Part A | 2.1
16 Part A Fundamentals
2.1 Atmospheric Processes
An instantaneous snapshot of the winds of the entire atmosphere would present an extremely chaotic view of
the flow. Atmospheric circulations can be characterized
as an enormously complex patterns of eddies within eddies. Thus, it is convenient to categorize the different
features of atmospheric circulations according to their
different sizes – or scales of motion.
The very small-scale eddies or whirls with time
scales of only a few seconds can be eliminated by
averaging the measured winds over periods of several minutes. Furthermore, observations are so widely
spaced in time and area that most must be considered
averages over horizontal distances of tens of kilometers, and vertical distances of tens of meters. Thus, even
such relatively large circulation phenomena as thunderstorms and tornadoes often cannot be resolved by the
usual weather station network of observations. Through
this kind of averaging the range of atmospheric motions
can be classified in terms of three general scales; microscale, mesoscale, and synoptic scales (Fig. 2.1).
On the microscale, we find small, short-lived eddies
(or turbulence), that are strongly affected by local conditions of both surface roughness and temperature. These
eddies are very significant as dispersers of pollutants. At
the large end of the microscale are tornadoes and waterspouts. Because microscale motions have such short
time scales, Earth rotation is not a significant factor.
Mesoscale phenomena of intermediate horizontal
size include such processes as land–sea breezes, thunderstorms, and squall lines. Earth rotation can be an
Hurricanes
Tornadoes
Waterspouts
0
1 0
Turbulence
Land–sea and
mountain–valley breeze
Thunderstorms
Extratropical
cyclones and
anticyclones
Long waves
in westerlies
Microscale (m)
Mesoscale (km)
Synoptic scale (km)
100
1
10
Space scale
Ti m e sc al e (s )
100
1000
10 000
100 000
Circumference of earth (40 000 km)
1 000 000
1 week
1 day
10 000
100
1
Fig. 2.1 Horizontal space and
time scales of atmospheric
circulations (after [2.1])
important factor affecting the larger mesoscale phenomena such as sea breezes.
Synoptic scale phenomena most notably represented by large-scale, low-pressure cyclones and highpressure anticyclones that control our day-to-day
weather changes – particularly at mid-latitudes. Historically these synoptic-scale features have been defined
every 3 h by the large-scale weather network of simultaneous observations. Earth rotation-related forces are
very important factors in the physics of synoptic-scale
systems, which persist for days to weeks. Synopticscale features that persist for weeks to months tend to
have planetary space scales and like jet stream loops
play an important role in determining the seasonal characteristics of the weather.
The vertical displacements distinguish these different scales of atmospheric motion. For example, synoptic scale phenomena are dominated by horizontal
motions; with vertical velocities generally only 1 to
2 cm=s. Even the great cyclonic storms in the middle
and high latitudes, have vertical velocities of the order of 50 cm=s. In the smaller, more intense mesoscale
circulations, horizontal and vertical velocities are more
comparable; with thunderstorm vertical velocities are
often of the order of 10 m=s. Microscale or turbulent
phenomena, with nearly equal horizontal and vertical
velocities, occur principally in a rather shallow layer
adjacent to the Earth’s surface and thus are important
factors in air–sea interactions. A detailed description of
the maritime atmosphere is provided in Chap. 3.
16 Part A Fundamentals
2.1 Atmospheric Processes
An instantaneous snapshot of the winds of the entire atmosphere would present an extremely chaotic view of
the flow. Atmospheric circulations can be characterized
as an enormously complex patterns of eddies within eddies. Thus, it is convenient to categorize the different
features of atmospheric circulations according to their
different sizes – or scales of motion.
The very small-scale eddies or whirls with time
scales of only a few seconds can be eliminated by
averaging the measured winds over periods of several minutes. Furthermore, observations are so widely
spaced in time and area that most must be considered
averages over horizontal distances of tens of kilometers, and vertical distances of tens of meters. Thus, even
such relatively large circulation phenomena as thunderstorms and tornadoes often cannot be resolved by the
usual weather station network of observations. Through
this kind of averaging the range of atmospheric motions
can be classified in terms of three general scales; microscale, mesoscale, and synoptic scales (Fig. 2.1).
On the microscale, we find small, short-lived eddies
(or turbulence), that are strongly affected by local conditions of both surface roughness and temperature. These
eddies are very significant as dispersers of pollutants. At
the large end of the microscale are tornadoes and waterspouts. Because microscale motions have such short
time scales, Earth rotation is not a significant factor.
Mesoscale phenomena of intermediate horizontal
size include such processes as land–sea breezes, thunderstorms, and squall lines. Earth rotation can be an
Hurricanes
Tornadoes
Waterspouts
0
1 0
Turbulence
Land–sea and
mountain–valley breeze
Thunderstorms
Extratropical
cyclones and
anticyclones
Long waves
in westerlies
Microscale (m)
Mesoscale (km)
Synoptic scale (km)
100
1
10
Space scale
Ti m e sc al e (s )
100
1000
10 000
100 000
Circumference of earth (40 000 km)
1 000 000
1 week
1 day
10 000
100
1
Fig. 2.1 Horizontal space and
time scales of atmospheric
circulations (after [2.1])
important factor affecting the larger mesoscale phenomena such as sea breezes.
Synoptic scale phenomena most notably represented by large-scale, low-pressure cyclones and highpressure anticyclones that control our day-to-day
weather changes – particularly at mid-latitudes. Historically these synoptic-scale features have been defined
every 3 h by the large-scale weather network of simultaneous observations. Earth rotation-related forces are
very important factors in the physics of synoptic-scale
systems, which persist for days to weeks. Synopticscale features that persist for weeks to months tend to
have planetary space scales and like jet stream loops
play an important role in determining the seasonal characteristics of the weather.
The vertical displacements distinguish these different scales of atmospheric motion. For example, synoptic scale phenomena are dominated by horizontal
motions; with vertical velocities generally only 1 to
2 cm=s. Even the great cyclonic storms in the middle
and high latitudes, have vertical velocities of the order of 50 cm=s. In the smaller, more intense mesoscale
circulations, horizontal and vertical velocities are more
comparable; with thunderstorm vertical velocities are
often of the order of 10 m=s. Microscale or turbulent
phenomena, with nearly equal horizontal and vertical
velocities, occur principally in a rather shallow layer
adjacent to the Earth’s surface and thus are important
factors in air–sea interactions. A detailed description of
the maritime atmosphere is provided in Chap. 3.