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Review question. How does hail form? What factors govern the ultimate size of hailstones?. Hail is produced only in cumulonimbus clouds where updrafts and downdrafts are strong and where there is an abundant supply of supercooled water.

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Review question

How does hail form? What factors govern the ultimate size of


Hail is produced only in cumulonimbus clouds where updrafts

and downdrafts are strong and where there is an abundant supply

of supercooled water.

Hailstones begin as tiny ice pellets that grow by collecting

supercooled cloud droplets as they fall through the cloud.

If a strong updraft is encountered, the hail may be carried

upward again and begin the descent anew.


Review question

For surface low pressure to exist for an extended period of time,

what condition must exist aloft?

In order for surface low pressure to exist for a reasonable period

of time, compensation must occur aloft. Surface pressure would be

maintained if divergence aloft occurs at a rate equal to the

inflow below.


Review question

Distinguish between macroscale, mesoscale and microscale

winds. Give an example of each.

Macroscale winds, the largest wind pattern, involve a time scale

of from days to years (hurricanes, mean global flow).

Mesoscale winds involve a time scale of from minutes to days

and include local winds such as the sea breeze as well or

weather phenomena such as tornadoes.

Microscale winds are very short-lived winds (from seconds to

minutes) and involves such things as turbulence (wind gusts).


Review question

What are two possible ways a blocking high might influence

the weather?

Blocking highs can block the eastward migration of cyclones,

keeping one region dry and keeping another region continually

under the influence of cyclone storms.

Another influence of blocking highs is their contribution to air

pollution episodes. This can happen as subsidence within an

anticyclone can produce a temperature inversion and light winds

associated with the center of an anticyclone do little to disperse

polluted air.


Synoptic Weather Forecasting

  • Until the late 1950’s synoptic weather forecasting was the
  • primary basis for weather prediction. Forecasters relied
  • heavily on synoptic weather maps to forecast.
  • Other methods of weather forecasting include:
  • Persistence Forecasting: yesterday’s weather today
  • Trend Forecasting: similar to persistence with slight changes in
  • previous day’s weather
  • Analog Approach: current weather conditions are matched with
  • records of similar past weather events (Model Output Statistics).
e lectronic n umerical i ntegrator a nd c omputer
Electronic Numerical Integrator And Computer

The ENIAC machine occupied a room thirty by fifty feet. The controls are at the left, and a small part of the output device is seen at the right.


Numerical Weather Prediction

  • Weather prediction models initiate with observations of the current state of the atmosphere and then use equations to compute new values at 5-10 minute intervals, called time steps. Predicted values serve as the next set of initial conditions for the next series of computations.
  • The weather models are not perfect representations of the
  • atmosphere. This is so for three basic:
  • inadequate representation of physical processes
  • errors in initial observations
  • inadequate model resolution

Numerical Weather Prediction

  • Weather prediction models initiate with observations of the current
  • state of the atmosphere and then use equations to compute new
  • values at 5-10 minute intervals, called time steps. Predicted values
  • serve as the next set of initial conditions for the next series
  • of computations.
  • The weather models are not perfect representations of the
  • atmosphere. This is so for three basic:
  • inadequate representation of physical processes
  • errors in initial observations
  • inadequate model resolution

Ensemble Forecasting

A forecasting technique that entails running several weather forecasts models, each beginning with slightly different weather information. The forecaster’s level of confidence is based on how well the models agree (or disagree) at the end of some specified time.

Scientific and computational limitations prevent us from constructing a perfect numerical model of real systems. In a chaotic system like the atmosphere errors in the initial conditions contribute to the eventual loss of predictive skill.

Ensemble forecasting, where not only one but a number of numerical integrations are carried out, was first introduced to assess initial error related variations in predictability


Ensemble Forecasting

Dr. Eugenia Kalnay

Distinguished University Professor, Department of Meteorology

University of Maryland

1965. Ph. D., 1971, MIT (under Jule G. Charney)


Ensemble Forecasting

By looking at a number of model runs the forecaster can rule out or give more consideration to a particular weather scenario as well as get a general feel for the variability of a weather pattern.

High confidence

Low confidence


Propagation of Errors

Random Error

Random error is the irreproducibility in making replicate measurements and affects the precision of a result. The distribution of random errors follows a Gaussian-shape “bell" curve. The precision is described by statistical methods such as a standard deviation or confidence limit.

Systematic Errors

Systematic errors are errors that produce a result that differs from the true value by a fixed amount. These errors result from biases introduced by instrumental method, or human factors.

Systematic errors can sometimes be identified and corrected.


Propagation of Errors

Giving that many time steps are needed to produce a forecast, the

problem of “propagation of errors” is a constraint on weather


Small errors may make

little difference in the

early stages of prediction,

can amplify dramatically

over time.

Yt = (a x Yt+1) – Y2t


Tools in Weather Forecasting

  • ASOS Automated Surface Observations
  • AWIPS Advanced Weather Interactive Processing Systems
  • NEXRAD Next Generation Radar (Doppler)
  • Satellites

Satellite Observations and remote sensing

The last 25 years have seen an explosion of remote weather

products. One advantage to these systems is that they provide

continuous data collection which allows observation changes

in the atmosphere over several time scales.







Challenge to design forecast models which

take advantage of constant data stream.



An electronic instrument used to detect objects (such as falling precipitation) by their ability to reflect and scatter microwaves back to a receiver.



(Next Generation Radar)

In 1988, the NEXRAD Agencies established the WSR-88D (Weather Surveillance Radar 88 Doppler) Radar Operations Center (ROC) in Norman, Oklahoma.

The NEXRAD network provides significant improvements in severe weather and flash flood warnings, air traffic safety, flow control for air traffic, resource protection at military bases, and management of water, agriculture, forest, and snow removal.



The wind profiler is a ground based array of multiple beam Doppler radar units which measures and displays wind information up to an altitude of 16 km. This instrument is generally used to detect low level wind shear.


Automated Surface Observing System (ASOS) / Automated Weather Observing System (AWOS)

Automated Weather Observing System (AWOS) is a suite of sensors, which measures, collects and broadcasts weather data to help meteorologists, pilots and flight dispatchers prepare and

monitor weather forecasts, plan flight

routes, and provide necessary

information for correct takeoffs and






  • The data collected by the AWOS may include:
  • Wind speed, direction, and gusts
  • Temperature and dew point
  • Cloud height and coverage
  • Visibility
  • Present weather (rain, drizzle, snow)
  • Rain accumulation
  • Thunderstorms and lightning
  • Altimeter
  • Fog, mist, haze, freezing fog


Polar Satellites

Polar orbiting satellites orbit the globe

at low altitudes (a few hundred km)

which allows them to complete one

pass in 100 minutes. With such a

quick orbit the satellite can capture

two sweeps of the globe in 24 hours.


Polar Orbiting Satellite ViewHigh Resolution Picture Transmission(from: Patrick Prokop, Meteorologist, WTOC-TV



Geostationary Satellites


Geostationary satellites were placed in orbit over the equator; they remain fixed over a point. To keep the satellite in place over Earth the satellite must orbit at a farther distance than

polar orbiters (35,000 kms). Some resolution is lost but very powerful satellites.


The TIROS Program (Television Infrared Observation Satellite) was NASA's first experimental step to determine if satellites could be useful in the study of the Earth






The images above show the stark contrast between the first image beamed down from

TIROS-1 on April 1, 1960 and the full-color full-Earth images that GOES-8 produces every

three hours. But, if it hadn't been for TIROS and the TIROS experiment, there would

be no GOES images today.



NOAA's National Environmental Satellite,Data and Information Service

NESDIS operates the satellites and manages the processing and distribution

of millions of bits of data and images these satellites produce daily. The

prime customer for the satellite data is NOAA's National Weather Service,

which uses satellite data to create forecasts for television, radio, and weather

advisory services.

NOAA's operational environmental satellite system is composed of:

geostationary operational environmental satellites (GOES) for short-range

warning and "nowcasting," and polar-orbiting environmental satellites (POES)

for longer term forecasting. Both kinds of satellites are necessary for

providing a complete global weather monitoring system. The satellites carry

search and rescue instruments, and have helped save the lives of about

10,000 people to date. The satellites are also used to support aviation safety

(volcanicash detection),

and maritime/shipping safety (ice monitoring and prediction).



Around the world...around the clock...NOAA proudly stands watch. As an integral part of worldwide search and rescue, NOAA operates the Search & Rescue Satellite Aided Tracking (SARSAT) System to locate those in distress almost anywhere in the world at anytime and in most conditions.The SARSAT system uses NOAA satellites in low-earth and geostationary orbits to detect and locate aviators, mariners, and land-based users in distress. The satellites relay distress signals from emergency beacons to a network of ground stations and ultimately to the U.S. Mission Control Center (USMCC) in Suitland, Maryland. The USMCC processes the distress signal and alerts the appropriate search and rescue authorities to who is in distress and, more importantly, where they are located. Truly, SARSAT takes the "search" out of search and rescue.  


Chapter 13 Air Pollution

  • Brief History of Air Pollution
  • Sources and Types of Air Pollution








By Charles F. Mabery

“….The essential constituents of the atmosphere include oxygen, nitrogen, and

carbon dioxide in constant proportions, nitrous and nitric acids, sulphurous and

sulphuric acids, dust, soot, moisture, ammonia, and certain other constituents in

variable proportions depending upon atmospheric conditions and local influences.

The allotropic form of oxygen, ozone is an important constituent of the atmosphere,

and probably also hydrogen dioxide….”


A Brief Historical Perspective

Air pollutants: Airborne particles and gases occurring in

concentrations that endanger the health and well being of

organisms or disrupt the orderly functioning of the environment.

The atmosphere’s composition of gases is always changing

(some gases are changing much faster than others). Natural

sources of air pollution have always existed. Some examples of

natural air pollution are: volcanic ash, salt particles from the

oceans, pollen and spores released by plants, smoke from forest

fires and windblown dust.


An important thing to consider when looking at atmospheric gases is not necessarily the concentration but rather the reactivity. Very reactive gases, even though they are not present in “high” concentrations are important.

The OH radical is one of the most important

species in atmospheric chemistry and global

change, as it controls the lifetimes of many

important atmospheric species, such as methane

and the alternative chlorofluorocarbons.

However, because of its high reactivity, the

concentration of the OH radical in the

atmosphere is very small (typically less than

0.5 parts per trillion by volume) and its lifetime

very short (less than 1 second). As a result,

accurate measurements of the OH radical in the

atmosphere are extremely difficult.


Air Pollution

  • Some types of air pollution are relatively new some have been
  • around for centuries.
  • Smoke pollution, associated with coal burning, was one the first
  • recorded types.
  • The problem is associated with a concentration of human activity.
  • With the dawn of the industrial revolution pollution became a
  • more serious issue with major cities experiences severe pollution
  • episodes. In extreme cases, widespread health effects were
  • experienced. In London in 1952 more than 4000 people died as
  • the result of a 5 day pollution episode. Most of the people who
  • died where old, had compromised respiratory systems, or heart
  • problems.

Factories were symbolic of economic growth and prosperity

“I want the sky to be filled with smoke of American industry and upon that cloud of smoke will rest forever the bow of perpetual promise.”

-Robert Ingersoll 1880

An intricate connection between economic development and pollution.


Natural air pollution can be accentuated by human activities

Dust storm in 1937 occurred because the soil was plowed

and vulnerable to strong winds.


Sources and Types of Air Pollution

  • Pollutants can be grouped into two categories: primary and
  • secondary.
  • Primary pollutants are emitted directly from identifiable
  • sources. They pollute the air immediately upon being emitted.
  • Secondary pollutants are produced in the atmosphere when
  • certain chemical reactions take place among primary pollutants.
  • See table 13-1.