Understanding the weather
- Basic considerations
- Pressure systems
- Air masses
- Seasonal variations
- Mountain influence
- Sources of information
- Improving your weather knowledge
An ability to understand, anticipate and interpret weather changes is desirable for any outdoor person. But for leaders, this ability is essential. Think about the proportion of equipment in your pack that is carried to cope with the weather! It is as important as good route planning and a good knowledge of equipment. Your weather knowledge must be practical, so that it will guide you in what to do. The weather in southeast Australia can bring snow to the mountain tops in summer and it can mean the difference between an enjoyable walk through Tasmanian mountains and a trip in which hypothermia leads to loss of life. Desert storms can flood usually dry rivers, and dust storms can turn day into near night. The wet season in tropical northern Australia can bring astounding amounts of rain. To know your weather, understanding a few basic principles together with habitual observation of everyday weather, is all that is required.
From the bushwalker or skitourer’s point of view, the basic elements of weather are:
- precipitation (rain, snow, hail etc)
An understanding of some of the basics and the broad reasons for the vagaries of Australian weather will assist your weather prediction skills in the field.
Weather occurs as a result of movement of the atmosphere. The majority of weather phenomena are caused by the vertical movements of parcels of air:
- rising air expands, cools, and sheds moisture as cloud droplets, rain, hail or snow
- sinking air is compressed, warms and can carry more moisture – clouds dissipate as water droplets evaporate into invisible water vapour in the warming air.
Winds occur at ground level because of differences in the weight of air above different locations. The weight of air at any location on the ground’s surface is called the air pressure. If the pressure at one location is greater than the pressure at an adjoining location, the surface air will move from the high pressure to the low pressure. This is surface wind. The various atmospheric movements attempt to equalise the weight of atmosphere above points at sea level. Consequently maps of these weights or pressures, at sea level, can show much of the broad-scale horizontal and vertical motion of the atmosphere. The lines drawn on weather maps connect points of equal pressure and are called isobars. Winds do not flow directly from high to low pressures. The rotation of the earth causes the winds to flow almost parallel to the isobars but:
- outwards from high pressures
- inwards towards low pressures.
Regions on a weather map where isobars are closer together mean that pressure differences between adjoining locations are higher, and so the winds will be stronger.
The daily weather maps show regions of high and low pressure from which general predictions can be made. At ground level, air flows outwards from a high pressure system. So the column of air in the high-pressure system is generally sinking towards ground level. As air sinks, it is compressed (by the weight of air above) and warmed. Clouds evaporate and the weather is generally sunny.
On the other hand, at ground level air flows into areas of low pressure. So the column of air in a low-pressure system rises and expands. The air cools and any water content tends to condense into clouds. Clouds may form and, depending on the amount of water vapour in the air, rain and other forms of precipitation may occur.
The weather patterns over Australia show interspersed areas of high and low pressure, as shown in Figure 18.1. These generally move from west to east.
Wind direction, shown by arrows, is predominantly parallel to the isobars. It spirals outwards from highs (where air is sinking and warming) and inwards to lows (where air is rising and cooling). Wind travels clockwise around lows and anticlockwise around highs. Thus, if you face into the wind, the low is on your left. Usually the isobars are closer around lows, indicating stronger winds. Knowing that Australian weather systems generally move from west to east, you can infer from a strong northerly wind that a cold front may be on the way, and conversely that a cold southerly wind heralds improving weather. However, beware of the occasional low pressure system moving down the east coast of Australia. This can bring worsening weather with southerly and southeasterly winds, particularly in the northeastern regions of Victoria and southern New South Wales.
In the equatorial regions, particularly in summer, the ocean waters warm significantly. In areas of low pressure, moisture-laden air rises. At the condensation layer, cloud forms and the heat released by the condensing vapour causes upward moving air currents. More heat is released as more vapour condenses, large thunderstorms are created and the monsoon rains are experienced. This leads to periods of very high humidity and temperature, known as the ‘build up’, typically in November–Decem-ber. This is then followed by several weeks of very heavy, monsoon rains.
The incredible energy stored in weather events in northern Australia can lead to cyclones—very deep lows with destructive winds reaching 200 km/h at times. Cyclones form over the sea, and generally lose energy as they pass over land, making coastal areas and offshore islands at most risk. Cyclone weather alerts are issued at times of high, direct threat from a particular storm. These must never be ignored by outdoor enthusiasts—the energy of cyclones can devastate whole communities. If walking in cyclone prone areas during the cyclone season (typically November–March) always carry a transistor radio to monitor weather forecasts.
Within southern Australia, the patterns of highs and lows are established to the south and west of the continent. They flow over Australia from west to east. Within central and northern Australia, ‘heat’ lows are produced by contact between land that is heated to summer temperatures of above 40°C. Air in contact with the land warms and rises. Often there is no moisture associated with these lows.
Knowing the sea-level wind movement along the weather map isobars, you can determine the likely route taken by the air which will be reaching you. The air temperature you experience, and the moisture content of this air, has been established over many days and is reasonably constant over hundreds of kilometres, so will provide useful information on the likely temperature and potential for rainfall. Figure 18.2 shows the main types of air mass that affect Australia:
- Generally prevailing westerlies: moist air over western coastal areas which condense where raised by topography.
- Cyclonic tropical: very strong, moist winds producing heavy rain over affected northern coastal areas
- Tropical continental: very hot, dry air which produces heatwave conditions when it moves south (hot northerlies in summer). The air has been heated by contact with the dry, hot land in central Australia.
- Southern maritime: a reasonably moist air stream with temperatures determined by the southern oceans over which it has passed.
- Polar maritime: very cold, partly moist air, originating over the Antarctic Ocean. It produces sudden cold snaps and heavy snow falls.
The temperature of an air mass is predominantly affected by conduction and convection, and a little by radiation. All ski tourers will have experienced the biting southerly wind under a cloudless sky. Convective temperature changes are evident in the large, fluffy ‘cottonwool’ cumulus clouds, and the thunderstorm (cumulonimbus) clouds.
At the surface, air exchanges heat with the ground. During a cloudless day, the ground absorbs radiation from the sun and heats up the surface layer of air via conduction. A parcel of air, heated by contact with the ground, rises (or convects upwards) and expands. The parcel cools. Water vapour may form a cloud at a ‘condensation level’, the height of which depends upon the amount of water vapour in the air. The resulting flat-bottomed cumulus clouds may develop into thunderstorms as the heat released by the condensation drives the air parcel higher. The tops of thunderstorm clouds may reach the tropopause at 9000–12 000 m.
Conductive cooling of an air mass occurs on clear nights. The earth cools by radiating energy into space. The air in contact with the ground cools. If there is much wind, this cooling effect is distributed through many hundreds of metres of the atmosphere, and little change in air temperature is noticed. However, on still, clear nights, particularly during the passage of the centre of a high pressure system, only the bottom few metres of the atmosphere will be cooled. Depending upon the moisture content of the air, fog may occur and, depending on the initial ground temperature, cooling may proceed to freezing point producing frost. In hilly country, the cooled air will drain into valley floors producing frost hollows. In such conditions, a warmer night will generally be experienced up on the valley sides.
A layer of cloud acts as a blanket, slowing the radiative cooling and generally resulting in warmer nights. Similarly, a tent pitched under a crown of trees will escape the frost that covers a tent pitched in the open.
Fronts represent the interface between different air masses and are generally accompanied by substantial changes in weather. The most common front in southern Australia is the cold front when a colder air mass is displacing a warmer one (Figure 18.3). Cold fronts, which are usually linked to low-pressure systems, occur in two forms:
- Type A: a deep, rapidly moving, cold wedge of air, lifting unstable warm air. As the front approaches, warm humid northwestery winds freshen, carrying scattered low cloud. High clouds increase and become darker and lower. The
- ‘anvil’ head often present with thunderstorm clouds may be visible. Light rain quickly turns to heavy showers and thunderstorms with associated gusty winds. The winds turn cooler and west to southerly as the front passes.
- Type B: a shallow, slow moving wedge of cold air lifts moist, stable warm air. Because of the slow movement and the shallowness of the wedge, the vertical motion of the warm air is relatively slow. High clouds extend well ahead of this type of front. The frontal cloud extends back behind the front for many kilometres and rain falls over a wide area.
The whole process of the Type B front passing a particular locality is slower than that of Type A. The high cirrus clouds may precede a front by 12 to 36 hours.
There is a general seasonal variation in the paths traversed by the pressure systems and in their relative intensities (Figure 18.4). Hot summer conditions are indicated by high pressure systems moving south of the continent, whereas winter conditions are typified by high pressure systems moving across continental Australia.
A party venturing into mountainous areas has to be prepared for weather conditions vastly different to those in low-lying country. The following aspects of weather are affected by mountains.
Air pressure at the tops of mountains is less than at the bottom. Air flowing over them will therefore be forced into an area of lower pressure and will expand and cool. In cooling, it sheds moisture and can produce cloud, rain and snow. Have you caught glimpses of sun filled valleys through mist swirling around a summit? On the lee side of the mountain, the descending air increases in temperature and clear periods are more frequent, as shown in Figure 18.5. If precipitation occurs, the clouds will dissipate at a higher level on the lee side than they formed on the windward side. If you have a choice of routes to leave the high country in bad weather, head for the lee side, with the wind on your back.
Temperature and sunshine
If the air is well mixed (as in windy conditions), the air temperature decreases with increasing altitude at a rate of 0.6–1.0°C per 100 m for humid air, and about 1.0°C per 100 m for dry air. For example, if the temperature at sea level is 10°C, at 1500 m it will be below freezing point—so take warm clothing for a summer weekend on mountains above 1800 m or so.
As atmospheric dust and moisture content decreases with increasing altitude, on a clear day the direct sunlight at 1200 m contains twice the amount of ultraviolet light as at sea level. So in high country, without clouds, ‘slip on a shirt, slap on a hat and slop on sunscreen’.
Wind is the movement of air to equalise atmospheric pressure. Because of friction at sea level, winds are gusty but have reduced velocity. At high altitudes, they are stronger and steadier. Knowledge of wind speed is important, and vitally so in winter conditions, where even a light breeze can reduce the environmental temperature (the temperature as felt by the human body) to an uncomfortable level. The wind speed on open land above 1000 m is about two to two and a half times stronger than on more sheltered low-lying land. Environmental temperature is also affected by humidity: the raw conditions of winter and the mugginess or oppressiveness on a sultry summer’s day are well-known effects.
To cool down, you need to lose heat to the environment, mainly through conduction to the air layer surrounding your body. If this does not provide sufficient cooling, your body perspires, and the evaporation helps the cooling process. In both cases, the rate of evaporation, and hence effectiveness of cooling, is influenced by wind speed and humidity in the air. Wind chill is a measure of the effectiveness of wind speed as a coolant. Air with an actual temperature of 4°C and a wind speed of 24 km/h, has a wind chill temperature of –5°C; and if moving at 48 km/h, of –11°C. Wind chill is discussed further in Chapter 21.
The efficiency of cooling depends on the amount of surface skin area exposed to the air flow. In extreme conditions, leaders should monitor:
- thin, tall walkers who, while having very efficient cooling in summer, may be prone to hypothermia in winter (particularly children)
- shorter, squat walkers who may have trouble shedding heat in summer, but who are efficient storers of body heat in winter.
Large mountains may strongly affect rainfall patterns. Moisture-laden air approaching a mountain range is forced to rise. It expands as it rises (because of the decreased weight of air above) and therefore cools. This cooling can force cloud formation and precipitation on the windward side and tops of the range. As the air descends the lee slope, it is compressed and warms. It can contain more moisture and so precipitation can cease and clouds disappear, as shown in Figure 18.5. A rain shadow exists on the lee side of the mountains and can give rise to a different vegetation regime.
In southeast Australia, rain-bearing winds normally come out of an arc from west to south. The Great Dividing Range provides a barrier for many of the less intense, low-pressure systems, with rain precipitating on the southern and western faces. The southeast coast of New South Wales and Victoria often experiences relatively warm weather during cold and wet westerly or northwesterly winds elsewhere. These winds rise up the windward side of the mountains, cooling and shedding moisture. The relatively dry winds warm as they descend the lee side and move over the coastal regions. The coastal hills of southwestern areas of Western Australia are wetter than areas inland from the highest points of the ranges, and the Adelaide Hills in South Australia cause a rain shadow on the plains to their east towards the Murray River.
The air’s humidity governs the formation of clouds, fogs, dew and so forth. It also greatly affects the comfort of bushwalkers. Humidity measurements provided by the Bureau of Meteorology, state the amount of water vapour in the air as a percentage of the maximum amount that the air can absorb. Remember that water vapour is invisible, and only becomes visible as cloud, fog or precipitation when it condenses. The maximum amount of water vapour that a parcel of air can contain depends on its temperature and pressure (or height). If a parcel of air at 100% humidity cools, some of its water vapour will condense and become visible as cloud or fog.
Low humidity air has the capacity to absorb water from objects it moves over. High humidity air is unable to absorb extra moisture. When walking, our bodies generate heat which must be dissipated by perspiration. We cool as it evaporates. However, in periods of high humidity, the evaporation process is very inefficient and cooling is difficult. The air cannot absorb much additional moisture. Our bodies can overheat and become dehydrated as excess fluids are perspired. These are risks in tropical northerly regions, particularly when high temperature is often combined with high humidity and little wind.
At the other extreme, ski tourers in winter are faced often with very low humidity air as winds from the south move north, warm and increase their ability to absorb moisture. Low humidity air removes much more water from lungs than does humid air, increasing the need for frequent drinks during heavy exercise.
The difference in humidity between the outside air and air within a Gore-Tex jacket affects the ability of Gore-Tex fabric to pass water vapour to the outside. Water vapour passes from areas of high humidity to low humidity. Gore-Tex works exceptionally well in low humidity, particularly on skiing trips. In high humidity, when perspiring freely, you can feel as wet inside your Gore-Tex jacket as it is outside it.
Leaders need to be aware that human skin will be dried out during trips in low humidity in the snow. The cold weather can restrict blood flow, leading to frost bite and chilblains. Conversely, asthma attacks and hayfever can increase during high pressures as allergens such as pollen are carried aloft in the rising air currents.
Sources of information
Seek the best available information before departing on a trip. The weather information in newspapers can be relatively old, and papers vary significantly in the coverage provided. The best are quite good, with four-day forecasts and predicted weather maps. Most radio weather broadcasts with news segments are cursory and usually relate to main population areas only.
The Australian Broadcasting Corporation (ABC) regularly has a meteorologist giving detailed comments on current and impending weather, usually during a program for primary producers (check with the ABC for times and stations). Some of the TV channels are providing good weather coverage with their evening news presentations. They all review a current weather map, and some give outlooks for up to four days ahead. With all of the above, you have to interpret likely mountaintop weather from the information given.
The Bureau of Meteorology provides a valuable source of information on an internet site at www.bom.gov.au which gives current analysis, forecasts on surface level maps and regional forecasts as well as applicable warnings.
By far the best source of information is obtained by phone from the Bureau of Meteorology’s Regional Forecast Centre in each state. When calling, you can request a forecast for the specific area you are interested in, and information can also be faxed to you.
Improving your weather knowledge
To develop weather knowledge, follow this procedure:
- Listen frequently (at least once a day) to the weather forecast.
- Listen for key words that might herald bad mountain weather: e.g. front, trough, depression, low pressure, falling pressure, increasing cloud/wind.
- Conversely, the following words frequently suggest good mountain weather: calm or light winds, high pressure, anticyclone, rising pressure, clear skies, fog and/or frost warnings.
- Read the daily weather forecast and then deliberately observe the weather. Become familiar with the terms commonly used to describe the weather. At the same time, try to identify clouds by reference to pictures.
- Having practised the above for some weeks, try looking at the weather maps and make a forecast before reading the text. Check with the given forecast.
- Next, try to make forecasts by observing the weather first. Check your opinions against maps and text.
- Finally, do not look at the paper, just go out and make a personal forecast from what you can see and what you know has happened recently. Write it down, and then see whether you are accurate.
The key to all forecasting is to look into the approaching weather. This applies equally when reading the weather maps at home and anticipating what large changes may occur, or when in the outdoors looking into the wind and in the direction of the approaching weather to anticipate the next shower before it arrives.
Although maps and regional forecasts give knowledge of the weather over a wide area, they nearly always refer to low-lying ground. The ability to interpret the local mountain situation in the light of this information is a practical skill which all leaders should cultivate.
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Crowder R. 1995. The wonders of the weather. In: Manual of Meteorology, Part 1 – General Meteorology. Bureau of Meteorology, AGPS, Canberra.
Langmuir E. 1984. Mountaincraft and Leadership. Official Handbook of the Mountain Leader Training Boards of Great Britain and Northern Ireland. Edinburgh, UK.
Tapper N. & Hurry L. 1988. Australia’s Weather Patterns – an Introductory Guide.