ESS 111 – Climate & Global Change

Lecture 1

Structure of the Atmosphere

Global Wind Belts

Where is the atmosphere?

Everywhere!Completely surrounds EarthHeld to Earth by gravitational attraction

What makes up the atmosphere?

Water Vapor

Location of this in the atmosphere is highly variable Significantly influences climate & weather

How?

Atmospheric Thickness

No defined top to the atmosphereThe atmosphere is very shallow—and is less than 2% of the Earth’s thickness

Over 90% ofatmosphere inthe lowest 16km& is where nearlyall weather occurs

Temperature Basics Temperature – measure of average kinetic energy (motion) of individual molecules in matterThree temperature scales (units): Kelvin (K), Celsius (C), Fahrenheit (F)

All scales are relative

degrees F = 9⁄5 degrees C + 32degrees K = degrees C + 273.15

Temperature Layers

Due to surface heating (Longwave, Latent heat, Sensible heat)

Due to ozone absorption of sunlight

Due to Solar winds, Cosmic rays

Temperature decreases with height in the Troposphere

Density & Pressure

Density & Pressure

Lower layers of atmosphere are compressed by air above it

This compression increases pressure & density of the lower layers of the atmosphere

What is atmospheric pressure?

Weight of the overlying airTaller the column of air above an object, the greater the air pressure exerted on that object

Standard Atmospheric Pressure

1013.25 mb1013.25 hPa29.92 inches of Hg

The Layers of the Atmosphere

Thermosphere

Troposphere

Lowest region of the atmosphereContains ½ of the Earth’s atmosphere

density density

Troposphere

Depth of tropopauseBetween the Troposphere & Stratosphere is the tropopauseHeight is variable – Thermal expansion & contraction

How do we determine where the tropopause is located?

Stratosphere

Temperature increases with an increase in altitude

What is this called?

Why is there a temperature inversion in the stratosphere?

Temp Inversion – temperature warms with height instead of cooling w/ heightOzone

Gas that absorbs ultraviolet (UV) solar energyIncreases the temperature of the air surrounds ozone

Mesosphere

Temperature decreases with an increase in altitudeWhere meteors burn up while entering the Earth’s atmosphere

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

Vertical Structure of the Atmosphere

Space shuttle Endeavour straddles mesosphere & stratosphere

Planetary WindsWell-defined pressure patterns exist across the Earth that induce the global wind patterns on the planet

Idealized Single-Cell Convection Model for a Planet

Features of the circulation pattern:

•horse latitude•trade winds•doldrums•prevailing westerlies•polar easterlies•polar front

The Three-Cell Model

Intertropical convergence zone (ITCZ) -- surface low pressure with clouds and rain

Subtropical high -- Air subsides (dry climate)

Hadley cell -- tropical convection cell

Ferrel cell -- southwesterly winds at surface

Polar cell -- northeasterly winds at surface

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

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 thisregion 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.

Idealized Pressure Belts

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 theinteraction (convergence) of the polar easterlies and the westerlies

Polar Highs – located over the poles! The process which producesthe polar highs is different than the process which produces thesubtropical highs. Surface cooling is the principle reason the polar high.

The ITCZ is a band of clouds across the tropics

ITCZ

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

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 cellsThe Aleutian, Icelandic, and Tibetan lows

Siberian, Hawaiian, and Bermuda-Azores highs

ITCZ (low)

Vertical structure and mechanisms

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

Ferrel Cell (dynamical): Dynamical response to Hadley and polar cells

Polar Cell (thermal): Driven by heating at 50 degree latitude and cooling at the poles

Average atmospheric air pressure and wind patterns in January

Average atmospheric air pressure and wind patterns in July