What set the atmosphere in What set the atmosphere in motion?motion?

Review of last Review of last lecturelecture

Earth’s energy balance at the top of the atmosphere Earth’s energy balance at the top of the atmosphere and at the surface. What percentage of solar energy and at the surface. What percentage of solar energy is absorbed by the surface?is absorbed by the surface?

Atmospheric influences on radiation (3 ways)Atmospheric influences on radiation (3 ways)

What cause the greenhouse effect? What are the What cause the greenhouse effect? What are the major greenhouse gases? Why is methane important? major greenhouse gases? Why is methane important?

The three types of atmospheric scattering. What The three types of atmospheric scattering. What causes the blue sky? Why causes the reddish-orange causes the blue sky? Why causes the reddish-orange sunsets? sunsets?

Basic characteristics of global temperature Basic characteristics of global temperature distribution (T decreases poleward, isotherm shifts distribution (T decreases poleward, isotherm shifts seasonally, T over land > over ocean in summer).seasonally, T over land > over ocean in summer).

Atmospheric ThicknessAtmospheric Thickness

No defined top to the atmosphereNo defined top to the atmosphere The atmosphere is very shallow—and is The atmosphere is very shallow—and is

less than 2% of the Earth’s thicknessless than 2% of the Earth’s thickness

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

Temperature LayersTemperature Layers

Pressure EssentialsPressure Essentials Pressure – force exerted/unit area (weight above Pressure – force exerted/unit area (weight above

you)you)

units - Pascals (Pa) or millibars (mb) (1 mb = 100 Pa)units - Pascals (Pa) or millibars (mb) (1 mb = 100 Pa) Average surface pressure over globe: 1013.2 mb.Average surface pressure over globe: 1013.2 mb.

Atmosphere is mixture of gases -> partial pressure.Atmosphere is mixture of gases -> partial pressure. Dalton’s Law: sum of partial pressures equals total Dalton’s Law: sum of partial pressures equals total

pressure pressure

Pressure gradient (pressure difference between two Pressure gradient (pressure difference between two locations/distance) gives rise to a force (pressure locations/distance) gives rise to a force (pressure gradient force), which sets the air in motion. gradient force), which sets the air in motion.

The Equation of State (Ideal Gas Law)The Equation of State (Ideal Gas Law)

Describes relationships between pressure, temperature, and density (Start w/ molecular movement in sealed container Pressure proportional to rate of collisions between molecules and walls).

At constant temperatures, an increase in air density will cause a pressure increase (Add more molecules increase density increase rate of collisions raise pressure)

Under constant density, an increase in temperature will lead to an increase in pressure (Raise temperature increase speed of molecules increase rate of collisions raise pressure)

Pressure = density x temperature x 287 J kg-

1 K-1

[ p = ρTR]

Pressure decreases with height

Why? Because downward gravity force is balanced by vertical pressure gradient (called hydrostatic equilibrium)

ΔΔp/p/ΔΔz = z = ρρgg

Pressure decreases non-linearly w/ height (Why? Because air is compressible, so denser near the surface)

Vertical pressure distribution: Hydrostatic Vertical pressure distribution: Hydrostatic equilibriumequilibrium

ρρgg

ΔΔp/p/ΔΔzz

Movie time!Movie time! Weather: Weather: WindWind

Horizontal pressure distribution and horizontal pressure gradient

Pressure maps depict isobars, lines of equal pressure Through analysis of isobaric charts, pressure gradients are apparent

Steep pressure gradients are indicated by closely spaced isobars Typically only small gradients exist across large spatial scales (4%

variation at continental scale), smaller than vertical gradients

Horizontal pressure gradients are small compared to vertical pressure gradients

Horizontal Pressure Gradients and wind

The pressure gradient force initiates movement of atmospheric mass, wind, from areas of higher to areas of lower pressure

Horizontal wind speeds are a function of the strength of the pressure gradient

SLP and winds plotted on same chart

Notice the strong winds in Ohio – due to tight pressure gradient

Forces Affecting the Speed and Direction of the Wind

Horizontal pressure gradients responsible for wind Horizontal pressure gradients responsible for wind generationgeneration

Three factors affect wind speed and/or direction Three factors affect wind speed and/or direction ((velocityvelocity):):

1.1. Pressure Gradient Force  (PGF)Pressure Gradient Force  (PGF)

2.2. Coriolis Effect  (CE) Coriolis Effect  (CE)

3.3. Friction Force (FF)Friction Force (FF)

FFCEPGFt

V++=

ΔΔ

1. Pressure 1. Pressure GradientGradient

ForceForce(PGF)(PGF)

• pressure gradient: high pressure low pressure

• pressure differences exits due to unequal heating of Earth’s surface

• spacing between isobars indicates intensity of gradient

• flow is perpendicular to isobars

2. The 2. The CoriolisCoriolis Effect Effect

objects in the atmosphere are influenced by the Earth’s rotationobjects in the atmosphere are influenced by the Earth’s rotation Rotation of Earth is counter-clockwise looking down from N. Pole.Rotation of Earth is counter-clockwise looking down from N. Pole.

results in an ‘apparent’ deflection (relative to surface)results in an ‘apparent’ deflection (relative to surface) deflection to the right in Northern Hemisphere (left in S. Hemisphere)deflection to the right in Northern Hemisphere (left in S. Hemisphere) Greatest at the poles, 0 at the equatorGreatest at the poles, 0 at the equator Increases with speed of moving objectIncreases with speed of moving object and distance and distance CE changes direction not speedCE changes direction not speed

FFCEPGFt

V++=

ΔΔ

Winds in the upper air:Geostrophic Balance

• Now the wind speed/direction is simply a balance between the PGF and CE. This is called GEOSTROPHIC BALANCE. • Upper air moving from areas of higher to areas of lower pressure undergo Coriolis deflection• Air will eventually flow parallel to height contours as the pressure gradient force balances with the Coriolis force

Friction is very small in the upper air:

FFCEPGFt

V++=

ΔΔ

Winds near the surfaceWinds near the surface

Friction slows down wind speed and reduces Coriolis deflectionFriction slows down wind speed and reduces Coriolis deflection

Friction is important for air within ~1.5 km of the surface (the so-Friction is important for air within ~1.5 km of the surface (the so-called called planetary boundary layer).planetary boundary layer). It varies with surface texture, It varies with surface texture, wind speed, time of day/year and atmospheric conditions. wind speed, time of day/year and atmospheric conditions. Friction above 1.5 km is often small (often called the Friction above 1.5 km is often small (often called the free free atmosphereatmosphere), except over regions with storms and gravity ), except over regions with storms and gravity waves.waves.

The third term (friction) must be considered:The third term (friction) must be considered:

Pressure Gradient + Coriolis + Friction Forces

w/out Friction(geostrophic balance)

w/ Friction

High pressure areas (High pressure areas (anticyclonesanticyclones) ) clockwise airflow in the clockwise airflow in the Northern Hemisphere (opposite flow direction in S. Northern Hemisphere (opposite flow direction in S. Hemisphere)Hemisphere) Characterized by descending air which warms creating clear Characterized by descending air which warms creating clear

skiesskies

Low pressure areas (Low pressure areas (cyclonescyclones) ) counterclockwise airflow in counterclockwise airflow in N. Hemisphere (opposite flow in S. Hemisphere) N. Hemisphere (opposite flow in S. Hemisphere) Air converges toward low pressure centers, cyclones are Air converges toward low pressure centers, cyclones are

characterized by ascending air which cools to form clouds characterized by ascending air which cools to form clouds and possibly precipitationand possibly precipitation

Cyclones, Anticyclones, Troughs and Ridges

Cyclones, Anticyclones, Troughs and Ridges on weather charts

HighestLevel

LowestLevel

•Isobars usually not closed off at highest levels – Troughs (low pressure)/Ridges (high pressure)

•Isobars usually closed off at lowest levels – Cyclones, Anticyclones

SummarySummary Four layers of the atmosphere, what separate them?Four layers of the atmosphere, what separate them? Definition of pressure and its unit. Definition of pressure and its unit. Definition of pressure gradient. Pressure gradient sets Definition of pressure gradient. Pressure gradient sets

the air in motion.the air in motion. Equation of state (Relationship between P, Equation of state (Relationship between P, ρρ, and T), and T) Vertical Pressure Distribution. How does pressure Vertical Pressure Distribution. How does pressure

change with height? What is the hydrostatic change with height? What is the hydrostatic equilibrium?equilibrium?

Summary (cont.)

Know 3 Forces that affect wind speed /directionKnow 3 Forces that affect wind speed /direction Especially work on Coriolis force, as this is the hardest Especially work on Coriolis force, as this is the hardest

to understand. Which direction is air deflected to by to understand. Which direction is air deflected to by Coriolis force?Coriolis force?

What is the geostrophic balance? At which level is it What is the geostrophic balance? At which level is it valid? Difference between upper level and surface valid? Difference between upper level and surface windswinds

Does cyclones correspond to high or low surface Does cyclones correspond to high or low surface pressure? Is the air moving clockwise or counter-pressure? Is the air moving clockwise or counter-clockwise around them? How about anticyclones?clockwise around them? How about anticyclones?