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ESS 111 – Climate & Global Change
Lecture 1 Structure of the Atmosphere Global Wind Belts
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Where is the atmosphere?
Everywhere! Completely surrounds Earth Held to Earth by gravitational attraction
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What makes up the atmosphere?
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Water Vapor Location of this in the atmosphere is highly variable
Significantly influences climate & weather How?
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Atmospheric Thickness
No defined top to the atmosphere The atmosphere is very shallow—and is less than 2% of the Earth’s thickness Over 90% of atmosphere in the lowest 16km & is where nearly all weather occurs
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Temperature Basics Temperature – measure of average kinetic energy (motion) of individual molecules in matter Three temperature scales (units): Kelvin (K), Celsius (C), Fahrenheit (F) All scales are relative degrees F = 9⁄5 degrees C + 32 degrees K = degrees C
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Temperature Layers Due to Solar winds, Cosmic rays
Due to ozone absorption of sunlight Temperature decreases with height in the Troposphere Due to surface heating (Longwave, Latent heat, Sensible heat)
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Density & Pressure
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Density & Pressure Lower layers of atmosphere are compressed by air above it This compression increases pressure & density of the lower layers of the atmosphere
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What is atmospheric pressure?
Weight of the overlying air Taller the column of air above an object, the greater the air pressure exerted on that object
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Standard Atmospheric Pressure
mb hPa 29.92 inches of Hg
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The Layers of the Atmosphere
Thermosphere
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Troposphere Lowest region of the atmosphere
Contains ½ of the Earth’s atmosphere density density
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Troposphere
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Depth of tropopause Between the Troposphere & Stratosphere is the tropopause Height is variable – Thermal expansion & contraction
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How do we determine where the tropopause is located?
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Stratosphere Temperature increases with an increase in altitude
What is this called?
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Why is there a temperature inversion in the stratosphere?
Temp Inversion – temperature warms with height instead of cooling w/ height Ozone Gas that absorbs ultraviolet (UV) solar energy Increases the temperature of the air surrounds ozone
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Mesosphere Temperature decreases with an increase in altitude
Where meteors burn up while entering the Earth’s atmosphere
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Thermosphere First exposed to the Sun's radiation and so is first heated by the Sun Air is so thin that a small increase in energy can cause a large increase in temperature
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Vertical Structure of the Atmosphere
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Space shuttle Endeavour straddles mesosphere & stratosphere
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Planetary Winds Well-defined pressure patterns exist across the Earth that induce the global wind patterns on the planet
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Idealized Single-Cell Convection Model for a Planet
Features of the circulation pattern: horse latitude trade winds doldrums prevailing westerlies polar easterlies polar front
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The Three-Cell Model Polar cell -- northeasterly winds at surface
Ferrel cell -- southwesterly winds at surface Subtropical high -- Air subsides (dry climate) Hadley cell -- tropical convection cell Intertropical convergence zone (ITCZ) -- surface low pressure with clouds and rain
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Observed Distribution of Pressure and Winds
(a) An imaginary uniform Earth with idealized zonal (continuous) pressure belts (b) Actual planetary winds belts on Earth taking into account continents and ocean currents
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Idealized Pressure Belts
Equatorial Low- warm air rising creates cell of low pressure. Intertropical Convergence Zone (ITCZ)- referred to as the convergence zone because this region is where the trade winds converge. Ascending air leads to cloud formation which makes this region clearly visible on satellite imagery. Subtropical Highs- These zones are caused primarily by Coriolis deflection which restricts upper-level winds from moving poleward. Subsiding air and divergent winds at the surface cause warm, cloud- free weather (many large desert areas are located along this latitudinal belt). Subtropical Highs tend to persist throughout the year, with the center of the high migrating, and are regarded as semi-permanent pressure systems.
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Idealized Pressure Belts (cont.)
Subpolar Low – located around 50 to 60 latitude. Associated with the polar front. The belt of low pressure is formed by the interaction (convergence) of the polar easterlies and the westerlies Polar Highs – located over the poles! The process which produces the polar highs is different than the process which produces the subtropical highs. Surface cooling is the principle reason the polar high.
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The ITCZ is a band of clouds across the tropics
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The three-celled model vs. reality:
Hadley cells are close approximations of real world equatorial winds Ferrel and polar cells do not approximate the real world winds very well at all Model is unrepresentative of westerly flow aloft Continents and topographic irregularities cause significant variations in real world wind patterns compared to the model
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Northern hemisphere semi-permanent cells
Semi-Permanent Pressure Cells are large areas of higher or lower atmospheric pressure than the surface average They may be thermally induced (rising warm air or subsiding cold air) or they may be caused dynamically by converging or diverging wind patterns) They fluctuate seasonally Northern hemisphere semi-permanent cells The Aleutian, Icelandic, and Tibetan lows Siberian, Hawaiian, and Bermuda-Azores highs ITCZ (low)
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Vertical structure and mechanisms
Polar Cell (thermal): Driven by heating at 50 degree latitude and cooling at the poles Ferrel Cell (dynamical): Dynamical response to Hadley and polar cells Hadley Cell (thermal): Heating in tropics forms surface low and upper level high air converges equatorward at surface, rises, and diverges poleward aloft descends in the subtropics
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Average atmospheric air pressure and wind patterns in January
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Average atmospheric air pressure and wind patterns in July
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