Saturday, July 28, 2012

Rain from Clouds

Water is continually evaporating and condensing in the sky. Clouds contain water vapor and small drops of condensed water (cloud droplets). Most of the condensed water in clouds does not fall as precipitation

Potential rain forms when water droplets condense on dust, salt, or smoke particles, which act as a cloud seeding nucleus.

When the water droplets become large enough that their fall speed exceeds the updraft speed in the cloud, they fall toward the ground. If the water drops do not evaporate as they travel earthward, they reach the ground as rain.

Virga, oil on canvas by Crystal Foreman Brown
Virga is the term for visible streaks or shafts of water that fall from clouds but evaporate before reaching the ground.
After the last of the rain had fallen from the sky and come to earth — leaving the sky clear and the earth damp and gleaming — the world below grew joyful in the cool left by the rain, and the greater clarity of life that returned with the blue of the heavens furnished each soul with its own sky.
~ Fernando Pessoa (1888-1935) from The Book of Disquiet.

Rain Drop Size

Size, fall velocity, and particle density (drops per square foot of air) vary for different types of rain (shown below).

Median diameter
Fall Velocity
Drops per sec.
per sq. ft.
Light rain.041.2415.726
Moderate rain.151.6018.746
Heavy rain.602.0522.046
Excessive rain1.602.4024.076
Source: Lull, H.W., 1959, Soil Compaction on Forest and Range Lands, U.S. Dept. of Agriculture, Forestry Service, Misc. Publication No.768

Rain is grace; rain is the sky descending to the earth; without rain, there would be no life.
~ John Updike (1932-2009)


Saturday, July 21, 2012

Avalanche Forces

An avalanche is a sudden snowslide. An accumulation of snow, gravity, and a slope are all that's needed.

Avalanches are triggered by natural phenomena like new snow, or by man-made phenomena like snowmobilers, skiers, and gunshots.

Avalanches are classified by type as loose snowor slab avalanches, and by moisture content:
  • dry loose snow avalanche;
  • wet loose snow avalanche;
  • dry slab avalanche; and
  • wet slab avalanche.

A slab avalanche in Gallatin National Forest

Slab Physics

An idealized slab avalanche is expressed as up- and down-slope forces in a free body diagram (shown below).

By Jeff Levison, The Alaska Avalanche Information Center
The free body diagram has the following variables:
Gcenter of gravity
wweight of the slab
ffrictional force
Nnormal force
Fccompressive force from the down-slope snow
θslope angle

The stability of the slab is a balance between stress and strength.
stress = strength
That is, stability is a balance of the strength of the down-slope snow Fc and the frictional force f between slab snow and base snow against the stress caused by the slope-adjusted weight w sin θ of the slab:
w sin θ = Fc + f
The balance between stress and strength is often precarious.
Even the largest avalanche is triggered by small things.
Vernor Vinge
From the equation, one can imagine an imbalance triggered by:
  • More snow - which would add to the slab's weight w, or
  • A temperature change - which might lessen the frictional force f.

Word Origin

Avalanche is French word derived from the verb avaler meaning to descend or go down.
No snowflake in an avalanche ever feels responsible.
Computer Models

3-dimensional computer models can simulate avalanches. Computer models use topographic data and prevailing snow conditions. Such models are idealized, but serve an objective measure of avalanche risk.

An animation of the Salezer avalanche near Davos Dorf, Canton Grisons, Switzerland was compiled from the results of a RAMMS (Rapid Mass Movements) computer simulation.

Simulation of Salezer avalanche near Davos Dorf,
Canton Grisons, Switzerland.


Saturday, July 14, 2012

Traveling & Standing Waves in Water

Traveling Waves

I recall jerking a garden hose, then seeing the bend I'd made travel away from me down a finite length of the hose. This retreating bend, or pulse, is a traveling wave.

Waves are caused by a disturbance - an input of energy - that travels through spacetime while the energy of the disturbance is transferred and dissipated.

A traveling wave is seen when the wave is not confined to a given space along the medium such as ocean waves approaching a shoreline.
The sea pronounces something, over and over, in a hoarse whisper; I cannot quite make it out.
~Annie Dillard
Ocean waves can travel thousands of miles before reaching land.

From the Wave

It mounts at sea, a concave wall
   Down-ribbed with shine,
And pushes forward, building tall
   Its steep incline.

...opening stanza from a poem by Thom Gunn

Standing Waves

A standing wave is stationary. It is confined in some way by natural or man-made contraints.

A vibrating guitar string is an example of a standing wave. A guitar string has a number of frequencies at which it will vibrate (i.e., the harmonics of the guitar string). The frequency at which a guitar string vibrates depends on the tension of the string, the linear density of the string and its length.

Standing Waves in Rivers

Standing waves in rivers are caused by a high volume of water constricted by flowing into and over a structure like a submerged boulder and creating a wave behind it known as a hydraulic jump. Thought another way, when flowing water encounters a zone of slower moving water a abrupt rise occurs in the surface of the water.

Standing waves can been ridden like the river surfers on the Eisbach in Munich (below). The Eisbach river surfers catch the wave by launching into it from the sides, then facing upstream while traversing its stationary wave front.

The Eisbach wave is about 3' high. Enthusiasts have modified the wave to boost its height and to break more cleanly by submerging planks attached to an upstream bridge.

Surfing a standing wave gives the sensation of traveling over water while not actually moving. A standing wave can be surfed for as long as one maintains the right position and balance.
How could drops of water know themselves to be a river? Yet the river flows on.
~Antoine de Saint-Exupery

Further Reading

Saturday, July 7, 2012

The Nature of Sea Levels

Sea Surface by Petr Kratochvil
Sea level is the average height of the ocean's surface. Sea levels have fluctuated over time.


Mean Sea Level is a way to express the elevation of the sea with wave motion and tide changes averaged out.

Geologic observation and artifacts narrate the story of the oceans.

Constant Change

Sea level fluctuations are preserved in the geologic record.

Sedimentary and fossil records are often the key to estimating historic sea levels. Coral reefs are also accurate markers because they grow just below the prevailing sea level of the time.

There is sedimentary and fossil evidence that sea level was more than 21 meters (70 ft) higher 400,000 years ago than it is today:
...sedimentary and fossil evidence in the walls of a limestone quarry in Bermuda that documents a rise in sea level during an interglacial period of the Middle Pleistocene in excess of 21 meters above its current level. ~ Dramatic Rise In Sea Level And Its Broad Ramifications Uncovered

Ocean and Continents

Western Interior Seaway
The notion of the familiar borders between oceans and continents seems static within the time scale of a human lifetime. Over a geologic timescale, oceans and continents appear ever-changing.
Even Castles made of sand, fall into the sea, eventually.
Jimi Hendrix
The Western Interior Seaway was an inland sea that divided North American into two landmasses during the Mid-to-Late Cretaceous period. Vast carbonate deposits indicate the Seaway was warm and tropical.

The shallow Western Interior Seaway supported abundant marine life and early birds.