Interpreting weather changes in the field
- What does the wind tell me?
- What do clouds tell me?
- Weather and altitude
- Making practical use of weather knowledge
Often the current weather and, more importantly, the weather expected during the next few hours, impacts on decision making. A competent leader must be able to anticipate risk elements that could jeopardise the safety of party members. An ability to interpret weather changes in the field aids this decision making. To camp on the side of the ridge, or to push on to the next recognised camping spot? To seek shelter from the noon sun or to urge the water-depleted party onwards? To continue walking in the light mist or to put on full wet-weather gear?
What does the wind tell me?
The isobars on a weather map provide general wind directions. However, local conditions can produce winds quite different from those expected. In open areas where isobaric wind flow is expected, you can often work out the location of a low pressure by facing the wind. In Australia, the low will be on your left. If you face north into the wind, the low is to your left and hence to the west, so worsening weather should be planned for. If the wind is from the south, the low pressure is to your left and moving eastward, and the weather is likely to be improving. Winds getting stronger, particularly from the north, indicate that the low is approaching. Isobars are normally closer together around low-pressure systems. Understand how the wind changes direction as a front moves through. A northerly wind will shift back to a westerly then to a southwesterly.
In the outdoors there are many practical situations where an accurate prediction of weather can make conditions more comfortable and safe. Some examples include:
- Having observed the imminent arrival of a front, in what direction do you face the door of your tent?
- Where do you place your tent with respect to a campfire?
- Your party camped in unpleasant conditions the night before. A thunderstorm forced meals to be cooked in tents. This morning, there is a light scattering of snow and the previous day’s northerly wind has been replaced by a cold southerly. You tell your party to rug up and smile, because the weather is getting better. The low is now to the east and a high pressure system is approaching.
- With a cloudless sky on a windless late afternoon, a snow-covered clearing is considered for a campsite. Some of your party members do not have very warm sleeping bags. There is a high probability of very low temperatures during the night, as the earth cools down due to radiative cooling. You might advise your party to seek additional shelter by camping under tree cover. Water bottles should be kept in the tent to prevent freezing.
- Another hot, windless day and your party has a relatively short walk along the beach at a coastal park. Is it better to walk in the morning or midday or afternoon? You might decide to wait for a sea breeze. During summer days, the land heats up more than the water. By midday there may be a strong rising current of air above the land, which is replaced by a cool breeze blowing in from the sea—a pleasant walking environment.
- You are to meet your party early on the first day of a walk at a carpark at the top of a river system. The weather is fine with the high pressure centred overhead. The forecast is for a calm cloudless night. Do you drive up the river valleys to the meeting point the evening before or early in the morning? A still cloudless night leads to frosts and fogs, particularly in river valleys. You might choose an evening trip.
- Noting the morning fog, you know that the night has been clear and windless. The high pressure has not moved on and the day is likely to be fine and sunny.
What do clouds tell me?
Clouds are the most visible indicators of weather and impending changes. Take a small weather reference text with you on trips and learn to identify different cloud types and become familiar with their movement. There are three main cloud levels: high, mid and low. In southern Australia, high-level clouds, such as cirrus or altocumulus, moving in from the west may indicate an approaching frontal system that could arrive in the next 12 to 36 hours. In tropical areas, such clouds may mean a quicker change for the worse.
A high flying plane leaving long-lasting contrails in the air indicates relatively high levels of moisture, also a forewarning of potential bad weather coming from the west.
Fluffy cotton-ball cumulus clouds may appear during sunny afternoons, as the warming earth heats the air which rises. Depending on moisture content, these fair weather clouds will appear. If the air is humid, it becomes unstable. The heat released by condensation drives the air to greater heights, producing more condensation. This feedback process continues and large cumulonimbus thunderheads develop, extending from low to very high levels. Watching this process can be exhilarating. The wise leader, noting this process, will try to get his or her party away from exposed locations. Chaotic winds within thunder clouds generate static electricity that discharges as lightning. In the tropical north, this procedure can be repeated day after day in hot, moist conditions.
You can roughly estimate how far away the storms are, by counting the seconds between a lightning flash and the following thunder. Noise from thunder travels about 1 km in about 3 seconds. Use these distance estimates to plan your party’s next movements. Move off the tops of ridge lines and exposed slopes. Be prepared for sudden rain squalls and potential hailstorms. A green tinge to the base of a cloud or a whitening of rain may indicate approaching hail or snow.
Another indicator is the colour of the sky: ‘A red sky at night is a shepherd’s delight; a red sky in the morning is a shepherd’s warning’. If it has been a cloudy, miserable day and just as the tents are being erected, the rain stops and the sky turns red, this indicates a break in the cloud to the west, and higher air pressure, probably associated with improving weather. If, however, a morning sky is deep red, it may mean rain or a cloud-filled sky for most of the day.
While pitching a tent next to a dry creek bed or a small meandering stream and watching the thunderheads and lightning play about the distant hills, you should consider the impact the of mountains on precipitation, and may reassess the location of your camp as thoughts of flash floods occur.
Coronas or haloes about the moon indicate the presence of mid- to high-level clouds and are often a sign of advancing rain or storms. The larger the halo, the higher the cloud, and so the further off the worsening weather.
Care must be taken when venturing into the forests of southern Australia in summer. Forest fires can cause much damage. The alert leader will look out for pyro-cumulus clouds. A bush fire will generate heat, causing air to rise in a column of smoke. With sufficient heat, the smoke column will rise to the condensation layer leading to the formation of dense white cumulus cloud. After a thunderstorm, a party in the bush should be wary of fires ignited by lightning.
Weather and altitude
The experienced leader prepares the party for altitude-induced changes in weather. In normal conditions, the ambient air temperature will decrease at about 1°C for every 100 m climbed. Warmer clothing will be required at the top.
As well, the thickness of air above decreases, and the protection it offers from sunburn decreases. Additional sunscreen may be required. The relative lack of oxygen could cause problems for the less fit members of the party, although this is unusual at Australian altitudes.
In alpine areas, the wise leader may ask party members to put on additional clothes prior to leaving the protection of snow gums for the open alpine slopes, particularly in cold, windy conditions. The additional windchill in the open can be substantial, particularly if clothes are damp from mist or perspiration.
Making practical use of weather knowledge
The ability to make better decisions from correct interpretation and prediction of weather conditions can make a huge difference to the comfort and safety of outdoor groups. The leader needs to practise observation skills for clues about what the weather is likely to do, and then consider the implications for the trip planned. Many trips can be modified to improve everyone’s comfort through relatively simple decisions.
Severe weather advice
In 2009, following that year's devastating Black Saturday bushfires, the Victorian Outdoors Sector, with the support of the Victorian State Government, developed guidelines for the management of outdoor activities which may be affected by severe weather. The resulting Guidance Note is provided as part of Victoria's Adventure Activity Standards and can be viewed here.