Where: Where warm waters abound and during the times of highest SSTs When: August and September in the Northern Hemisphere; January to March in the Southern Hemisphere

Hurricane Characteristics Hurricanes consist of: A central eye surrounded by large cumulonimbus thunderstorms occupying the adjacent eye wall Pressure differences into the center of the storm are about twice as great as the average mid-latitude cyclone, resulting in strong sustained winds

Eye and Eye Wall Eye: an area of descending air and light winds; about 25 km (15 mi) in diameter on average; air in the eye to be warmer than elsewhere A shrinking eye indicates storm intensification Eye wall: comprised of the strongest winds, the largest clouds, and the heaviest precipitation with rainfall rates as high as 2500 mm/day (100 in.)

Hurricane Formation Start: Tropical disturbances Often begin in the eastern ocean basins as disorganized clusters of thunderstorms Some form in association with mid-latitude troughs migrating toward lower latitudes; Some from ITCZ-related convection. Most associated with easterly wave.

Hurricane Formation Tropical depression: below 37 mph Tropical storm: further intensification to wind speeds of 60 km/hr (37 mph) Hurricane: when winds reach or exceed 120 km/hr (74 mph) A high percentage of depressions become tropical storms and an even higher percentage reach hurricane status

Conditions Necessary for Hurricane Formation Hurricanes form only over deep water layers with surface temperatures in excess of 27 oC (81 oF) Coriolis force is an important contributor, and as such, hurricanes do not form equatorward of 5o Strong vertical shear must be absent

Destruction by Hurricanes Winds Heavy rainfall Storm surge is responsible for a large percentage of damage along coastal regions (e.g., hurricane Camille caused a storm surge of 7 m (23 ft) along the Mississippi coast) High surf occurs atop the surge, increasing damage

Destruction by Hurricanes Winds and surge are typically most intense in the right front quadrant of the storm. Why? Combination of wind speeds and the speed of the storm’s movement.

Destruction by Hurricanes The right front quadrant also produces the greatest frequency of tornadoes within the hurricane.