MEA 130 Exam 2 Review

What is used to measure air pressure?

Millibars (mb), Hectopascal (hPa), and inches of mercury (in. Hg)

Station pressure

Observed pressure that is the true barometric pressure; varies greatly depending on elevation

Pressure…

Always decreases with height and is heavily influenced by elevation

Mean sea level

Average surface of the oceans

Isobars

Lines of constant pressure

What does sea-level pressure take into account?

Elevation

What way do atmospheric phenomena travel?

Horizontally along isobaric surfaces

Isobaric

Constant pressure

Contour lines

Show the height of isobaric surfaces above sea level; are closer when height changes quickly and farther when it changes slowly

200 Mb isobaric surface

Upper-level jet stream

500 Mb isobaric surface

Steering flow, troughs, and ridges

700/850 Mb isobaric surface

Temperature and moisture advection

Cold air aloft

Isobaric surfaces sink to lower altitudes

Warm air aloft

Isobaric surfaces rise to higher altitudes

Lower heights indicate…

Lower pressure and colder temperatures aloft

Higher heights indicate…

Higher pressure and warmer temperatures aloft

Ridges

Elongated area of higher pressure aloft

Troughs

Elongated area of lower pressure aloft

Net force acceleration of air

High to low pressure

Pressure Gradient Force (PGF)

The Net force directed from high to low pressure; main driving force behind wind

What is the main driving force behind wind?

Pressure gradient force (PGF)

Coriolis Force

An apparent force that's caused by Earth's rotation, which makes air curve

What direction does the coriolis force deflect the wind in the Northern Hemisphere?

To the right

Why do winds rotate around low/high pressure systems?

Earth's spin

What are winds in the upper levels?

Geostrophic; balanced by PGF and Coriolis Force and flows parallel to isobars

Frictional forces cause…

winds to turn towards the lower pressure

Winds around low pressure are __ and around high pressure are __ in the NH

Counterclockwise; clockwise

Upper-level winds are

Geostrophic; counterclockwise around troughs and clockwise around ridges

Geostrophic

Parallel to height contours

Cyclonic

Winds around low-pressure systems and troughs

Anticyclonic

Winds around high-pressure systems and ridges

Cyclonic flow is … in the Northern Hemisphere

Counterclockwise

Anticyclonic flow is … in the Northern Hemisphere

Clockwise

“Low/high pressure systems” are actually referring to…

Surface systems

Surface maps analyze…

Surface conditions and sea-level pressure

Air converging around a surface low will…

rise → diverge → and spread out aloft

Air converging around a surface high will…

Spread out → sink to replace it → and converges aloft

Wind gust

Rapid fluctuations in wind speed

Wind vanes

Instruments that measure wind direction

Anemometers

Instruments that measure wind speed

Wind speed can be measured in…

Knots (kts), miles per hour (mph), and meters per second (m/s)

Wind direction

The direction the wind comes from, measured in degrees

Southerly wind

Wind comes from the south and goes toward the north

Northeasterly wind

Wind comes from the northeast and goes toward the southwest

Wind barbs

Symbols that indicate speed and direction

Onshore wind

Wind coming from the ocean towards the coast/land

Offshore wind

Wind coming from the coast/land to the ocean

Upslope wind

Wind that moves up a mountain

Downslope wind

Wind that moves down a mountain

Scales of motion

Classification of atmospheric circulations

Microscale

Smallest scale of atmospheric motions, seconds to minutes, a few meters to 1 km in diameter

Examples of microscale

Turbulence, frictional forces, thermals, small tornadoes

Turbulence

Irregular/disturbed flow that produces gusts and eddies

Wind speeds are faster over the … due to frictional forces

Ocean

Mesoscale

Systems the size of cities or states, a few km to several hundred km in diameter, lasts minutes to hours (sometimes days)

Examples of mesoscale

Thunderstorms, large tornadoes, local wind systems, sea breeze, and convective systems

Synoptic scale

Shows large map features, spans from hundreds to thousands of sq km, can last days to weeks

Examples of synoptic scale

Cyclones, anticyclones, and fronts

Global/planetary scale

Largest scale, tens of thousands of km, can last over a month

Examples of global/planetary scale

Global winds, jet stream, long waves, and semi-permanent pressure centers

How is a thermal circulation formed?

Winds blow toward the warm air at the surface and toward cold air aloft

Sea breeze

Surface air blows inland during the day; a coastal, daytime circulation that causes coastal thunderstorms in the summer

Land breeze

Surface air blows offshore at night; a coastal, nighttime circulation

Katabatic winds

Cold, dense air blowing downslope

Chinook winds

Winds on the downwind side of North-South mountain ranges, air warms by descending down the mountain

Santa Ana winds

Warm, dry wind that blows into Southern California, air warms as it sinks from the plateaus, helping spread wildfires

Dust devils

Mircoscale, occur on hot, clear days, 80+ mph winds not related to tornadoes

Haboob

Intense dust storm carried by downdrafts from thunderstorms

Monsoons

Synoptic scale, seasonally changing winds

General circulation of the atmosphere

The average flow of air around the globe that is caused by the unequal heating of Earth’s surface

Single cell model

One large circulation per hemisphere, PGF dominates, shows the importance of Earth’s temperature gradient but not the atmospheric circulation

Three cell model

Explains global atmospheric circulation and climate zones

Westerlies

The dominant westerly wind in the mid-latitudes

Hadley cell

Rising air at the equator, sinking air at 30 degrees

Intertropical Convergence Zone

The boundary that separates the northeasterly trade winds in the NH and southeasterly trade winds in the SH, just above the equator, and is associated with frequent large thunderstorms and rainfall amounts

Rainfall most common:

Near ITCZ and between 40 and 55 degrees around the polar front

Rainfall least common:

Near 30 degrees or subtropical highs and polar regions

Jet streams

Narrow bands of strong winds in the upper levels of the troposphere, found at the tropopause, are mostly westerly

Jet streams can bring…

Cold air south, warm air north, moist air from humid regions, and dry air from dry regions

El Nino

Reversal of the normal circulation, the east Pacific becomes more stormy and the west Pacific becomes less stormy, and trade winds weaken

La Nina

Enhancement of the normal circulation, the east Pacific waters are colder, and the west Pacific waters are warmer