Chapter 5 Air Pressure

Driving Question What is the significance of horizontal and vertical variations in air pressure?

Air Pressure Air pressure is a measure of the force that air exerts on a surface Weight per unit area of the column of air above that location –Weight = mass * acceleration due to gravity Average air pressure at sea level –1.0 kg/cm 2 –14.7 lb/in 2 The air pressure at any point is the same in all directions

Air Pressure Measurement Barometer Aneroid   Mercury

Mercurial Barometer More accurate than aneroid Invented by Torricelli in 1643 One meter (39 inches) long glass tube sealed on one end Open end is inverted in pool of mercury Height of mercury changes as pressure changes Adjustments are required for temperature and latitude

Aneroid Barometer Flexible evacuated chamber with a spring inside As pressure changes allow the chamber to flex This causes movement in gears which display the pressure

Air Pressure Tendency The change in air pressure with time –Rising: continuing fair or clearing weather –Falling: approaching inclement weather –Steady: no change Barograph –An instrument that provides a continuous trace of air pressure with time Altimeter –An aneroid barometer that is calibrated to measure altitude or elevation

Air Pressure Units Millibars, inches of Mercury, Pascals

Air Pressure Meteorologists often express altitude in terms of pressure (850mb map) Worldwide range in air pressure averages between 970mb and 1040mb Lowest: 870mb (25.69in Hg) in Typhoon Tip near Guam Highest: mb (32.01in Hg) in Siberia

Variations in Air Pressure With Altitude The maximum air density is at the surface Number density: the number of gas molecules per unit volume –Decreases with altitude Thinning of the air with altitude is associated with decline in air pressure 50% atmosphere’s mass below 3 miles 99% atmosphere’s mass below 20 miles

The Standard Atmosphere: a model of the real atmosphere averaged across all latitudes for all seasons characterized by 1.Sea level air temperature of 15 o C 2.Sea level pressure of mb

Variations in Air Pressure With Altitude Denver (mile high city) has an average air pressure 83% less than Boston –Altitude sickness –Lexington is about 900 feet above sea level, so actual pressure is less than “fixed” pressure In very sparse air (thermosphere) temperature is very high, but heat transfer is low

Horizontal Variations in Air Pressure On a surface weather map, variations in pressure due to altitude are removed by determining what the pressure would be at that point if that point were at sea level Easier to observe variations in pressure from one place to another by day and hour

Influence of Temperature and Humidity Generally, temperature has a greater influence on density and pressure than water vapor Air pressure drops more rapidly with altitude in cold (more dense) air than warm (less dense) air Dry air is more dense that humid air!!

Influence of Temperature and Humidity Cold, dry air masses are more dense and produce higher surface pressures than warm, humid air masses Change in air pressure is usually accompanied by a change in air mass

Influence of Winds Divergence (convergence) are caused by winds blowing away (toward) a location

The Gas Law (Equation of State) Variables of State: temperature, pressure, and density p = ρRT –Pressure (p) –Density (ρ) –Gas Constant (R) –Temperature (T) In the atmosphere these 3 variables are constantly changing

The Gas Law P = ρRT Pressure is held constant –If T increases then ρ decreases –If ρ increases then T decreases Temperature and Pressure are inversely proportional

The Gas Law P = ρRT Density is held constant –If T increases then P increases –If P increases then T increases Temperature and Pressure are directly proportional

The Gas Law P = ρRT Temperature is held constant –If P increases then ρ increases –If ρ increases then P increases Density and Pressure are directly proportional

Basic Understandings (Ch. Review) Air pressure is the weight of a column of air acting on a given surface area At any point in the atmosphere the air pressure is equal in all directions A barometer measures air pressure – there are two types: mercurial and aneroid

Basic Understandings (Ch. Review) Pressure and Density decrease with altitude –50% of atmosphere is below 3.5 miles –99% of atmosphere is below 20 miles Barometer readings are adjusted to sea level to remove the influence of elevation

Basic Understandings (Ch. Review) Cold air masses are denser and exert higher pressure than warm air masses Dry air masses are denser than humid air masses Temperature has a greater influence on density and pressure than humidity Variables of state: temperature, pressure, and density

Basic Understandings (Ch. Review) Important changes in weather often accompany relatively small changes in air pressure at the Earth’s surface. High or rising pressure indicates fair weather Low or falling pressure indicates inclement weather