Vertical Structure of the Atmosphere and Pressure

Everything is made of molecules! –The ground –Clouds –Your chairs –Your cars –You and me –The atmosphere (air) See video: QD0 QD0 Magnify one billion times

A larger volume of air will contain more molecules and will thus weigh more

Atmospheric Pressure Imagine that each air molecule is a gumball Weight of column of air is like weight of column of gumballs At surface (bottom of machine), there are the most molecules (gumballs) above you o Greatest weight o Greatest pressure As you go up through the atmosphere, there are less and less molecules (gumballs) above you, so weight of column above that level (pressure) decreases ALWAYS Pressure ALWAYS decreases with height

How Much Does Air Weigh? Imagine square columns of air 1 inch on each side, extending from the surface all the way up to the top of the atmosphere How much will each column weigh? –Depends on how many air molecules are in column!

How Much Does Air Weigh? Imagine square columns of air 1 inch on each side, extending to the top of the atmosphere

Size Matters! Now imagine columns of air 2 inches on each side. Weight increases by factor of 4. 4ES 18 lb 26 lb 39.6 lb 49.2 lb 57.6 lb 58.8 lb

Pressure = Weight/Area Pressure = Force/Area Independent of area of column. Nice.

Pressure Always Decreases with Height Columns of air above lower elevations ALWAYS weigh more than columns above higher elevations. Why?

Pressure and Height Pressure change with altitude: 1 mb per 10 m (near surface) –Go up 850 m (2800 feet, about half a mile) and average pressure is same as in the eye of Hurricane Katrina at landfall –Why? Less atmosphere is above you (less weight of air!) ALWAYS Atmospheric pressure ALWAYS decreases with height

Density = Mass / Volume –Units = kg/m 3 Gravity pulls most air molecules closest to surface Density always decreases with height Air Density

How Do We Measure Pressure? Barometers Units: o Millibars (mb) o Pounds per square inch (lb/in 2 ) o Inches of mercury (inHg) Mean sea-level pressure o mb o in Hg o 14.7 lb/in 2

Surface Weather Map Lower pressure generally means a stronger storm More molecules above highs Fewer molecules above lows On weather maps, pressures typically around 1000 mb

Pressure Facts Please don’t memorize the records. It is a waste of your time. You should know range of typical pressure values Instead, understand what pressure means!

Pressure and Height Shouldn’t pressure in mountainous areas (e.g., Colorado) always be lower than pressure at sea level?

Pressure and Height Pressure difference due to weather: o Varies from day to day, but typically 30 mb or so across Continental USA Pressure difference due to elevation: o Roughly constant with time, about 300 mb between sea level and tops of highest mountains Meteorologists must correct pressure to sea level to remove this variability in order to get a clear picture of the weather!

Non-Corrected (Station) Pressure (Katrina) LL H H H L

Station Pressure (Yesterday) L H L H H

Yesterday’s Weather Map Station pressure bears no resemblance to yesterday’s surface weather map Must correct pressure to sea level to see changes in pressure due to weather!!

Sea-Level (Corrected) Pressure (Yesterday) L H H L

Sea-Level Pressure (Katrina) L L H H H L L

Atmospheric Temperature We use temperature to divide the atmosphere into vertical layers Lapse rate – The rate at which the temperature decreases with height Inversion – Layer in which the temperature increases with height Inversion Lapse rate ˗ Lapse rate +

Troposphere Lowest layer (8-12 km deep) of the atmosphere Contains all of the earth’s weather Temperature usually (but not always) decreases with height o Sun heats ground, ground heats air!

Atmospheric Soundings (Lab This Week!) Troposphere/lower stratosphere ONLY Inversion layer in troposphere

Tropopause Marks the top of the troposphere Approximately 8-12 km above surface Acts as lid on weather o Inversion very stable Level of jet stream Difficult to breathe because air is so thin

Stratosphere Temperature increases with height (inversion layer) Ozone layer o Ozone absorbs UV radiation, warming the stratosphere If you looked up from the stratosphere, you would see black space, not blue sky Stratopause marks the top of the stratosphere

All clouds (weather) in troposphere –Tallest thunderstorm clouds extend to tropopause (8-12 km) Stratosphere largely void of clouds and weather Atmosphere slowly fades away to space. –Above middle stratosphere, air is so thin, atmosphere fades to black –There is no well- defined “top” of atmosphere Chris on Twitter

Mesosphere Name means “middle atmosphere” Temperature decreases with height o Lack of ozone Air very thin: < 0.1% of surface pressure Mesopause marks top of mesosphere – coldest temperatures in atmosphere

Thermosphere Temperature increases with height o Oxygen absorbs gamma rays and cosmic rays Air is very thin o Air molecules can travel 1 km before colliding with another molecule o You will freeze regardless of temperature High energy particles from space strikes air molecules, creating aurora borealis (Northern Lights)

Weather Balloon Flight through the Atmosphere Attach an HD camera to a weather balloon Balloon will ascend to about 100,000 feet, then burst o This is in the upper stratosphere o