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.

Physical Principles: The Surprising Composition of Clouds

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

Composition & Structure of the Atmosphere

Introduction

·       Meteorology = the study of the atmosphere and the processes (such as cloud formation, lightning and wind movement) that cause what we refer to as the “weather”

·       Weather is distinct from climate in that the former deals with the short-term phenomena and the latter with the characteristic long-term patterns.

The Atmosphere, Weather and Climate

·       Atmosphere = a mixture of gas molecules, small suspended particles of solid liquids, and falling precipitation. 

·       Climatology

o   Relies on averages taken over a number of years in order to gauge typical atmospheric conditions for locations across Earth’s surface.

o   Also want to know about variability of the weather elements.

o   Frequencies of occurrences of weather events (such as extreme heat, hail or lightning) are also aspects of climates.

o   Concerned with changes in Earth’s climate and the factors responsible for those changes.

Thickness of the Atmosphere

·       ­How high is the sky?

o   There is no definitive answer. However, because Earth’s atmosphere becomes thinner at higher altitudes, even at heights of several hundred kilometers above sea level, there is some air and, hence, an atmosphere. But we have no way to establish its upper boundary because there is no universally accepted definition of how much air in a given volume constitutes the presence of an atmosphere.

Horizontal Winds	Vertical Winds
-       Primary motion over large areas -       Are typically hundreds to thousands of times greater than vertical wind speeds	-       Hardest to detect and forecast -       Determine much of the atmospheric behavior

Composition of the Atmosphere

·       The atmosphere is composed of a mixture of invisible gases and a large number of suspended microscopic solid particles and water droplets.

·       Molecules

o   Molecules of gases can be exchanged between the atmosphere and the Earth’s surface by physical processes (i.e. volcanic eruptions) or by biological processes (i.e. plant and animal respiration).

o   Can be produced and destroyed by purely internal processes (i.e. chemical reactions between gases)

o   Steady State / Equilibrium Condition = input rate is equal to the output rate

o   Dynamic Equilibrium = molecules cycling in and out of the atmosphere

o   Residence Time = average length of time that individual molecules of a given substance remain in the atmosphere

·       Homosphere

o   The lowest 80 km (50 mi) of the atmosphere

o   Vertical motions are more important than gravitational settling thus processes (other than settling) under gravity must explain any variations present.

o   Reflects the homogenizing role of wind and other motions

o   Gases are often categorized as being permeant or variable, depending on whether or not their concentration is uniform.

§  Permanent Gases = found everywhere in nearly the same proportion.

§  Variable Gases = distribution is uneven in both time and space.

·       Heterosphere = layer of the atmosphere (above the homosphere) where gases segregate according to molecular weight

·       Permanent Gases

o   Make up more than 99% of the atmosphere

o   Nitrogen (most abundant gas)

§  78% of all permanent gasses volume or 75.5% of their mass

§  Largely unreactive

§  Occurs primarily as paired nitrogen atoms bonded together to form single molecules denoted N₂

§  Isotopes = variants of an element with different neutron counts

o   Oxygen (second most abundant gas)

§  21% of the volume of the atmosphere and 23% of its mass

§  Crucial to the existence of virtually all forms of life

§  Dynamic Oxygen = paired oxygen atoms

o   Nitrogen + Oxygen = 99% of all permanent gases

·       Variable Gases

o   Water vapor (most abundant) is 1% of the total volume

§  Condenses to a liquid at relatively low levels in the homosphere

§  Not uniformly distributed with altitude—at higher altitudes, water vapor is even more rare

§  Hydrologic Cycle (water cycle)

§  Source of moisture to form clouds

§  Very effective absorber of energy emitted by the Earth’s surface (radiant energy) thus making it one of the “greenhouse gases”