INTERPRET WEATHER CONDITIONS IN THE FIELD

DESCRIPTION

This unit covers the knowledge and skills for interpreting weather signs to predict future weather conditions and their impact on outdoor recreation activities

OBJECTIVES

On completion of this unit, you should: • have an understanding of topology and other local factors that will affect the weather • have an ability to understand current weather and predict likely future conditions • be able, based on likely future conditions, to decide whether to continue, modify, or abandon paddling activities

BACKGROUND.

All weather is driven by temperature differences across the surface of the Earth: on a local scale sea breezes and the like, on a planetary scale the global circulation.

SEA BREEZES

Land generally heats (and cools) faster than the ocean. On a summer day air above the land will become warmer than land over the sea. It therefore expands, and being less dense, will be buoyed upwards by cooler air from the sea: that cooler air flowing in is the sea breeze. Aloft, there will be movement in the opposite direction. (Note that warm air does not rise of its own accord: the movement is brought about by pressure changes caused by the differences in density between warm and cool air.)

Sea breezes can reach considerable distances inland, and can be quite strong: 20 to 25 kn is not unusual late in the day. The direction will be directly on-shore as the breeze starts, but will change during the day as the Coriolis effect deflects the airflow.

Strength and direction will also be influenced by the winds of the general circulation: if the two are in the same general direction the result can be quite strong. At night, the situation is reversed, and breezes blow from land to sea, although not as strongly as during the day.

TOPOGRAPHIC WINDS

Valleys tend to become warmer than surrounding hills, and breezes tend to flow up the slopes and up the valleys during the day. Strength and direction are influenced by the general circulation. At night, the flow is reversed, down the valley.

Hills and valleys also tend to channel the general circulation, wind tending to blow along valleys rather than across. Sea paddlers should be careful of winds channelled down valleys opening on to cliff coastlines, which might otherwise give some shelter.

DOWNSLOPE WINDS

Downslope winds are caused by the general circulation and occur on the lee side of mountains and hills, especially where the upwind slope is gradual and the downwind steeper. The Adelaide gully wind and Perth scarp wind are the best known in Australia. They can be quite gusty

EDDIES

As with water flow in a stream, eddies can form in flowing air behind obstructions large and small. Expect gusts and turbulence downwind of hills and islands in strong winds. Wind flowing down hills and over cliffs can generate strong downdrafts, sufficiently strong to blow paddlers over.

SOUTHERLY BUSTER

The Southerly Buster (or Burster) occurs along the south-east coast of the continent, and is marked by the sudden onset of strong southerly winds, often gale force, along the coast. The temperature can fall 10 – 15˚C in less than half an hour. The Buster occurs when a cold front is slowed across the south-eastern ranges but surges along the coast. Southerly Busters are most frequent in November and December, and forecasts of the phenomenon are becoming increasingly accurate.

EAST COAST

LOWS East Coast Lows occur in autumn or winter over New South Wales and southern Queensland. They usually begin at night near the coast where there is a large temperature difference between ocean and land, and rapidly develop strong winds and produce heavy rainfall.

LOCAL INFLUENCES

Places along the coast tend to have more moderate temperatures and temperature range than those farther inland. They will also have sea breezes in summer. If the prevailing wind is onshore, they will have a higher rainfall than inland.

Inland, temperature ranges can be extreme: it may be 40˚C in the day, but freezing at night. Mountains and hills have their effects, with higher places being cooler. Slopes facing the prevailing maritime winds will receive higher rainfall than leeward slopes: the ‘rain shadow’ effect. In winter in alpine areas and Tasmania cold fronts will often bring snow as well as other precipitation.

WIND OVER OPEN WATER

Wind over water will generate waves, the ‘sea’. The height of that sea will depend on the strength of the wind, the distance across water it blows, the ‘fetch’, and the time. The water can look deceptively smooth from the upwind shore of a lake or the sea, but the whitecaps will not be far out. Tide and sailing direction books may have tables for the likely sea states in your area.

SEASONAL NORMS

SUMMER

Highs tend to be centred south of the continent, and may be stationary in the Tasman for several days, blocking the movement of other systems. Cold, fronts, for instance, will be slowed, and may miss the mainland. Conditions will be hot and dry over Victoria and South Australia due to the overland trajectory of the air, but the east coast may be wet in the on-shore stream. Sea breezes will be common along the coasts. Thunderstorms will be common in the north, with tropical cyclones between November and April.

AUTUMN

Highs are moving north, and winds generally become lighter.

WINTER

Highs now track cross the continent, and fronts reach well inland, bringing wind and rain and showers (and snow on the highlands). Southern parts of the continent receive their greatest rainfall. The generally offshore winds in the north mean that weather is generally dry

SPRING

Highs are moving south, but fronts and lows still bring wind and precipitation.

HIGH

In the southern hemisphere the circulation is anticlockwise. Barometric pressure is high. Air in the centre is descending, but is relatively calm. There may be stratus or cumulus cloud. With no cloud or traces of cirrus, expect fine, warm weather.

LOW

Air is circulating clockwise, and air in the centre is rising. Barometric pressure is low. Cloud and showers are likely. The tropical cyclones of northern Australia are smaller, but violent versions, without fronts.

ISOBARS

Isobars join places of equal barometric pressure. As with contour lines on a topographic map, the closer the lines, the steeper the gradient. In this case the steep gradient indicates strong winds.

Winds are shown on the charts with arrows, with the number of tails indicating wind strength. Precipitation is shown by shading.

COLD FRONTS

Cold fronts are associated with low pressure systems. Ahead of the front, the wind will be northerly, conditions will be warm (or even hot, because of the overland trajectory of the air), and the barometric pressure will be falling. The cloudbase will be lowering, and there may be rain. If the front is a vigorous one, winds can be strong. Behind the front, the wind will be southerly (strong with a vigorous front), the air will be colder, there may be showers, and the barometric pressure will be rising. With some fronts there may be thunderstorms.

TYPICAL SEQUENCE OF CLOUD AND WEATHER WITH COLD FRONTS

(Afer Colls and Whitaker)

AIR MASSES

A body of air with similar characteristics of temperature and humidity is referred to as an air mass. An air mass that has been over the continent for some time will tend to be warm and dry: a continental air mass. An air mass from over the ocean, a maritime air mass, will be moist, especially if it is from the tropics. Knowing the source of an air mass, you can predict its characteristics, and the likely weather from it.

WIND

Wind is measured, and forecast, at 10m above the surface, averaged over 10 minutes. The highest wind speeds may be as much as 40% greater than the average: something to remember in windy conditions. Wind speeds are given in knots for marine and aeronautical users, kilometres per hour for others.

CLOUDS

Clouds are not just shapes in the sky: they are indicators of what is happening in the atmosphere, and a guide to future weather. Clouds form when air is cooled, usually by being lifted. That lifting may be by flowing over coastlines or hills (orographic), by convection, or by the passage of a front. Air can also be cooled by radiation (clear nights, leading to dew or fog) or by flowing over a cold surface, as when warm air flows from the sea over land. When the air is cooled to saturation moisture condenses: the Dew Point is the temperature at which condensation begins. The shape of the cloud is determined by the way in which it is formed, e.g. convection produces cumulus, orographic stratus, and so on.

Ten types of cloud are recognised, and 27 subtypes. That is beyond the scope of this module, which will concentrate on a few main types.

LOW CLOUDS

CUMULUS

Scattered cumulus is usually a sign of fine weather. However if large clouds develop in the afternoon there is the possibility of showers. Cumulus that grows to a great height in warm humid air, becoming cumu lonimbus, means showers, perhaps a thunderstorm. In winter, hail and snow may be the result. In the tropics, thunderstorms are a daily occurrence in the wet season.

MIDDLE LEVEL COUDS

ALTOCUMULUS AND ALTOSTRATUS

With northerly winds, and the cloud thickening, expect a change within 10 to 20 hours. On the other hand, with southerly winds, expect any rain to decrease.

ALTOCUMULUS

Altocumulus on summer mornings may mean thunderstorms later in the day. Keep an eye out for developing cumulus and cumulonimbus

HIGH CLOUDS CIRRUS

These high feathery clouds are an indication of fine weather if winds are southerly over the southern half of the country. If winds are from the north, increasing cirrus means that a front is on the way, within perhaps 20 to 30 hours. Cirrostratus clouds will often produce a halo around the Sun or Moon as the ice crystals refract the light.

RAIN, OR SHOWERS?

Rain falls from stratiform clouds: it can continue for some time. Showers come from cumuliform clouds: they may be heavy, but for a short time only as the cloud passes overhead.

INSTRUMENTS

Although you can see clouds and sense wind strength and direction, and temperature we have no way of sensing barometric pressure. A serious expedition may want to have a small barometer or altimeter in its equipment. A rising barometer indicates fine weather is approaching, a rapidly falling barometer is a sign of bad weather on its way. It is the rise or fall, and the rate, that is significant, not the wording on the dial. If you want to keep records, you could also carry thermometer and some form of anemometer. Several small electronic types are now available. Certainly carry a radio so that you can receive current forecasts, which you will then need to interpret for your locality.

CONCLUSION

The Bureau of Meteorology makes its forecasts using computer models based on the information from observers, automatic stations, satellites and so on. They are generally reliable for about three days. You will have much less information: only what you can see in the sky and measure at one spot with whatever instruments you have. But that limited amount should be sufficient form a mental model to guide your decisions about proceeding with, modifying, or abandoning an activity. It was sufficient for sailing ship captains to start shortening sail when cirrus appeared in the sky.

ACKNOWLEDGEMENT

This resource was written and illustrated by Peter Carter