Cloud and Meteorological Optics Images*

by: Russell D. Sampson

Camera: Nikon Coolpix 2000

*If you would like to use these images please contact me.  They are free of charge only for educational purposes.

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A 'Normal' Primary and Secondary Rainbow. 

A 'normal' primary and secondary rainbow taken from Edmonton Alberta, Canada in the summer of 2005.  The rainbow is produced by sunlight refracting and reflecting off of spherical raindrops.  The primary rainbow is produced by a single internal reflection inside the raindrop.  The fainter and wider secondary rainbow is produced by a double internal reflection.  The double internal reflection is the reason the colour sequence of the secondary rainbow is the mirror image of the primary rainbow (i.e. red of inside and blue on the outside). 

A Post-sunset Primary Rainbow. 

This image was taken at 22:01 Mountain Daylight Time on July 13, 2005 from a fifth floor apartment on 112 Street and 84 Avenue in Edmonton Alberta Canada.  The image was taken about 4 minutes after sunset which was nominally predicted to occur at 21:57 MDT.  The unrefracted altitude of the Sun at the time of the image was -1.29 degrees and the approximate refracted altitude of the centre of the Sun was about -0.6 degrees.  Notice how much thinner this rainbow is compared to the more full-spectrum rainbow on the left.  This is caused by the increased reddening of the sunlight as it experiences maximum Rayleigh scattering through an optical path that is nearly tangential to the surface of the Earth.  As a result, all the colours except the red are 'filtered' out of the sunlight leaving a nearly monochromatic rainbow.  Also notice that the bottom of the rainbow is cut off - or at least extremely dim.  This is caused by the shadow of the Earth. Raindrops at this low altitude are too low to 'see' the Sun, while at a higher altitude the drops are still illuminated by sunlight even though the Sun has set as seen from the surface.  Finally, notice how the intersection of the arc of the primary with the horizon is almost at right angles.  With the Sun just below the horizon the anti-solar point would actually be above the apparent horizon and the centre of radius of the rainbow would therefore be slightly above the horizon.

Telephoto Image of a Primary Rainbow from a Sprinkler. 

This image was taken at maximum optical telephoto of the Nikon Coolpix 2000.  Notice that numerous drops inside the rainbow arc are spectrally coloured.  This may indicate that non-spherical drops are present.  This has been suggested as a cause of rare non-circular rainbows.  The non-spherical shape of the drops may be momentarily caused by collisions or fission of drops.  It has also been suggested that they may be produced by the force of air resistance against larger diameter drops.  

Telephoto Image of a Primary Rainbow from a Sprinkler. 

In this image the 'artificial' primary rainbow is more dramatically coloured.  Notice that the blue and violet region is faintly visible - something not normally seen in natural rainbows.  This is due to the fact that natural rainbows are most often seen when the sun is at a relatively low altitude.  The longer optical path length though the atmosphere filters the shorter wavelengths by Rayleigh scattering thus removing the blue and violet from the direct sunlight.   

Summer Cirrus Clouds and a 22-degree Halo . 

The colourful circle of light around the sun is caused by randomly oriented hexagonal ice crystals that act as millions of prisms.  Even though this image was taken in the summer the cirrus clouds are high enough above the surface to experience temperatures of 20 to 50 degrees below zero Celsius.  This image was taken from Edmonton Alberta, Canada in the summer of 2005.  The camera is pointed almost exactly east so the time of the day is early morning.  Notice that the colour sequence is reversed from the primary rainbow.  This is due to the fact that no internal reflection occurs in the 22-degree halo - the sunlight simply enters one face of the ice crystal and out another.

Altocumulus Clouds with Wave Structure

Notice the reddish colouration to the sky nearest the horizon.  This is caused by Rayleigh scattering and is the same reason the low altitude sun (sunrise and sunset) is reddish.  Also note the crepuscular rays at the lower right near the horizon caused by scattered sunlight passing through openings in the cloud deck.  Another optical phenomenon can be seen in the borders of some of the clouds.  These borders are brightly lit by forward scattering of sunlight and is sometimes referred to as the "silver lining" of the clouds.  The wave structure of the clouds would appear to indicate that the air is stable near the cloud deck.  As parcels of air rise they experience negative buoyancy thus forcing the parcel downward.  Like a weight on a spring  the parcels oscillate in and out of the lifting condensation level (LCL) as they travel in the prevailing winds. This image was taken from Edmonton Alberta, Canada (summer 2005).  The camera is pointed almost exactly east so the time of the day is early morning. 

Orographic Cloud over an Island

This image was taken over one of the Gulf Islands between Victoria B.C. and Seattle, WA. just after 0800 PDT on August 19, 2005.  The surface winds have lifted humid air over the topography of the island forcing the air to rise to its lifting condensation level and forming a stationary cloud that follows the shape of the island. 

Mountain Shadows in the Air

These mountain peaks just east of Seattle are casting long shadows into the air as the sun rises.  The aircraft is flying towards the sun. 

Cloud Colouration

Notice the dramatic variations in cloud colouration.  The foreground cloud is dark due to a shadow cast on the cloud by a cloud behind the camera (the sun is also behind the camera).  The cloud behind this cloud is in full sunlight and therefore appears brighter.  Also note the distinct reddish colouration of the very distant clouds near the horizon.  This is caused by Rayleigh scattering.  These cumulus clouds have sharp cauliflower edges indicating an early growth stage with vigorous updrafts.  This also implies that the cloud is still comprised mostly of liquid water droplets rather than ice crystals. This image was taken from Edmonton Alberta, Canada in the summer of 2005.  The camera is pointed almost exactly east and the time is the afternoon. 

Cloud Layers

Notice the two distinct layers of clouds.  The lower layer is made of fair weather cumulus clouds (made of liquid water droplets) and the upper layer is cirrus clouds most likely composed of ice crystals (although the rather bumpy morphology of these clouds may indicate the presence of liquid water droplets).  This image was taken from Edmonton Alberta, Canada (summer 2005).  The camera is pointed almost exactly east and the time is the afternoon.

Summer Cumulonimbus. 

Notice the sharp edges to the cloud indicating continued growth, vigorous updrafts and the presence of liquid water droplets.  A rain-shaft can be seen below the cloud.  The flat base of the cloud indicates the location of the lifting condensation level (LCL) and is the height above the ground where a rising parcel of air would cool adiabatically to the parcel's dew point temperature - and thus condensation would occur.  This image was taken from Edmonton Alberta, Canada (summer 2005).  The camera is pointed almost exactly east and the time of day is in the afternoon. 

Summer Cumulonimbus with Cirrus Anvil. 

This image is of the same cumulonimbus cloud pictured on the left but a few minutes later.  Notice the feathery border of the anvil which indicates that the cloud droplets are turning to ice crystals as they approach the extremely cold temperatures near the tropopause..

Cirrus Clouds. 

The wispy appearance of these high altitude clouds indicates that they are made of ice crystals.  From mid latitudes these clouds are visible at all times of the year and often indicate the leading edge of a front and thus the possibility of a change in the weather.   In towering cumulus clouds the transition between liquid droplets to ice crystals can been seen when the cloud changes from a cauliflower surface to a more feathery surface like the cirrus shown above.

'Heiligenschein' from an Airplane . 

Notice that just above the centre of the frame the trees are unusually bright.  This bright area surrounds the shadow of the aircraft - which is too small to see from this altitude.  Most often this effect is seen as a glow around the shadow of a person's head cast onto grass.  It is often caused by light passing through dew drops and then backscattered off of blades of grass behind the drops.  However it is also caused by shadow hiding (also known as the opposition effect) where the angle of the sunlight is such that no shadows are visible.  The brightening at the anti-solar point has also been shown to be caused by an effect called coherent backscatter where the backscattered light experiences constructive interference off of a granular surface.  The appearance of coherent backscatter is a function of the surface properties of the material.  Therefore, the exact cause of a specific heiligenschein display is complex and may involve more than one mechanism.