Weather exam 2

Earth's axial tilt / obliquity

Approximately 23.5

Sun directly over the Tropic of Capricorn

Winter solstice

Northern Hemisphere winter

Occurs when the Northern Hemisphere tilts away from the Sun

When Earth is closest to the Sun

Perihelion

When Earth is farthest from the Sun

Aphelion

Order of sunrise stages

Astronomical sunrise occurs first, followed by nautical sunrise, then civil sunrise

Fundamental cause of global wind circulation

Differences in solar energy receipt

Equatorial region

Typically referred to as the ITCZ (Intertropical Convergence Zone)

Typical wind direction from 60–90 degrees North and South

Easterly

Boundary between warmer tropical air and colder polar air

Polar Front

Three-cell model and Coriolis

If global circulation were driven only by temperature gradients, there would be one circulation cell in each hemisphere. Earth's three-cell structure is associated with the Coriolis effect

Typical number of air masses over the United States at one time

5

Air mass

A large body of air with similar horizontal characteristics of moisture and temperature

Air mass descriptors

The class material identifies moisture and temperature as the key horizontal characteristics used to describe an air mass. Air mass dominant over the southeastern USA, especially in summer: mT (maritime tropical)

Warm air moving into cold air

The boundary is called a warm front

Cold fronts

Cause uplift when they meet warm air

Occluded front

A front in which a mass of warm air is cut off from the surface

First step in classical cyclogenesis

A polar front separates masses of warm and cold air

Last step in classical cyclogenesis

Occlusion occurs

Anticyclone

A high-pressure system

Station model: light rain

Two black dots side by side

Station model: 75% black central circle

Cloud cover ranges over 70 to 80 percent of the sky

Three stages of an air-mass thunderstorm

Towering cumulus, mature, dissipating

Air-mass thunderstorms

Extinguish themselves through the formation of extensive downdrafts

Thunderstorms in the United States are most common in

Florida

Lightning

A discharge of static electricity

Most lightning discharges

Occur within clouds

Typical cloud-to-ground lightning event

Several distinct steps that look to the human eye like a single lightning strike

Principal hazard of microbursts to aircraft during takeoff or landing

Sudden changes in lift and tailwinds

NOT a reason for flooding from thunderstorms

Downdrafts

Month when tornadoes are most likely in the United States

May

Mesocyclone formation

The horizontal tube of air must be tilted upward

Thunderstorm ingredients

Moisture, a lifting mechanism, and instability

Thunderstorm ingredient: moisture

Moisture is one of the required ingredients for thunderstorm development

Thunderstorm lifting mechanism: convection

Convection is a lifting mechanism for thunderstorms

Thunderstorm lifting mechanism: orographic lifting

Orographic lifting is a lifting mechanism in which terrain forces air upward

Atmospheric instability

Unstable air means rising air, the atmosphere becomes more unstable when it is colder with height and when dry air is above humid air

Developing thunderstorm stage

Also called the towering cumulus stage, updrafts dominate

Mature thunderstorm stage

Strong updrafts and induced downdrafts occur, heavy rain and an anvil top are present

Dissipating thunderstorm stage

Downdraft dominates

Severe thunderstorms

Can occur as a series of storms called a mesoscale convective system (MCS) or mesoscale convective complex (MCC)

Pennsylvania thunderstorm note

The severe-weather slides state that mostly air-mass thunderstorms occur in Pennsylvania, not MCSs

Downdrafts and storm systems

Downdrafts can propagate to other storms, and the whole system can rotate

Hail formation

Updrafts cause riming

U

S

record hail stone listed in the slides

Vivian, South Dakota, July 23, 2010, 8-inch diameter, 1

94 pounds

Microburst

A concentrated downdraft that moves straight down and then spreads outward, winds can reach 100 mph

Lightning: basic definition

Static electricity, a huge electrical discharge

Lightning voltage listed in the slides

More than 100 million volts

Lightning duration listed in the slides

A fraction of a second

Lightning temperature effect

Lightning heats air to more than 54,000°F

Lightning charge separation

Friction from rising and sinking hail and water helps separate charge, lighter hail or drops are carried toward the top and carry positive charge, while negative charge develops at the bottom of the cloud

Cloud-to-ground lightning sequence

A positive charge on the ground is attracted, a stepped leader comes from the cloud and a return stroke comes from the ground

Thunder

Always occurs with lightning, although you may not hear it

It results from rapid expansion and contraction of air

Heat lightning

Lightning whose thunder may not be heard

Lightning deaths

The slides list about 100–200 deaths annually, with most in Florida

Lightning safety

Stay indoors or in your car, do not touch anything, stay away from isolated or tall trees

If your hair stands on end, crouch alone on the balls of your feet to reduce the path available for current

Tornadoes

Associated with severe thunderstorms, especially supercells, the slides identify intense low pressure and wind shear as associated conditions

Mesocyclone

Forms when the horizontal tube of air is tilted upward

Hurricane: names and locations

Hurricane: Atlantic and eastern Pacific

Typhoon: western Pacific

Tropical cyclone: Indian Ocean

Relative size: tornado

Less than 7 miles wide

Relative size: thunderstorm

About 10–100 miles wide

Relative size: hurricane

About 200–400 miles wide

Relative size: midlatitude cyclone

About 500–800 miles wide

Hurricane structure

Low-pressure center, strong pressure gradient, high winds, spiraling thunderstorms, warm core, ocean water, condensation, low temperatures aloft, and cirrostratus clouds

Hurricane eye

The center of the hurricane, about 15 miles in diameter in the slide example, has clear sky, light wind, and descending air

Hurricane eyewall

The most intense part of the storm

Hurricane rain bands

Spiraling bands of thunderstorms around the hurricane

Hurricane formation: easterly wave

A disruption in the easterly trade winds, associated with the west coast of Africa

Convergence on the eastern side of the wave creates a tighter gradient, rising air, and a tropical disturbance

Easterly wave movement

Typically moves 10–20 mph and takes about 7–10 days to cross the Atlantic

Hurricane formation: warm water

Warm ocean water enhances development, the slides specify a deep warm layer around 81°F (28°C)

Previous hurricanes can mix the water layer

Instability and condensation are also involved

Hurricane season

June 1 to November 30

Saffir-Simpson: tropical depression

Less than 37 mph

Saffir-Simpson: tropical storm

38–73 mph

Saffir-Simpson: Category 1

74–95 mph

Saffir-Simpson: Category 2

96–110 mph

Saffir-Simpson: Category 3

111–130 mph

Saffir-Simpson: Category 4

131–155 mph

Saffir-Simpson: Category 5

More than 156 mph

General hurricane movement: easterly trades

Move storms from east to west

General hurricane movement: Bermuda High

Helps guide hurricanes from south to north

General hurricane movement: westerlies

Can move storms from west to east

Hurricane movement

Overall movement depends on easterly trades, the Bermuda High, westerlies, and specific weather conditions

Storm surge

A rise in coastal water level caused by winds piling water toward the coast and low pressure raising the water level

Most dangerous part of a hurricane

Storm surge

Storm surge especially dangerous when

It coincides with high tide, especially around a new or full moon, and in low-lying areas

Hurricane right-front quadrant

The most severe weather occurs here, including strong wind, heavy rain, tornadoes, and storm surge

Right-front quadrant conditions

The slides associate the most severe quadrant with warm water and note that there is no land

Hurricane naming

All depressions are given names, names are alphabetical using 21 letters, there are 6 rotating lists, a supplemental list is used if names run out, male and female names alternate, famous names can be retired

2005 Atlantic hurricane season

28 named storms (record at the time), 7 Category 3 or stronger, strongest was Wilma at 882 mb and 185 mph, damage exceeded $100 billion (record at the time), and fatalities were 2,048

Hurricane Katrina

August 23–31, 2005, lowest pressure 902 mb, highest wind 175 mph, damage $160 billion, listed as costliest in U