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J
is for ...
Jet
Stream
This
is a very long, narrow and fast-flowing current of air which usually
circulates west-to-east at the level of the tropopause i.e. the
boundary between the lowest level of the atmosphere, the troposphere,
and the stratosphere - usually about 30,000 feet.
The
jet stream is characterised by quick changes in wind speed , both
horizontally and vertically, and the minimum core speed is in excess
of 60 knots. Speeds of well over 150 knots are frequently experienced.
There
are two main types of jet stream: polar and subtropical. The position
of the polar jet stream is highly variable within the temperate
latitude belt, often looping across the British Isles, while the
subtropical jet tends to be more consistent at lower latitudes.
Aircraft
often try to use the quick jet stream flow to save fuel on, and
hasten, eastbound flights: for example if you were to fly from New
York to Newcastle. However, turbulence is often associated with
the regions around the jet stream so it can be a bumpy ride at times!
Jacob’s
Ladder
Jacob’s
ladder, and ‘sun drawing water’ are both common terms
used to describe the optical phenomenon known as crepuscular rays.
These occur when sunbeams break through gaps in a layer of low cloud.
They appear slightly luminous as light reflects off water droplets
or dust particles in the air.
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| Crepuscular
rays caused by sunbeams |
Other
pale blue or white rays can occur in an upward direction, separated
by dark streaks which are shadows cast by irregularly-shaped clouds.
Red
or orange rays spreading up from the horizon at dusk are light reflecting
off dust or other pollutants. The greenish areas separating the
rays are shadows cast by clouds or hills below the horizon.
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