The South African Weather Service defines a heat wave as “when for three days the maximum temperature is five degrees higher than the mean maximum for the hottest month”.
Cape Town experiences a Mediterranean type climate, which is characterized by the cool, wet winters and hot, dry summers. One of the main characteristics of similar regions is that they all situated near large bodies of water. This gives the areas moderate temperatures with a comparatively small temperature range between the summer maximum and winter minimum. The adjacent water bodies are also responsible for the cooling effect resulting in the coastal areas, which in many cases does not allow temperatures to reach their maximum highs.
During the summer months, coastal lows (locally known as berg winds), sometimes move over the area, causing the temperature to increase to sometimes uncomfortable levels. The hot air blows into Cape Town from the Karoo interior mainly in February and early March.
What is the Heat Index?
The heat index is a measure of how hot it really feels when the effects of humidity are added to high temperature. To alert the public to the dangers of exposure to extended periods of heat and the added effects of humidity a Heat Index table is used to correlate measured temperature and humidity into a apparent temperature. The SA Weather Service offers gradiated heat maps showing this temperature.
The Heat Index is a calculated value based on air temperature and humidity. To calculate a specific value for a previous date, you will need to know the air temperature and humidity. To use the Heat Index Table below, find the temperature on the left of the chart. Read across until you reach the desired relative humidity. The number which appears at the intersection of the temperature and relative humidity is the Heat Index.
Note that the Heat Index under direct sunlight will be 8 °C higher than the number shown in the chart.
What is the discomfort index?
This index evaluates the impact of heat stress on the individual taking into account the combined effect of temperature and humidity. The formula used by the SA Weather Service to calculate the discomfort index is:
Discomfort Index = (2 x T) + (RH/100 x T) + 24
T is the dry-bulb or air temperature in degrees Celsius and RH is the percentage relative humidity.
This index gives the following degrees of discomfort:
- 80-90 - moderately uncomfortable
- 90-100 - very uncomfortable
- 100-110 - extremely uncomfortable
- 110 and more - hazardous to health
During a heat wave, the public are urged to:
- Drink plenty of water. Water is the best liquid for hydration during a heat wave.
- Never leave children unattended, especially outside.
- Never leave children or pets unattended in motor vehicles.
- Take care of pets and other animals – they should not be directly exposed to the sun
- Be aware of the dangers of heat exhaustion.
- Stay indoors in the coolest room of your home as much as possible, and splash your face with cold water to cool down.
- Keep rooms cool by using shade cloth or reflective material on the outside of the window.
- If it is safe to do so, open windows at night when the air is cooler.
- Cyclists are advised not to cycle as they can sustain heat-stroke and cardiac problems.
- Listen to alerts on the radio and television for up-to-date weather reports.
- Avoid hiking and walks, especially from 10:00 to 16:00. If you have to hike during this time, take along plenty of liquids, use an adequate sun-protection, wear suitable clothing and notify somebody of your intended travel plans.
Tourists visiting Cape Town who are not acclimatised to the heat are asked to take extra precautions to avoid swelling of the ankles, inflammation of the skin, sharp pains due to loss of water, dizziness, nausea, diarrhoea and fainting.
When heat wave conditions occur authorities, places of safety and care-givers should be on high alert, especially regarding the wellbeing of children and the elderly.
The Disaster Risk Management Centre offers the following advice during heat wave conditions:
- Monitor those at high risk. Infants and children up to four years of age are sensitive to the effects of high temperatures. They rely on others to regulate their environments and provide adequate liquids.
- People who are 65 years of age and older may not compensate for heat stress efficiently, and are less likely to sense and respond to change in temperature. People who are overweight may be prone to heat sickness because of their tendency to retain more body heat.
- Those who are physically ill, especially with heart disease or high blood pressure, or who take certain medications for conditions such as depression, insomnia or poor circulation, may be affected by extreme heat.
- Visit adults at risk at least twice a day and closely watch them for signs of heat exhaustion or heat stroke. Infants and young children need much more frequent watching.
- Air conditioning saves lives during periods of extreme heat. If your residence does not have air conditioning, seek some relief in public spaces such as shopping malls and libraries.
- Use a fan. Don't place the fan directly in front of a window because it may pull hot air in. Try placing the fan so that it blows in the room and out the window instead.
- Drink plenty of water and other fluids, but avoid alcohol. Drinks containing alcohol can actually worsen dehydration. Your intake of fluids should be increased even if you are at rest.
- During the hottest hours of the day, stay inside. If possible stay inside an air-conditioned building. The hottest hours of the day are typically from mid-morning to mid-afternoon.
- Dress lightly and when sleeping use lightweight breathable covers.
- Move your exercise routine to early morning or later in the evening - even swimming. Swimming pool dangers related to heat exposure do occur.
- Never ever leave anyone in a vehicle while you run to do a quick errand – no child or pet should be left in a car. It's never safe. The inside of a vehicle can overheat quickly and become an oven. People can succumb to heat exposure and death very quickly in a closed vehicle.
For updated weather forecasts visit SA Weather Service or call the Weatherline at 082 162.
Symptoms of heat stroke can sometimes mimic those of heart attack or other conditions. Sometimes a person experiences symptoms of heat exhaustion before progressing to a heat stroke.
Signs and symptoms of heat exhaustion:
- Muscle cramps and aches
Some individuals can develop symptoms of heat stroke suddenly and rapidly without warning. Different people may have different symptoms and signs of heat stroke. Heat stroke is a form of hyperthermia.
Signs and symptoms of heat stroke:
- High body temperature
- The absence of sweating, with hot red or flushed dry skin
- Rapid pulse
- Difficulty breathing
- Strange behaviour
A patient must be referred to a medical doctor if any of these signs are present.
By drafting your own Family Emergency Plan and identifying all your risks, including those hazards affecting your own environment, you can make a positive contribution to preparedness.
City appeal to avoid causing runaway fires
The City also appeals to the public to refrain from throwing cigarette butts out of motor vehicle windows. This is often the cause of runaway fires, especially in hot and windy conditions.
Further to this, any fires for cooking should not be made unless they can be controlled. No fires are allowed on the mountain fringe or urban edge areas except in specifically demarcated and pre-authorised areas. If you are unsure whether fires are allowed in an area, do not make a fire there.