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Many people think a mirage is only seen if you are suffering
from heat stroke, but this is not the case. A mirage is not
an illusion, but a real phenomenon that can be photographed.
There are two main
types of mirages, 'inferior' and 'superior'. An inferior image
is one which is seen below an object (such as the sky or the
sun), and a superior image is seen above it. Both are caused
by the bending or 'refracting' of light.
When light passes through
one medium to another, its speed changes. This causes the
light to bend, or 'refract'. This can be seen by placing a
pencil in a glass of water, which seems to bend where it enters
the water. Refraction also happens when light passes through
air of a different density, such as when the temperature changes.
Most people are familiar
with the inferior mirage and have seen 'pools of water' on
the road on a hot day. In this case the refraction is caused
by a large temperature difference near the road. The light
bends so much that an image of the sky is seen on the road.
A superior image is less common than an inferior mirage, as
it also includes a setting or rising sun. The sun that we
see setting or rising is a mirage of the actual sun, which
is really below the horizon.
A superior image is
formed by a warmer layer of air above some colder air, called
a 'temperature inversion'. When the temperature difference
is large enough, the warmer air refracts the light and amazing
images can be seen. A common superior mirage is a ship that
looks like it is sailing in the sky.
A superior mirage could
explain some apparent UFO sightings. Refracted light from
the headlights of cars, gives the impression that there are
lights in the sky. Due to the unpredictability of mirages,
these lights can seem to move quickly through the sky and
suddenly vanish.
In April 1977, people
in Grand Haven, USA saw lights across Lake Michigan. The nearest
town was Milwaukee, 75 miles away. A temperature inversion
across the lake allowed this faraway city to be briefly visible.
With an average April temperature of around 13C, this mirage
at least would not have been due to heat stroke!
Other features in
the Weather Basics series
Air Pressure
Clouds
Day and Night
Fog and Mist
Fronts
Frost
High Pressure
Humidity
Jet Streams
Lightning
Low
Pressure
Northern Lights
Rain or Showers?
Thunder
Watercycle
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