Focus on Aviation: Microbursts

Understanding Weather and Climate (7th Edition) (MasteringMeteorology Series) by Edward Aguado, James E. Burt 

Clouds

            The first widely accepted system for cloud classification was devised by English naturalist Luke Howard in 1803. It divided clouds into four basic categories:

1.    Cirrus—thin, wispy clouds of ice

2.    Stratus—layered clouds

3.    Cumulus—clouds having vertical development

4.    Nimbus—rain-producing clouds

Generalized Cloud Chart:

Our current classification scheme is a modified version of Howard’s, retaining his four categories and also allows new combinations. The ten principle types of clouds that result are then grouped according to their height and form:

High Clouds (greater than 19,000 ft)	Cirrus (Ci)	Thin, white, wispy clouds of ice resembling mares’ tails.
	Cirrostratus (Cs)	Extensive, shallow clouds somewhat transparent to sunlight, producing a halo around the Sun or Moon.
	Cirrocumulus (Cc)	High, layered cloud with billows or parallel rolls.
Middle Clouds (6,000 ft to 19,000 ft)	Altostratus (As)	Extensive, watery, layer clouds composed of water droplets. Allows some penetration of sunlight but Moon or Sun appears as bright spot within cloud.
	Altocumulus (Ac)	Shallow, mid-level cloud containing patches or rolls, often arranged in bands. Generally more opaque and having less distinct margins than cirrocumulus.
Low Clouds (below 6,000 ft)	Stratus (St)	Uniform layer of low cloud ranging from whitish to gray.
	Nimbostratus (Ns)	Low cloud producing light precipitation. Produces darker skies than altostratus.
	Stratocumulus (Sc)	Low-level equivalent to altocumulus with some vertical development.
Clouds with Extensive Vertical Development (may extend through mmm moo atmosphere)	Cumulus (Cu)	Detached billowy clouds with flat bases and moderate vertical development. Sharply defined boundaries.
	Cumulonimbus (Cb)	Clouds with intense vertical development with characteristic anvil. May be tens of thousands of meters thick. Appear very dark when viewed from below. Can create violent weather.

Unusual Clouds:

─     Lenticular Clouds—waves formed by the passage of air over a topographic barrier.

─     Banner Clouds—isolated atop mountain peaks

─     Mammatus—found in margins of cumulonimbus clouds, formed by downdrafts, and sometimes are distorted by complex motions.

─     Nacreous Clouds—stratus clouds only observed at high latitudes.

─     Noctilucent Clouds—in the mesosphere, can illuminate the sky at high latitudes during the twilight hours. Noctilucent clouds

Other:

─   Aircraft Contrails—A type of ice cloud, know as contrails, is frequently caused by jet aircraft. The very hot engine exhaust contains considerable water vapor as a result of fuel combustion, and turbulence in the wake of the aircraft rapidly mixes the exhaust with the cold, ambient air. The mixing of warm, moist air with cold air can lead to saturation and, in this case, the rapid formation of ice crystals.

Focus on Aviation: Density Altitude and Aircraft Performance

Understanding Weather and Climate (7th Edition) (MasteringMeteorology Series) by Edward Aguado, James E. Burt 

Focus on Aviation: Lightning and Aircraft

Understanding Weather and Climate (7th Edition) (MasteringMeteorology Series) by Edward Aguado, James E. Burt

Meteorological Measurements

Ø  Surface Measurements

̶        Most locations report hourly automated measurements

̶        Non-automated measurements are reported every 3 hours

̶        U.S. national weather service stations are called ASOS (Automated Surface Observing Systems)

̶        Federal aviation administration and department of defense are called AWOS (Automated Weather Observing Systems)

̶        ASOS reports cloud height and amount, visibility, precipitation type, intensity, and accumulation, obstructions to vision such as fog or haze, dewpoint temperatures, and wind direction, speed, and character (gusts, squalls)

̶        Data available on the internet

̶        Original data are coded using Meteorological Aviation Report format called METAR

̶        Data also plotted on meteograms (graphs that show how several atmospheric properties change with time)

̶        Meteorologists use meteograms to analyze the structure of hazardous weather events and track their progress as they move between stations

Ø  Rawinsondes

̶        Rawinsondes are primary instrument packages used to make these measurements (through the depth of the troposphere and well into the stratosphere)

̶        Is a balloon-borne instrument system that measure pressure, temperature, dewpoint temperature, wind direction, and speed?

̶        Launched worldwide twice a day at 0000 UTC and 1200 UTC

̶        Data collected are plotted on a diagram that depicts pressure on the vertical axis and temperature (Celsius) on the horizontal axis

Ø  Radar

̶        A device that transmits pulses of microwave energy

̶        When microwaves encounter objects, such as raindrops and hailstones, some of the energy is scattered back toward the antenna.

̶        The antenna gathers its energy (radar echo) and passes it through other electronic device called a receiver

̶        The amount of energy that returns to the radar depends on 3 parameters:

1.    Size of precipitation particles

2.    Type of particles (ice crystals, rain, hail)

3.    Number of particles in the beam

̶        Radar reflectivity allows meteorologists to estimate the rain rate

̶        To display radar reflectivity meteorologists use logarithmic or decibel scale which is denoted as dBZ

Ø  Wind Profiling

̶        Type of Doppler radar that operates with very high frequency (VHF) and ultra high frequency (UHF) radio bands.

̶        Antenna is an array of cables called a Phased Array Antenna

̶        Array transmits electromagnetic radiation with a slight time delay from one side to the other across the array - delay creates a beam of radiation that points in a specific direction

̶        By using three beams and trigonometry a vertical profile of the wind can be obtained

̶        RASS (Radio Acoustic Sounding System) transmits an acoustic (sound) wave in the vertical direction

̶        Speed of sound is related to the temperature of the air through which the acoustic wave is passing

̶        WSR-88D (Doppler) radar are obtained through mathematical manipulation of the radial velocity measurements all around the radar

̶        WSR-88D can also detect motion within about 20 km of the radar site

Ø  Satellites and Satellite Imagery

̶        Found in 2 types of orbits (Geostationary orbits and Low-Earth orbits)

̶        Geostationary Orbits: circular orbit lying in the earth’s equatorial plane in which a satellite has the same rotational velocity as the earth, must be 35,800 km up

̶        Low-earth Orbits: several hundreds of thousands of km up, near polar orbit (crosses the equator), view small part of the earth, clear view of polar regions

̶        Three primary channels are used when weather monitoring:

1.    Visible: measures solar radiation reflected from the earth or atmosphere at a frequency visible to the human eye (like sitting on a satellite viewing the earth in black and white)

2.    Infrared: Tuned to be sensitive to infrared electromagnetic radiation that the earth emits. Lower clouds are warmer so they give off more radiation then the cooler higher clouds in the troposphere

3.    Water Vapor Channels: Measures radiation at a specific infrared frequency that is sensitive to radiation emitted from water vapor molecules in the atmosphere, allows meteorologists to monitor during both clear and cloudy situations

Ø  Commercial Aircraft Measurements

̶        Various types of systems used are collectively named AMDAR (aircraft meteorological data reporting) systems

̶        Meteorological data is reported to the National Weather Service (NWS) and is used to develop weather forecasts

Ø  Lightning Detection

̶        NLDN (national lightning detection network) and CLDN (Canada LDN) map out the location of all cloud-to-ground lightning strikes in the US and Canada

̶        Consists of remote, ground based sensing stations

̶        Direction, time of arrival, and other characteristics of electromagnetic waves created

̶        Triangulates using three or more receivers to determine the position, time, strength, and polarity