Basic Meteorology Jeff Herring Environmental Studies Program Prescott College Diana Anderson Center...

Basic Meteorology

Jeff HerringEnvironmental Studies Program

Prescott College

Diana AndersonCenter for Environmental Sciences and Education

Northern Arizona University

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Contact Information

Dr. Diana Anderson Center for Environmental Sciences and

Education, NAU E-mail: [email protected]

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Outline of Topics

Energy Balance at the Earth’s Surface Why Do We Have Seasons? Why Does Air Move? What Are Clouds? Atmospheric Stability Air Pollution Dispersion

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Energy Balance at the Earth’s Surface

Must consider energy from sun (solar radiation), energy emitted by ground, clouds, greenhouse gases (infrared or IR radiation)

Some solar radiation reflected by clouds or ground; fraction reflected is “albedo”

Heat redistributed by air, water motion

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Average Energy Balance in the Atmosphere

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Energy Balance

On average, areas of Earth closer to equator (0 deg. latitude) get more incoming solar radiation than they lose in IR radiation

Areas near poles lose much more IR than they gain in solar radiation

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Why Do We Have Seasons?

What do you think?

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Why Do We Have Seasons? (cont.)

Common misconception Seasons caused by changes in distance

between Earth and Sun But what about Australia?

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Why Do We Have Seasons? (cont.)

Primary cause of seasons is change in incoming solar radiation due to tilt of Earth’s axis

Let’s act it out using a globe and a light source

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Why Does Air Move?

Consider horizontal motion of air separately from vertical motion

Most wind is horizontal; vertical wind speeds typically much less than horizontal wind speed

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Why Does Air Move? (cont.)

To understand air motions, consider concept of pressure Air pressure: measure of total force of

collisions with air molecules Another conceptualization: weight of column

of air overhead If air pressure different at two locations at same

elevation, air will “blow” from higher pressure to lower pressure

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Vertical motions of air also important to formation of clouds, dispersion of air pollution, etc. Factors causing vertical motion very different than

those causing horizontal motion Pressure decreases rapidly as altitude increases, but pressure

difference balanced by gravity

Vertical air motions can be caused by turbulence, wind flowing over mountains, larger scale weather patterns, local heating and cooling

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Localized solar heating causes air near ground to expand Air may then be less

dense than surrounding air; so buoyant (“floats” upward like hot air balloon)

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What Are Clouds?

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What Are Clouds? (cont.)

Droplets of liquid water or solid ice crystals suspended in air

We see reflection of sunlight off cloud droplets or crystals

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In atmosphere, anything that causes air to cool can cause a cloud to form Most common way: air

expands and cools as it rises

Unsaturated air parcels cool by about 10°C for every 1 km they rise (“adiabatic lapse rate”)

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Fog usually due to cooling of air at night or as it moves over a cooler surface

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Atmospheric Stability

Key factor in determining severity of air pollution is stability of atmosphere

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Atmospheric Stability (cont.)

Stable atmosphere: vertical air motion inhibited

Unstable atmosphere: vertical motion triggered by local heating or turbulence is enhanced

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Best way to determine stability of atmosphere: launch weather balloon

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Using this data, make plot of vertical temperature profile

How does this compare to the April 27 profile?

Balloon Launch at FlagstaffJanuary 21, 2002, 5 amAltitude (m) Temperature, Celcius

2192 -11.12362 0.82657 22743 1.63075 0.23658 -2.93800 -3.73962 -5.14267 -7.74877 -12.94990 -13.95576 -17.75680 -17.35791 -17.95831 -18.1

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To understand, compare rate of atmospheric temperature change

with height (environmental lapse rate or ELR)

with rate at which temperature of rising parcel of

air changes (adiabatic lapse rate or ALR)

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Given that ALR is –10ºC per km, estimate what would happen to parcel of air pollution At ground that is 8ºC warmer than

surrounding air At 5 a.m. on January 21

A parcel of air will rise if warmer than surrounding air at same altitude

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Temperature inversion occurs when temperature increases with height

Temperature inversions tend to trap air pollution by limiting maximum mixing depth (MMD) into which pollution can be dispersed

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Inversions often form near ground at night due to radiation cooling

Higher altitude inversions can be caused by large-scale sinking of air

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Air Pollution Dispersion

Effects of pollution on human and ecosystem health depend on pollution concentration

Rate of dispersion of non-reactive pollutant determines concentration “The solution to

pollution is dilution”

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Air Pollution Dispersion (cont.)

Worst episodes of air pollution have occurred during winter Ground level radiative

cooling causes fog to form

Fog reflects solar radiation, preventing ground from rewarming

Resulting inversion can be very low and persistent

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October 25-31, 1948, Donora, Pennsylvania: strong inversion caused pollution to accumulate 20 people died from exposure, including 17 in

14-hour period

December 5-9, 1952, London, England: fog inversion caused SO2 concentrations up to 1.3 ppm (almost 10 times EPA limit). Several thousand people died, probably from

combination of exposure to fine particles and SO2

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Ventilation describes degree to which atmospheric motion dilutes air pollution Ventilation high in

periods of wind or when atmospheric stability is low

Stagnation will occur when wind speed is low and stability is high

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Pasquill Stability CategoriesSurface wind speed at 10 m (m/s)

Day Night

Incoming Solar radiation Cloud Cover

Strong Moderate Slight Thinly Overcast Mostly Cloudy

< 2 A A-B B

2-3 A-B B C E F

3-5 B B-C C D E

5-6 C C-D D D D

>6 C D D D D

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Our goal: predict air pollution concentrations given knowledge of sources and meteorology

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Air Pollution Dispersion (cont.) One approach: assume each individual

plume behaves in Gaussian manner Results in concentration profile with bell-

shaped curve

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Gaussian behavior extends in two dimensions

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Air Pollution Dispersion (cont.) This assumption allows us to calculate

concentrations downwind of source using this equation

where c(x,y,z) = contaminant concentration at the specified coordinate [ML -

3], x = downwind distance [L], y = crosswind distance [L], z = vertical distance above ground [L], Q = contaminant emission rate [MT-1], y = lateral dispersion coefficient function [L], z = vertical dispersion coefficient function [L], u = wind velocity in downwind direction [L T-1], H = effective stack height [L].

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The Bad News: This equation would get pretty ugly to solve, especially with multiple sources to consider

The Good News: Computers can do the work for us!

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Other Meteorology Resources www.wrh.noaa.gov/Flagstaff/ Access to local National Weather

Service Office for local forecasts, satellite imagery, and radar. Provides link to main NWS homepage and thereby to other regional forecasts

www.unisys.com: Almost all of information available to any weather forecaster. Previous sites more user-friendly, but this one provides nitty-gritty weather details. Best feature is access to output from all major computer forecast models. Great satellite photos, too

www.rap.ucar.edu/weather/upper Link to view most recent skewT/log-P diagrams from the U.S. networ.

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Other Meteorology Resources (cont.)

www.wrcc.dri.edu/: Western Region Climate Center at the Desert Research Institute. Huge amount of information on climate in the Western U.S. Note especially their pages on El Nino and their historical summaries (http://www.wrcc.dri.edu/climsum.html) of western U.S. climate

www.intellicast.com: Another good, all-purpose weather web site. Best part of site excellent quality of radar data, which differentiates between snow, rain and mixed precipitation

wunderground.com Good site for quick forecasts and simple maps of weather conditions in the US. They also have some information for other countries

For more links, see the NAU weather station web page at: http://art.artsci.nau.edu/nauws

Basic Meteorology Jeff Herring Environmental Studies Program Prescott College Diana Anderson Center...

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