2.1 Climate zones

The climates prevailing around the globe vary greatly, ranging from the polar extreme to tropical climates. These are primarily influenced by the sun’s energy heating up the land and water masses. At the regional level, the climate is influenced by altitude, topography, patterns of wind and ocean currents, the relation of land to water masses, the geomorphology, and by the vegetation pattern.

Accordingly, the tropical and subtropical regions can be divided into many different climatic zones, but for practical reasons, in this publication three main climate zones are considered:

· the hot-arid zone, including the desert or semi desert climate and the hot-dry maritime climate
· the warm-humid zone, including the equatorial climate and the warm-humid island climate
· the temperate zone, including the monsoon climate and the tropical upland zone

The main climatic factors relevant to construction are those affecting human comfort:

· air temperature, its extremes and the difference between day and night, and between summer and winter temperatures.

· humidity and precipitation

· incoming and outgoing radiation and the influence of the sky condition

· air movements and winds

Fig 2/1 World climatic zones

2.1.1 The hot-arid zone

This zone is situated in two belts at latitudes between approximately 15° and 30° North and South of the equator. Its main characteristics are the very hot summer season and a cooler winter season, and the great temperature difference between day and night.

Temperature in summer

In the hot season the air temperature rises quickly after sun rise up to a mean maximum well above 40°C, with a recorded maximum of 58°C. At nighttime the temperature falls by about 20°C.

Temperature in winter

In the cool season the mean maximum lies at about 30°C and falls at night by about 10 to 20 °C or more, according to altitude. In addition ground frost is possible at night.

Coastal areas

In the maritime region the temperatures are somewhat less extreme but in the hot season the mean maximum temperature also reaches about 40°C and drops at night by 10 to 15°C . In the cool season the mean maximum lies at about 25°C with a similar drop at night.

Humidity and precipitation

The relative humidity is very low in the continental areas and varies between 10% and 55%. In the coastal areas, however, it can reach up to 90% which, together with the high temperature, makes the climate very uncomfortable. Precipitation is scarce, irregular and unreliable.

Radiation

The sky is mostly clear, with some haze in the coastal regions, allowing a very strong solar radiation during the daytime. A considerable release of the heat stored during daytime takes place in the form of radiation toward the cold night sky.

Wind

The winds which vary greatly are usually caused by thermals created by humidity and temperature differences. During the daytime they are often strong and violent with a tendency to evolve to sand or dust storms. In the coastal regions a regular wind pattern exists, blowing landward from the sea during the daytime and seaward at night.

2.1.2 The warm-humid zone

This zone covers an area around the equator extending from about 15° N to 15°S. There is very little seasonal variation throughout the year.

Temperature

The air temperature varies very little throughout the year or between day and night. It reaches a mean daytime maximum between 20°C and 32°C and a nighttime minimum between 21°C and 27°C.

Humidity and precipitation

The relative humidity varies between 55% and 100%, but generally lies around 75%.
Precipitation is high throughout the year and often occurs in the form of torrential rains with heavy winds and storms.

Radiation

The sky is fairly cloudy throughout the year; in coastal regions, however, it is often clear. Accordingly, the solar radiation is to a great extent diffused and partly reflected by the high vapour content. Thus at night the accumulated heat is not readily dissipated.

Wind

The wind velocity is generally low except during rain squalls, when usually one or two dominant wind directions prevail. In coastal regions, however, regular thermic winds provide relief from heat and humidity. Storms are common in this region.

2.1.3 The temperate, monsoon and upland zones

These climatic regions are generally located around the Tropic of Cancer and the Tropic of Capricorn. The climate is neither consistently hot and dry, nor warm and humid. Their characteristics change from season to season, alternating between hot, dry periods and periods of concentrated rainfall and high humidity.

Three seasons

Three main seasons can thus be distinguished:

· the hot and arid pre-monsoon season,
· the warm-humid monsoon period,
· the moderate or even cool winter period.

Temperatures in lowland areas

The lowland monsoon area is characterized by air temperatures which are highest in the pre-monsoon season, i.e. around 35 to 45°C in the daytime and a drop at nighttime of about 10 to 15°C. With the start of the monsoon rains the temperature drops considerably. In winter the lowlands have moderate temperatures.

Temperatures in upland areas

In the upland areas the temperature naturally depends on altitude. In winter night frost is possible. This can also happen in continental areas.

Humidity and precipitation

The relative humidity varies in the dry season between 20% and 55%, and in the wet period between 50% and 100%, depending on precipitation.

Radiation

The sky condition varies with the seasons. In the dry and cool season it is clear with intense direct solar radiation. In the hottest period the sky is rather hazy and radiation is more diffused. During the monsoon period, heavy and low clouds often cover the sky, alternating with periods of clear sky and intense solar radiation.

Wind

Winds are variable and influenced by topographical conditions. During the dry period winds are dusty and hot in lower areas. In mountainous regions, strong and regular valley winds of thermic origin occur in the afternoon.

Fig 2/2 Diagrams showing the typical mean temperature curves for the three zones over 24 hours during the hot and cool seasons.

2.1.4 Microclimate

The above described division into three climatic zones is very generalized, since many areas exist with differing climates or a combination of types. Local conditions, however, may also differ substantially from the prevailing climate of a region, depending on the topography, the altitude and the surroundings which may be either natural or built by humans.

Cold air pool

A phenomenon often observed is that of the cold air pool. Because cold air flows downwards, similar to water, it causes cold air “lakes” in depressions and in the bottom of valleys if there are insufficient outlets. This occurs especially at night, but can also prevail over a longer period and can prevent air circulation. In urban areas which are located in such depressions this phenomenon favours the development of smog conditions.

Local wind

Local wind conditions strongly influence the climate. They are determined mainly by the topography. When a wind blows over an obstacle such as a hill or tree, its velocity on the windward side is greater than on the wind-protected leeward side, and is greatest on the crest.

Water bodies

Large water bodies such as lakes and seas generally have a balancing effect on the temperature in the adjacent areas due to the great thermal storage capacity of the water. Water is also a source of local winds because it accelerates thermic air movements.

Urbanization

Heavy urbanization of an area (townships) generally increases the temperature compared to the rural surroundings. Differences of up to 10°C are possible. Wind velocity and its ventilation effects are generally decreased, but the channeling effect of narrow streets can also cause the opposite to occur.

Altitude

Altitude is a major factor influencing air temperature. As a rule of thumb, the temperature is reduced by 2°C for every 300 m increase in altitude.

Ground surface

The properties of the ground surface cover also influence the climate. Bare or denuded surfaces store little or no humidity, but absorb solar heat radiation and heat up. Surfaces covered with vegetation heat up much less, and thus have a regulating effect on the temperature and increase humidity. The more intense the vegetation, the greater is its balancing effect.

Response to microclimate

While considering the general climatic characteristics may be sufficient in working out the rough concept of a building, the individual site conditions, as observed according to the above criteria, need to be considered in designing the details. If possible, these factors should already be considered when selecting the construction site.