Pressure & Winds

Sea-Level Pressure

Average sea level = 1013.25

Four different types of fronts

Cold (best lifter of air) Warm Stationary Occluded (worse lifter of air)

Near-surface Winds

Near the surface, friction becomes to more important Friction acts to slow winds speeds down Surface winds do not flow parallel to isobars They cross isobars at an angle as they blow from High to Low pressure.

Cold Front Characteristics: Contact with the front

Temperature: cooling suddenly Winds: variable and gusty Precipitation: heavy rain or snow, hail sometimes

Other Forces

The PGF initiates motion In the absence of other forces the air will move from High to Low Pressure Friction and the Coriolis force affect the speed and direction of winds

Winds around Low Pressure

They flip cause of the coriolis effect Air still convergences Air is rising = clouds/precipitation

Weather Associated with Cold Front

Thunderstorms (severe) Strong winds behind the front Precipitation is typically heavy along the front Ahead of front, high temperatures Ahead of front, moist air (if talking about cold front east of Rockies)

1.Which of these locations best represents a trough? 2.Where are we likely to find the fastest winds? 3.Where are we likely to find high pressure? 4.Where are we most likely to find a low pressure system forming down at the surface? 5.Where are we likely to find rising air

1.Trough 2.Ridge 3.Site of Convergence (area between ridge and trough) 4.Exit Region of the Trough 5.Exit Region of the Trough

Friction

A force that acts to slow down moving objects It is important near the earth's surface but not at higher levels in the atmosphere Building, trees, etc apply friction to wind Only applies to the surface to the 300ft

Pressure Gradient

A pressure gradient is a change in pressure over some distance PG=change in pressure/change in distance *Be able to interpret Pressure Gradient Map

Horizontal Gradient

A strong horizontal pressure gradient at he surface around a hurricane is approximately 1 mb per 6000 m.

Dryline

Acts as a front (lifting mechanism/ahead of the cold front) no change in temperature differences Air masses with similar temperatures, but strong humidity gradient exists

Corilois Force

Affects free-moving objects in the atmosphere It is caused by the rotation of the earth

Classifying Air Masses

Air Masses are classified by the area in which they originate and their temperature and moisture characteristics

Pressure Gradient Force (TRUE/FALSE)

Air always moves from high pressure towards low pressure The speed of the wind is directly proportional to the pressure gradient

Atmospheric Pressure

Amount of force applied by atmosphere over some area Decreases with height, exponentially Pressure varies b/c the mass of overlying air varies from place to place

Air Mass

An extremely large body of air whose properties of temp and moisture are fairly homogenous in all direction They have a major impact on our weather, especially humidity and temp.

Ridges

Are high pressure/ height areas

Troughs

Are low pressure/ height ares Equator ward bulge in N.H. mid level height pattern Exit region of the trough you get UL Divergence you get low pressure

Source Regions

Areas where air masses originate and take on their moist. and temp. Extensive and physically uniform Calm or light winds Located in high or low latitude areas

Horizontal Winds and Vertical Motion

As air moves toward the center of low pressure (Convergence), it must go somewhere It slowly rises and diverges (spreads apart) aloft Air moves away from center of high pressure (divergence) To replace the laterally spreading air, the air aloft converges and slowly descends.

Jet Stream & Temperature

Barrier between cold air mass and warm air mass

Mositure

Continental (c) = stable, dry Maritime (m) = unstable, moist, easy to saturate, high dew-point mT = Atlantic ocean, gulf of Mexico, Gulf Stream cT= form in the desert in the southwest P is heavier than T

Surface Winds

Crossing the isobars (Friction) Depends on the smoothness of the surface andthe latitude

Upper Level Winds

Does not cross the isobars Coriolis Force

Why do surface winds cross isobars?

Friction reduce wind speed Lower wind speed weakens CF CF can't balance the PGF and winds will cross the isobars towards the Low PGF will ultimately be balanced by the sum of the frictional force and the Coriolis force

Front

Great lifting mechanisms The dividing line (transition zone) between the air masses No air mass replacement in the summertime

Upper Level Convergence

High pressure = sinking/unsaturated air Low level divergence

Low-level jet

Important for severe weather; regional specific to the southeast and the great plains Forms at night near (900-850mb level) Generally flows from south-to-north Mostly occurs in spring/summer

How do air masses get their characteristics?

Like a child, an air mass develops an "accent" based on where it was "raised"

Upper Level Divergence

Low pressure at the surface Rising air causes for clouds and precipitation Surface winds occur within the first 3000-5000 ft

Troughs & Ridges

Many low and high pressure system are not closed cells but elongated areas

Winds around High Pressure

Northern Hemisphere = clockwise Southern Hemisphere = counterwise Upper level winds stay in between the ines

Forces that cause/effect the wind

Pressure Gradient Force (PGF)* Most important Coriolis Force/Effect (CF) Friction

Wha about Vertical Pressure Gradients?

Pressure decreases with altitude The avg vertical pressure gradient is 600 times greater than the extreme horizontal pressure gradient.

Pressure Gradient in the upper atmosphere

Pressure gradients and winds occur throughput the atmosphere We use heights instead of isobars to look at pressure gradients Arrows: get bigger as we go up (MAP)

Formation of Air Masses

Temp. & moisture characteristics of the atmos. arise in part from exchanges of energy and water vapor with surface.

Latitude

Temperature Polar (P) = high latitudes Tropical (T) = low latitudes

Jet Stream

Temperature gradient causes for Jet Stream A tube of air with fast winds Slows down in the summer, then the jet stream becomes zonal Tropopause- strong winds concentrate

Cold Front Characteristics: After the passing of the front

Temperature: cold and getting colder Winds: west to northwest Precipitation: showers then clearing

Cold Front Characteristics: Prior to the passing of the front

Temperature: warm Winds: south to southeast Precipitation:showers

Gravity

The force pulling down air

Why do we still have an atmosphere?

The huge vertical PGF is balanced by gravity which pulls the mass downward Normally, the vertical PGF and gravity are balanced

Winter/Summer Monsoon

The land is cold, the water is warm The air moves from high to low Monsoons are driven by the sun

Major Wind Systems (Monsoons)

Thermal induced *seasonal wind reversal Leads to seasonal shifts between hot, dry conditions and heavy precipitation Never have a monsoon in the Eastern Part of the U.S. East Asia Himalayans are great lifters of air

Upper Level Patterns

Variation in the upper level winds lead to areas of upper- air convergence and divergence

Occluded Front

When a cold front catches up to and overtakes a warm front, the boundary created between the two masses is called an occluded front or simply an occlusion Gravity always separates air masses so heaviest is on the bottom

500-mb Height Map

When the lines are close together thats where the fastest wind speeds are.

Zonal (winds on upper level charts)

When the winds blow parallel to lines of latitude Follows the lines of latitude And usually occurs in the summer

Meridional

When winds cross the latitude lines at a sharp angle Has a north south component Lots of troughs and rides, very wavy Occurs in the fall, winter and spring

Coriolis deflection

Zero at 0 degrees Maximum at 90 degrees Increases with speed of the moving object Occurs regardless of direction of motion Northern hemisphere deflects to the right

Pressure Gradient Force

it is the force that initiates air motion Distribution of pressure across the globe is extremely variable Distribution helps explain air motion or winds Air always moves from high pressure towards low pressure

Identifying Fronts

sharp horizontal temperature change sharp horizontal dew-point/humidity temperature change ***a shift in the wind direction presence of clouds & precipitation change in pressure