While the main driving force of weather is solar radiation, a second is energy from rotation. The Earth rotates once every 24 hours on its axis through the poles, although this goes unnoticed except for the apparent progression of the Sun, Moon, and stars across the sky. This rotation means that the surface of the Earth speeds eastward, much faster near the equator than at high latitudes. As warm air rises in equatorial regions and flows polewards, its momentum gives rise to strong, largely westerly winds high in the atmosphere. These often narrow jet streams loop in varying patterns around each hemisphere, causing the development and decay of large weather systems such as depressions and anticyclones. It follows that any forecasting system extending beyond one or two days must include the energy transfers that generate these winds high in the atmosphere.
Weather forecasting hinges upon knowing how weather systems will develop and move. The first essential requirement is to find out what is happening now. To forecast for an hour or two, information from the local area may often be sufficient. Attempts to forecast beyond two days or so require global observations. Weather observations are made on land and at sea, at the surface and in the upper atmosphere. Many are direct measurements from conventional instruments, but increasing numbers are made remotely by radar and satellite. Information is collected from all nations of the world, checked, plotted on charts, and stored in computers.
The laws of physics and the mathematical equations governing the motion of fluids have been well known for more than a century. They incorporate the principles of conservation of momentum, mass, energy, and water, and include laws of motion applied to a fluid on a rotating sphere as well as the laws of thermodynamics, radiation, and gases. The Earth's size, rotation rate, geography, and topography are known, as are daily and seasonal variations of incoming solar radiation. Other factors include surface reflectivity (albedo), melting, evaporation, cloud, rain, friction, and sea temperatures. Many of these factors vary through the period of a forecast and must be updated accordingly.
The complex set of equations cannot be solved directly over the whole atmosphere. They are adapted to operate on the atmosphere at individual points, each representing an area of the Earth's surface. The model is applied to a large array of points, laid out as a grid in the model atmosphere. Each point includes several levels up through the atmosphere, and can be regarded as a "stack" of "parcels" of air, each of which represents a particular level over the area of a grid square.
The British Meteorological Office's Global Model is one of the most powerful current NWP models. Its grid comprises 288 points on each of 217 circles of latitude, with a stack of 19 levels at each. Thus the set of equations must be solved for well over a million "parcels" of air to advance the model a step in time. Every forecast starts with a "first guess" of the initial state of the atmosphere. This is based on a short-period forecast from a previous model run, adjusted by thousands of observations from around the world. Advancing the model in time can proceed only in short steps of ten minutes or so, because changes at each "parcel" influence its neighbours. The "time step" is repeated until the required forecast period is covered. A 24-hour forecast involves more than a trillion calculations, and currently takes about 5 minutes. Major NWP systems are continually being refined as understanding of the atmosphere improves, computing power increases, and mathematical techniques advance.
The grid spacing or horizontal resolution of the British model averages about 100 km (62 mi). This is important because it dictates the minimum size of atmospheric disturbance the model can be expected to forecast. Even the highest resolution model cannot be expected to predict a shower or thunderstorm with complete accuracy, but it should give a good indication of areas in which they might develop. Vertical model resolution is also important, because there are often important variations in wind and humidity over depths less than 1 km (.62 miles), especially near the Earth's surface and high in the atmosphere. For this reason model levels are unevenly spaced, being clustered at the top and bottom of the troposphere.
For greater detail over a smaller area of interest, it is possible to nest a higher resolution model within a Global Model. This avoids the extra computing needed with thousands of extra points over the whole globe. The British Model has a system rather like a Russian doll: the Global Model contains another with 50-km (30 miles) spacing which spans Europe and the North Atlantic, and that has within it a model with 15-km (10 miles) resolution over the British Isles.
There remains an important role for the forecasters. They must allow for weaknesses in the model, take account of later information, and use experience to add detail and value.
In meteorology, the formation of clouds caused by cooling of the air results in the condensation of invisible water vapour that produces visible cloud droplets or ice particles. Cloud particles range between approximately 5 and 75 microns (0.0005 and 0.008 cm/0.0002 and 0.003 in). The particles are so small that light, vertical currents easily sustain them in the air.
The different cloud formations result partly from the temperature at which condensation takes place. When condensation occurs at temperatures below freezing, clouds are usually composed of ice crystals; those that form in warmer air usually consist of water droplets. Occasionally, however, supercooled clouds contain water droplets at subfreezing temperatures.
The air motion associated with cloud development also affects formation. Clouds that develop in calm air tend to appear as sheets or stratified formations; those that form under windy conditions, or in air with strong vertical currents, have a towering appearance.
Clouds perform a very important function in modifying the distribution of solar heat over the Earth's surface and within the atmosphere. In general, because reflection from the tops of clouds is greater than reflection from the surface of the Earth, the amount of solar energy reflected back to space is greater on cloudy days. Although most solar radiation is reflected back by the upper layers of the clouds, some radiation penetrates to the surface of the Earth, which absorbs this energy and in turn reradiates it. The lower parts of clouds are opaque to this long-wave Earth radiation and reflect it back earthwards.
The result is that the lower atmosphere generally absorbs more radiative heat energy on a cloudy day because of the presence of this trapped radiation. By contrast, on a clear day more solar radiation is initially absorbed by the surface of the Earth, but when reradiated this energy is quickly dissipated because of the absence of clouds. Disregarding related meteorological elements, the atmosphere actually absorbs less radiation on clear days than on cloudy days.
Cloudiness has considerable influence on human activities. Rainfall, which is vital for growing food, begins in the formation of clouds. Visibility at flight levels was affected by clouds during the early days of air travel. With the development of instrument flying, which permits the pilot to navigate even in the midst of a thick cloud, this hazard has been alleviated.
The first scientific study of clouds began in 1803, when a method of cloud classification was devised by the British meteorologist Luke Howard. The next development was the publication in 1887 of a classification system that later formed the basis for the noted International Cloud Atlas (1896). This atlas is revised and modified regularly and is now used throughout the world.
Clouds are usually divided into four main families on the basis of their height above the ground: high clouds, middle clouds, low clouds, and clouds with vertical development, which may extend through all levels. The four main divisions are further subdivided into genera, species, and varieties, which describe in detail the appearance of clouds and the manner in which they are formed. More than 100 different kinds of clouds are distinguishable.
Common cloud terms and their meanings are :
Alto - high level Stratus / Strato - Layered Cumulus - Dome shaped, puffy
Cirrus - thread like, feathery Nimbus - Thick, dark
Features on the surface weather chart indicate
likely rainfall patterns as well as temperature distribution and wind strength.
In general, highs tend to be associated with subsiding (sinking) air and generally
fine weather, while lows are associated with ascending (rising) air and usually
produce rain or showers.
While cloud can exist without rain, the opposite
is not the case. Clouds form by the condensation of water vapour through cooling.
Causes of cooling include:
Convection, which may be caused
through air mass instability. It may be initiated by warming of low-level
air, forced ascent over mountainous country, or dynamic causes associated
with severe weather systems. Cumulus clouds often form as a result of convection.
The most exceptional forms are often associated with severe thunderstorms
and occasionally, tornadoes. Cumulonimbus, for instance, may reach altitudes
above 15,000 metres.
Systematic ascent of moist air
over large areas linked with large-scale weather systems such as low pressure
systems, including tropical cyclones. In mid-latitudes this systematic ascent
often occurs ahead of active fronts, or with 'cut off' lows. This type of
rain may be persistent and heavy and cause floods, especially if enhanced
by forced (orographic) ascent over mountains.
Orographic ascent which occurs
when air is forced upwards by a barrier of mountains or hills. Cloud formation
and rainfall is often the result. Australia's heaviest rainfall occurs on
the Queensland coast and in western Tasmania, where prevailing maritime airstreams
are forced to lift over mountain ranges.
Cold and warm fronts which
also cause systematic ascent. A cold front is the boundary where cold air
moves to replace, and undercut, warmer and less dense air. Associated cloud
and weather may vary enormously according to the properties of the air masses,
but tends to be concentrated near the front. As a typical cold front approaches,
winds freshen from the north or northwest, and pressure falls. After the front
passes, winds shift direction anticlockwise ('backing' to the west or southwest)
and pressure rises. Cold fronts are much more frequent and vigorous over southern
Australia than elsewhere. Warm fronts, relatively infrequent over Australia,
are usually found in high latitudes where they can occasionally cause significant
weather. They are often shown on weather charts over the Southern Ocean. Warm
fronts progressively displace cool air by warmer air.
Convergence lifting which
occurs when more air flows into an area at low levels than flows out, leading
to forced rising of large air masses. Convergence is often associated with
wave-like disturbances in tropical easterlies and may also occur with broad
tropical air masses flowing to the south. Given sufficient atmospheric moisture
and instability, it may cause large cloud clusters and rain.
In Australia, the most destructive winds over broad areas are generated by tropical cyclones. (Tornadoes, associated with some severe thunderstorms, have the potential to generate higher wind speeds, but areas affected are much smaller than these tropical storms.)
Tropical cyclones are low pressure systems in the tropics which, in the southern hemisphere, have well defined clockwise circulations with mean surface winds (averaged over ten minutes) exceeding gale force (63 kilometres per hour) surrounding the centre. Tropical cyclones exhibit a relatively clear eye, surrounded by dense wall clouds and a series of spiral rain-bands. The Bureau tracks cyclones with weather watch radar, special service reports and frequent satellite images. Figure 1 and Figure 2 show a tropical cyclone approaching, and crossing the Queensland coast near Rockhampton.
In both figures the wind speed and direction are graphically represented by "arrow tails" where a large feather represents 10 Knots and a half feather is 5 Knots. A solid triangle tail is 50 Knots. The arrow shafts show the wind direction at that place. Thus in Fig 2 the wind at Brisbane is almost easterly at 40 Knots.
|
BEAUFORT SCALE
|
Km / Hour
|
KNOTS
|
SAILOR'S DESCRIPTION
|
|
Force 0
|
Below 1
|
Below 1
|
Calm
|
|
Force 1
|
1 - 5
|
1 - 3
|
Light Air
|
|
Force 2
|
6 - 11
|
4 - 6
|
Light Breeze
|
|
Force 3
|
12 - 19
|
7 - 10
|
Gentle Breeze
|
|
Force 4
|
20 - 28
|
11 - 16
|
Moderate Breeze
|
|
Force 5
|
29 - 38
|
17 -21
|
Fresh Breeze
|
|
Force 6
|
39 - 49
|
22 - 27
|
Strong Breeze
|
|
Force 7
|
50 - 61
|
28 - 33
|
Moderate Gale
|
|
Force 8 *
|
62 - 74
|
34 -40
|
Fresh Gale
|
|
Force 9
|
75 - 88
|
41 -47
|
Strong Gale
|
|
Force10 #
|
89 -102
|
48 -55
|
Whole Gale
|
|
Force 11
|
103 - 117
|
56 - 65
|
Storm
|
|
Force 12
|
Above 117
|
Above 65
|
Hurricane
|
Warnings are issues by the Australian Weather Bureau whenever mean winds of 25K or more are expected.
* Gale warnings are issued when the mean wind is expected to reach 34K.
# Storm warnings are issued when the mean wind is expected to reach 48K.
<1K
1-3K
4-6K
7-10K
11-16K
17-21K
22-27K
