Meteorology Lec 6 - Air Pressure and Winds

Atmospheric Pressure

The force per unit area exerted by the weight of the air above a given point. It decreases with altitude and varies horizontally due to temperature differences.

Horizontal Pressure Variation

Caused by differential heating and cooling of air columns. Warm air aloft creates high surface pressure; cold air aloft creates low surface pressure, establishing a pressure gradient.

Gas Law

The relationship between air pressure (P), temperature (T), and density (ρ): P is proportional to T * ρ. At constant temperature, higher pressure means higher density. Cold air is denser than warm air at the same pressure.

Daily Pressure Variations

A regular diurnal cycle with two maxima (around 10 AM and 10 PM) and two minima (around 4 AM and 4 PM), most pronounced in the tropics.

Pressure Measurement

Measured with barometers. Units include millibars (mb) and kilopascals (kPa). Standard sea-level pressure is 1013.25 mb.

Station Pressure

The barometer reading at a location, corrected for temperature, gravity, and instrument errors, but not adjusted for altitude.

Sea-Level Pressure

Station pressure adjusted to mean sea level to allow for comparison between locations at different elevations. Pressure decreases ~10 mb per 100 m increase in elevation.

Isobars

Lines on a weather map connecting points of equal pressure. Drawn at 4 mb intervals.

Surface Map

A sea-level pressure chart with isobars. Used to identify high and low-pressure systems.

Upper-Level (Isobaric) Charts

Constant pressure charts showing height variations (contour lines). Low heights indicate lower pressure; high heights indicate higher pressure.

Newton's First Law (Wind Application)

Air will remain at rest or in constant motion unless acted upon by an external force.

Newton's Second Law (Wind Application)

The net force (Fnet) on an air parcel determines its acceleration (a). Fnet = ma. Key forces are PGF, Coriolis, friction, and centripetal.

Pressure Gradient Force (PGF)

The force that initiates wind flow, directed from high to low pressure, perpendicular to isobars. Stronger when isobars are closer together.

Coriolis Force

An apparent deflective force caused by Earth's rotation. It deflects moving objects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. It affects direction, not speed, and increases with wind speed and latitude.

Geostrophic Wind

A theoretical wind that flows parallel to straight isobars at a constant speed, where the PGF is balanced by the Coriolis Force. It is a good approximation for upper-level winds.

Gradient Wind

Wind that flows parallel to curved isobars (around highs and lows) at a constant speed, above the friction layer. The net force (centripetal force) is due to an imbalance between PGF and Coriolis Force.

Cyclonic Flow (NH)

Counterclockwise, inward spiral of air around a surface low-pressure center. Air converges at the surface and rises.

Anticyclonic Flow (NH)

Clockwise, outward spiral of air around a surface high-pressure center. Air diverges at the surface and sinks.

Friction Layer

The lowest part of the atmosphere (up to ~1000 m) where friction with the surface reduces wind speed and causes wind to cross isobars at an angle (~30°).

Buys-Ballot's Law

In the Northern Hemisphere, if you stand with your back to the wind, lower pressure is to your left and higher pressure is to your right. This relationship is approximate at the surface due to friction.

Convergence & Divergence

Convergence is the net inflow of air, leading to rising motion. Divergence is the net outflow of air, leading to sinking motion. The balance between surface and upper-level convergence/divergence determines if a pressure system strengthens or weakens.

Hydrostatic Equilibrium

The balance between the upward-directed pressure gradient force and the downward force of gravity. This balance explains why the atmosphere doesn't rush off into space.

Wind Direction

Defined as the direction from which the wind originates (e.g., a north wind blows from the north). Exceptions include upslope/downslope and onshore/offshore winds.

Prevailing Winds

The wind direction most frequently observed over a specific period. It significantly influences local climate and is crucial for urban planning.

Wind Rose

A diagram that shows the percentage of time the wind blows from different directions and at different speed ranges.

Anemometer

An instrument that measures wind speed, typically using rotating cups.

Wind Vane

An instrument that measures wind direction. The arrow points into the wind, indicating its origin.

Aerovane/Skyvane

An instrument that measures both wind speed (via a propeller) and direction (via a fin).

Radiosondes & Doppler Radars

Instruments used to measure vertical wind profiles (wind speed and direction at different altitudes) in the upper atmosphere