Basic Meteorology Jeff Herring Environmental Studies Program Prescott College Diana Anderson Center for Environmental Sciences and Education Northern Arizona University

2 Contact Information Dr. Diana Anderson Center for Environmental Sciences and Education, NAU

3 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

4 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

5 Average Energy Balance in the Atmosphere

6 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

7 Why Do We Have Seasons? What do you think?

8 Why Do We Have Seasons? (cont.) Common misconception Seasons caused by changes in distance between Earth and Sun But what about Australia?

9 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

10 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

11 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

12 Why Does Air Move? (cont.) 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

13 Why Does Air Move? (cont.) 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)

14 What Are Clouds?

15 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

16 What Are Clouds? (cont.) 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”)

17 What Are Clouds? (cont.) Fog usually due to cooling of air at night or as it moves over a cooler surface

18 Atmospheric Stability Key factor in determining severity of air pollution is stability of atmosphere

19 Atmospheric Stability (cont.) Stable atmosphere: vertical air motion inhibited Unstable atmosphere: vertical motion triggered by local heating or turbulence is enhanced

20 Atmospheric Stability (cont.) Best way to determine stability of atmosphere: launch weather balloon

21 Atmospheric Stability (cont.) Using this data, make plot of vertical temperature profile How does this compare to the April 27 profile?

22 Atmospheric Stability (cont.) 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)

23 Atmospheric Stability (cont.) 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

24 Atmospheric Stability (cont.) 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

25 Atmospheric Stability (cont.) Inversions often form near ground at night due to radiation cooling Higher altitude inversions can be caused by large-scale sinking of air

26 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”

27 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

28 Air Pollution Dispersion (cont.) 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 SO 2 concentrations up to 1.3 ppm (almost 10 times EPA limit). Several thousand people died, probably from combination of exposure to fine particles and SO 2

29 Air Pollution Dispersion (cont.) 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

30 Air Pollution Dispersion (cont.) Pasquill Stability Categories Surface wind speed at 10 m (m/s) DayNight Incoming Solar radiationCloud Cover StrongModerateSlightThinly OvercastMostly Cloudy < 2AA-BB 2-3A-BBCEF 3-5BB-CCDE 5-6CC-DDDD >6CDDDD

31 Air Pollution Dispersion (cont.) Our goal: predict air pollution concentrations given knowledge of sources and meteorology

32 Air Pollution Dispersion (cont.) One approach: assume each individual plume behaves in Gaussian manner Results in concentration profile with bell- shaped curve

33 Air Pollution Dispersion (cont.) Gaussian behavior extends in two dimensions

34 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].

35 Air Pollution Dispersion (cont.) 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!

36 Other Meteorology Resources 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 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 Link to view most recent skewT/log-P diagrams from the U.S. networ.

37 Other Meteorology Resources (cont.) 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 ( of western U.S. climate 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: