Where Wind Comes From * * * * * * * * * * * * * * * * *

Friction occurs in two forms that are important with regards to the weather and the atmosphere: molecular viscosity (much less important) and eddy viscosity (much more important); both these act opposite to wind direction and increases with increasing surface roughnessGravity accelerates air downward toward the surface but does not modify horizontal winds

These individual forces are then combined in hydrostatic equilibrium (the balance between gravity and the vertical pressure gradient), the geostrophic wind (an unaccelerated, horizontal wind that follows a straight path above the friction layer), the gradient wind (blows parallel to isobars and follows curved paths, also above the friction layer), and surface winds (which result from the effect of friction with the surface combined with the Coriolis effect, and act to direct wind across isobars and toward low pressure).

Hydrostatic Equilibrium Illustrated

Air Pressure gradients in the horizontal dimension can arise from the following: contrasts in air temperature (primary cause) and water vapor (or both, usually), and diverging and converging winds.

• Winds are monitored in several wayso Examples of instruments used include the wind vane,

the wind sock, a cup anemometer, and a hot-wire anemometer

o Wind speed and direction (= wind velocity…a brief description of what a vector is goes here)

o The Beauford scale is used to estimate the speed of the wind by observing its effects on the surface of water and on common, land-based objects.

• Wind is always named for the direction from which it comes

The Atmosphere’s Planetary Circulation * * * * * * * * * * * * * * * * *

• Chapter 18 is concerned with the genesis and characteristics of a variety of weather systems. • We examine these systems in order of decreasing spatial scale beginning with the largest scale, the global or planetary circulation. The principle components are:

– Intertropical Convergence Zone (ITCZ)– Trade winds– Subtropical Anticyclones– Westerlies– Subpolar lows– Polar front– Polar easterlies

Start with a layerof gas, add sun-light, then mix…

 The global pattern of circulation undergoes seasonal changes as follows:– In winter, temperatures favor high pressure is favored over

continents and low air pressure over oceans.

– This pattern is reversed in the summer

– Subtropical latitudes have seasonal monsoons that arise out of an interplay between several climate controls (solar heating on land versus the ocean, topography, seasonal shifts in global circulation and the ICTZ)

The average positions of the ITCZ in January and July

  Wave patterns in the westerlies and properties of the jet stream determine how smaller scale weather systems behave and what sort of weather a particular mid-latitude location receives– Zonal versus meridianal: zonal flow involves westerlies

blowing east to west with little amplitude; meridianal flow involves east to west prevailing winds with much amplitude north and south

– The shift between zonal and meridianal circulation fosters air mass exchange, poleward heat transport, and cyclone development and movement.

– Cutoff lows (cyclones) and anticyclones (highs) block the usual east-west progression of storm systems and may lead to extreme weather conditions (draught, temperature extremes, rainfall extremes)

Zonal Westerly Flow

Meridianal Flow

Cutoff Features

• El Niño and La Niña are air-sea anomalies that contrast with long-term average (neutral) conditions in the tropical Pacific, these anomalies can affect weather conditions worldwide.– El Niño—the lengthy period of unusually high

sea-surface temperature over a vast area of the eastern tropical Pacific

– El Niño—accompanied by weather extremes in the tropical latitudes and in other parts of the world.

– La Niña—conditions opposite of El Niño, again with global influences.

Effects of El NinoOn North America

El Nino as it appears in the data over the years…

Air Masses, Fronts, Cyclones, & Anticyclones* * * * * * * * * * * * * * * * *

• An air mass is a huge volume of air that is relatively uniform horizontally in temperature and humidity. – Air masses modify as they travel away from source

regions (where initial characteristics are acquired). – These source regions include maritime (ocean) and

continental (land), high latitudes (polar) and low latitudes (tropical)

– The four basic types: cold and dry, cold and humid, warm and dry, and warm and humid.

– The degree of modification depends on the stability of the air mass and the temperature of the surface over which it travels

• Where contrasting air masses meet, fronts and frontal weather develop. – Air that is forced to ascend along a front undergoes

expansional cooling that often spurs formation of clouds and precipitation.

– Four types of fronts: warm, cold, stationary, and occluded

– Warm frontal weather: broad cloud and precipitation shield on the cold side of the warm front

– Cold frontal weather: narrow band of convective clouds and brief rain / snow showers, or thunderstorms

Cold Front Weather

Warm Front Weather

Stationary Front Weather

• The life cycle of an extratropical cyclone demonstrates how air masses, fronts, and pressure systems are interrelated. – It starts as a wave along the polar front– It strengthens as the air pressure drops– Winds strengthen and frontal weather develops– The cyclone occludes as the faster-moving cold front catches

up with the slower moving warm front– The upper level low-pressure trough becomes vertically

stacked over the surface cyclone– The lifetime of this cycle varies considerably, with 1 to 4 days

needed to reach maximum intensity.– The maximum strength of a cyclone also varies tremendously

• Anticyclones follow cyclones. Anticyclones are fair-weather systems, may be warm core or cold core, and are accompanied by characteristic patterns of air mass advection.

• Land and sea (or lake) breezes, chinook winds, desert winds, and mountain and valley breezes are small-scale systems that are not significantly influenced by the Coriolis effect.

• Planetary- and synoptic-scale winds set the boundary conditions for local and regional circulation systems.

Thunderstorms and Tornadoes * * * * * * * * * * * * * * *

• A thunderstorm is a mesocscale weather system produced by strong convection currents that surge high into the troposphere.

• The three stages in the life cycle of a thunderstorm cell are cumulus, mature, and dissipating. – Cumulus stage occurs when the storm builds upward and

laterally, but no rain is falling yet– Mature stage begins when rain reaches the ground. At this time,

strong updrafts occur along downdrafts and the system peaks in intensity.

– Dissipating stage happens when subsiding (sinking) air spread throughout the entire cell and the clouds vaporize.

• We describe the special synoptic situation that favors development of severe thunderstorms, including atmospheric stability. – They develop along fronts within a mass of maritime tropical air– They can develop on mountain slopes– Via convergence of surface winds, or – Through intense solar heating of Earth’s surface

• Thunderstorm hazards include lightning, downbursts, flash floods, and hail. Some may become severe while others may not for various reasons, and some factors that are favorable for the occurrence of severe thunderstorms include:– The presence of a fast-moving, well-defined cold front associated

with a mature mid-latitude cyclone.– The jet stream causes dry air to sink over a surface layer of

maritime tropical air.– This layering effect favors explosive convection and development

of severe thunderstorms.

• Thunderstorms are most common over the continental interiors of tropical latitudes– Sea breeze convergence over Florida make thunderstorms more

common in that state than anywhere else in North America– At higher latitudes, colder airmasses tend to be stabilized, which

inhibits the convection necessary for thunderstorm development. – Thunderstorm hazards include lightning, downbursts, flash floods,

and hail (you can read more about these in your text)

• A tornado is a small mass of air that whirls rapidly around a nearly vertical axis and is made visible by water condensation and dust and debris drawn into the system

• The special combination of atmospheric conditions that favors tornado development occurs most frequently in spring in the central United States.– This is in a corridor that stretches from Texas northward to Nebraska and

eastward to Indiana and Illinois– Conditions favorable for tornadic development travel northward (with the

sun) from the Gulf Coast in early Spring to southern Canada by early summer)

• Based on rotational wind speeds reconstructed from property damage, tornadoes are rated from 1 to 5 on the Fujita-scale.

• Most North American tornadoes are spawned by supercell thunderstorms that develop along or ahead of well-defined cold fronts.

• Tornadoes also spin off gust fronts and are associated with land-falling hurricanes.

Cognitive Skill of the Week: Testing modelsA front is a conceptual model that is represented

graphically on a surface weather map. As is the case with all scientific models, a front approximates the transition zone between air masses that differ in temperature and or humidity. As part of this week’s investigation, you will analyze a meteorgram record of the changing atmospheric conditions at a particular location before, during, and after the passage of a cold front. By analysis of the meteorgram, you have an opportunity to compare the types of atmospheric changes (e.g., temperature, wind) that theoretically take place with the passage of a cold front versus what actually happens during a real cold front passage.

Affective Attribute of the Week: Objective

An objective learner acts and interprets information independently of personal feelings, opinions, or bias. In this week’s investigation, you are asked to be objective in comparing what you understand about fronts and frontal passages with atmospheric changes recorded during an actual frontal passage.