MEA 130 Chapter 8: Air Pressure and Winds

Pressure differentials

• Differences in air pressure between two locations

• Always moves from high pressure to low pressure

• The greater the pressure difference, the stronger the wind

• Ex: like two people pushing on opposite sides of a wall, the side with more force (higher pressure) moves toward the lower pressure side

• Net acceleration of air is always toward low pressure

How does warming/cooling a column of air lead to pressure gradients and circulations forming?

• Warming air makes it less dense, increasing the height of the air column → higher pressure aloft

• Cooling air makes it denser, decreasing the height → low pressure aloft

• Warm air aloft=high pressure, cold air aloft = low pressure

These horizontal differences in temperature → differences in pressure → pressure gradient force (PGF) → air moves from high to low pressure, forming circulation

What units do we use for measuring pressure?

• 1 millibar (mb)= -0.001 bar

• 1 hectopascal (hPA)=  100 pascals

• 1 millibar (mb)= 1 hectopascal (hPa)

• Inches of mercury (in, Hg) used in aviation

• 1013.25 mb= standard sea level pressure

What tool do we use to measure pressure?

Barometer

Station pressure

actual pressure measured at a specific elevation, affected by elevation, temperature and local density. 

Mean sea level pressure (MSL)

adjusted to sea level to remove elevation effects. Allows pressure comparisons between different locations

What features do we analyze on a surface analysis map?

• Plots sea-level pressure (constant altitude)

• Isobards: Lines of constant pressure (smoothed)

• Shows high(H) and Low(L) pressure systems

• Wind vectors shown as arrows

• Lows (Cyclones): counterclockwise and inward winds

• Highs (anticyclones): clockwise and outward winds

What features do we measure on upper air/isobaric charts?

• Constant pressure charts (e.g. 500 mb)

• Show height contours (altitude of a given pressure), temperature (isotherms), and winds

• Used to analyze atmospheric flow aloft (geostrophic winds)

On upper level charts, relate changes in temperature/pressure aloft to changes in height contours.

• Warm air aloft: isobaric surfaces are higher → higher contours

• Cold air aloft: isobaric surfaces are lower → lower contours

• Contours close together = strong height (pressure) gradient → strong winds

Ridge

elongated area of high pressure (warm air, high heights)

• form around highs

Trough

elongated area of low pressure (cold air, low heights)

• form around upper-level low

How does the wind flow around high/low pressure systems?

• Surface (NH):

• Low pressure (cyclones): counterclockwise, inward

• High pressure (anticyclones): clockwise, outward

• Southern hemisphere, opposite directions

• Upper levels: winds flow parallel to height lines (geostrophic) 

Pressure gradient force

Force that causes air to move from high → low pressure. The closer the isobars, the stronger the PGF and the faster the wind.

Coriolis force

Apparent force due to earth’s rotation that deflects moving air

• In the NH: deflects air to the right

• In the SH: deflects air to the left

• Increases with wind speed and latitude

• Affects direction, not speed

How does the pressure gradient force and coriolis force lead to both geostrophic winds and winds around pressure systems?

• Geostrophic balance: PGF (high→low) + coriolis (opposite direction) = balance → wind flows parallel to isobars

• In upper levels, wind = geostrophic

• Around surface lows/highs, friction alters this balance → winds cross isobars slightly inward/outward

How does cyclonic/anticyclonic flow differ in the northern/southern hemisphere?

NH: 

• Cyclonic (Low): counterclockwise, inward

• Anticyclonic (High): clockwise, outward

SH: opposite direction 

Where is frictional force impactful on winds and why?

Near the surface, friction slows wind, reducing coriolis effect → wind turns toward low pressure. Above 1km, friction is negligible → winds nearly geostrophic. 

What patterns of convergence and divergence are found around lows and highs at the surface?

• Lows: convergence → air rises → clouds/rain

• Highs: divergence → air sinks → clear skies

What patterns of convergence and divergence are found around lows and highs aloft?

• Lows: divergence aloft supports rising air below

• Highs: convergence aloft supports sinking air below