WeatherThe Earth's weather system represent complex interactions between the oceans, the land, the sun, and the atmosphere.

Weather is the condition of the atmosphere at a particular time and place. It refers to such conditions of the local atmosphere as

• temperature, • atmospheric pressure, • humidity (the amount of water contained in the atmosphere), • precipitation (rain, snow, sleet, & hail),• wind velocity.

Vertical Structure of the AtmosphereGeneral trends with increasing altitude:

Air pressure decreases. At any given altitude, the air pressure is caused by the weight of air above.

Constituent gases decrease in density. Because air pressure decreases with altitude, the amount of air per unit volume (density) also decreases with altitude.

Temperature decreases in the troposphere where weather occurs. Water vapor decreases dramatically

Composition of the Atmosphere The gaseous envelope that surrounds the planet. Because air pressure decreases with altitude, the amount of air per unit volume (density) also

decreases with altitude. The relative proportions of the gases in the air are essentially constant regardless of altitude.

Nitrogen, Oxygen, and argon make up 99.96% of the gases by volume.

Greenhouse EffectCarbon dioxide, water vapor, methane, greenhouse gases and chlorofluorocarbons (CFC's) are some

greenhouse gases.

Air Pressureo Air pressure decreases with altitude. At any given altitude, the air pressure is caused by the

weight of air above. This means that the air near the ground is compressed by the weight of the air above it.

o As air pressure decreases, air density decreases.

Water Vapor in the Atmosphere Water can exist in all 3 states at the normal range of earth temperature and pressure.Whenever matter changes from one state to another, energy is either absorbed or released.• From liquid to gas - evaporation - heat energy is absorbed • From gas to liquid - condensation - heat energy is released

The Hydrologic CycleWater continuously evaporates from oceans and other water bodies, falls as rain or snow, is transpired by plants, and flows through streams and groundwater back to the oceans.

Relative Humidityo Air is saturated when evaporation = condensation. Temperature dependent.o Saturation vapor pressure of air at any given temperature cannot be exceeded. o Relative humidity = the ratio of the vapor pressure in a parcel of air to the saturation vapor

pressure at the same temperature.

Therefore, relative humidity can be changed by...

• Changing the water vapor content.

– Add water, increase relative humidity

– subtract water, decrease relative humidity

• Changing the temperature.

– Increase temperature, decrease relative humidity

– Decrease temperature, increase relative humidity.

Adiabatic ProcessesUpward movement of air results from:• Convergence lifting - when flowing air masses of equal density converge and are forced upward.• Convective (Density) lifting - When warm, low-density air rises convectively and displaces cooler, denser air.• Orographic lifting - When flowing air is forced upward over a mountain range.• Frontal lifting - when two flowing air masses of different density meet.

• Warm front - Warm air mass advances rapidly.• Cold front - Cold air mass advances rapidly.

Atmospheric StabilityTwo assumptions:– Lifting processes force air upward.– Rising air does not mix substantially with the surrounding atmosphere.A parcel of air forced to rise will expand and cool adiabatically.

Stable air - if an air parcel that is forced aloft cools faster than the surrounding environment. If the lifting forced ceased, the parcel would have the density to sink.High pressure system – an area characterized by descending cooler dry air and clear skies.Cloud formation may occur at an altitude where the saturation temperature is reached (LCL), but clouds would be layered without much vertical development - fair weather clouds.

Unstable air - if an air parcel that is forced aloft cools slower than the surrounding environment. If the lifting force ceased, the parcel will continue to rise because it is warmer and more buoyant than its surroundings.Low pressure system – An area characterized by rising warmer and humid air and cloudy skies.If the air parcel rises to an altitude where the saturation temperature is reached (LCL), clouds with vertical development will form as the buoyant air rises on its own. (thunderstorm clouds).

TornadoesHow a Tornado Works - Associated with strong thunderstorms that develop when 3 main atmospheric conditions occur simultaneously in the central US 1) a northerly flow of warm, moist air from the Gulf of Mexico 2) a cold, dry air mass rapidly moving southward from Canada or the Rocky Mountains 3) strong easterly jet stream - These three air masses moving in different directions produce shearing conditions that are give thunderstorm clouds a "spin" - Funnel clouds begin to form, they may (or may not) touch down and develop into a tornado.

Warm moist air tropical air shoots upward as it meets colder, dryer polar air. As the warm moist air rises, it may meet varying wind directions at different altitudes due to a strong westerly jet stream. If these varying winds are staggered in just the right manner with sufficient speed, they will act on the upward rising air, spinning it like a top. The rotational cell sags below the cloud base to form a distinctive slowly rotating wall cloud. Strong tornadoes form within and then descend from the wall cloud.

Tornado Characteristics• About 70% of all tornadoes on Earth occur in the central and southern US.• One section of the nation is best at producing tornadoes. This area is called "Tornado Alley," (shown on the map).- Northern Texas and Oklahoma

• Occur mostly in late spring - early summer when conditions are best for tornado formation; but can occur anytime.• Can move at speeds up to ~60 mph and have max wind speeds of >300 mph.

• Tornado "magnitude" measured on the Fujita Scale (F1-F5); based on Damage

Wind• Wind is a horizontal air movement arising from differences in air pressure.• Horizontal pressure gradient is the driving force. Wind results when air flows from a place of high pressure to one of low pressure.• Magnitude is determined from the spacing of the isobars. The spacing of the isobars indicates the amount of pressure change over a given distance = pressure gradient.

• Compare to the slope of a hill

Coriolis Effect• Due to the rotation of the earth on its axis.• Deflects all free moving objects to the right of their path in the Northern Hemisphere and to the left in the Southern Hemisphere.• Strongest at the poles, nonexistent at the equator.• Deflection increases with wind speed.

Convergent and Divergent FlowIn the Northern Hemisphere

• Around a low pressure cell, an inward counterclockwise flow develops; Centers of low pressure are called cyclones = convergent flow

• Around a high pressure cell, an outward clockwise flow develops. Centers of high pressure are called anticyclones = divergent flow.

Vertical FlowHigh Pressure Center: Net downward movement of air and fair weather. Around a surface high air is spiraling outward, which leads to a downward flow of air at the center of the high and convergence aloft.

Low Pressure Center: Net upward movement of air, often resulting in cloud formation and precipitation. Around a surface low air is spiraling inward, which leads to an upward flow of air at the center and divergence aloft.

Hurricane A hurricane is a massive tropical cyclone with rotary winds that exceed 74mph blowing

counterclockwise around a relatively calm central area of very low pressure. Hurricanes in the Atlantic and East Pacific, typhoons in the west pacific, cyclones in the Indian

Ocean and Southern Hemisphere. Form in late summer and early fall when ocean waters are warmest.

Hurricane FormationThe process by which a tropical cyclone forms and subsequently strengthens into a hurricane depends on three conditions 1. A pre-existing disturbance with thunderstorms (typically emerging from the west coast of Africa) 2. Warm (at least 80ºF) ocean temperatures. 3. Light upper level winds that do not change much in direction and speed throughout the depth of the atmosphere (low wind shear)

Tropical systems are classified into four categories according to its degree of organization and maximum sustained wind speed.

Once hurricanes form, they are pushed west with the prevailing west-blowing winds in tropics. The Coriolis force causes storms north of the equator to travel in right-curving paths, and storms south of

the equator to travel in left-curving paths.

The Hurricane’s End• Strong vertical wind shear tears the hurricane apart.• Moving over cooler water can lead to weakening (Top Right).• Moving over land shuts off the moisture source and reduces surface circulation due to friction (Bottom Right).

Storm Surge Storm surges are like a “hill” of ocean water (sometimes as high as 20ft above sea level) pushed up by a hurricane. Storm surges are caused by two factors: low atmospheric pressure that pulls the ocean surface up, and the spiraling, converging winds that push the ocean water in toward the center of the storm. • 90% of all deaths in tropical cyclones result from storm surge.• Surge flooding.• Coastal Erosion.• Destruction of homes, buildings, roads, bridges, and piers.

FloodingHurricanes frequently produce huge amounts of rain. A typical hurricane brings at least 6 to 12 inches of rainfall to the area it crosses.

Winds and Wind DamagesWind damages include blowing in windows, doors, and walls, lifting off roofs, blowing down trees and power lines, and flying debris.Most damage is from winds.

The Saffir-Simpson Hurricane ScaleThe Saffir-Simpson Hurricane Scale is based on barometric pressure and average wind speed.

Hurricane NamesSince 1953, Atlantic tropical storms reaching tropical storm strength have been named from lists originated by the NHC. When first developed, the lists featured only women's names (figures!). Then in 1979 a six year rotating list with alternating male and female names was developed. The names of devastating storms are retired, and another name is selected by the WMO to replace it.