Thursday, May 29, 2008

Tropical Design Module 2: Climate Elements

Basic Concept:

"Weather" is the set of atmospheric conditions prevailing at a given place and time.

"Climate" can be defined as the integration in time of weather conditions, characteristics of a certain geographical location.

At the global level climates are formed by the differential solar heat input and the uniform heat emission over the earth's surface.

The movement of air masses and of moisture-bearing clouds is driven by temperature differentials and strongly influenced by the Coriolis force.

Classification of Climates:

Example: Those countries lying just above or below the equator, Southeast Asian Countries, Central America and the Amazon Basin in South America

Cool Temperate
Example: N.W. Europe, Canada, and parts of North America

Warm Temperate
Example: Mediterranean Countries

Cool Temperate
Example: N.W. Europe, Canada, and parts of North America

Example: Iceland, Greenland, Northern Russia, and China

For the purposes of building design a simple system based on the nature of the thermal problem in the particular location is often used.

Further Classification of Tropical, Sub-Tropical & Equatorial Climates:

A. Warm Humid (Tropical Island) overheating is not as great as in hot-dry areas, but it is aggravated by very high humidities, restricting the evaporation potential. The diurnal temperature variation is small.

B. Hot Dry (Arid/Maritime Desert) main problem is overheating, but the air is dry, so the evaporative cooling mechanism of the body is not restricted. There is usually a large diurnal (day - night) temperature variation.

C. Composite (Tropical Uplands)

Warm Humid:
High temp during the day, low diurnal change; Relatively high humidity: Heavy rains especially during monsoon season; Cloudy and glaring sky; Lesser ground vegetation
Hot Dry:
Very high temp during the day; large diurnal range; can be quite low in winter; Low and very low humidity; fairly constant throughout the year; Often low or very low precipitation; Little or no cloud. Cold and non-glaring sky; Sparse and often bare ground vegetation. Very high glare from ground. Rich soil which only requires water
Mixture of warm/humid and hot/dry.; 1/3 to 2/3 ratio of monsoon period

The general climate (macroclimate) is influenced by the topography, the vegetation and the nature of the environment on a regional scale (mesoclimate) or at a local level within the site itself (microclimate).

Tropical Climate: Philippines

Temperature – average mean temperature (dbt) 20 – 30 deg C
small diurnal temperature change/range 2-5 deg

Humidity Levels - 50% - 100% Relative Humidity

Wind Conditions – Slow Wind Flow; Average of 2 m/s

Prevailing Wind in the Philippines :
Amihan (NE) – November to April
Habagat (SW) - May to October

Sky Conditions – Overcast Sky most of the time; a lot of reflected heat/ solar gain

Precipitation – high during the year – average of 1000mm/yr

Many factors contribute to micro-climate, for instance, the location of hills, rivers, streams and lakes, the position of buildings and trees, whether the site is on coast or inland, in a town or in the rural areas, whether the location is above sea level, etc.

Some micro-climate phenomena are:
- land/sea breeze
- Courtyards
- Evaporative cooling
- Orientation
- Slope of land height in relation to air movement, rainfall and temperature

Urban Climate:
Almost every city in the world today is hotter - usually between 1 to 4 deg C hotter - than its surrounding area. This difference between urban and rural temperatures is called the "urban-heat-island" effect", and it has been intensifying throughout this century.

Elements of Climate Needed in Design:

A. DBT (Dry-Bulb Temperature) – measurement of the temperature of the air and as far as possible excludes any radiant temperature; measured in the shade.
instrument – silvered thermometer (in 0F or 0C)
- monthly mean of daily maxima (deg C)
- monthly mean of daily minima (deg C)
- standard deviation of distribution

B. Wind
– direction, frequency and force of the wind throughout the year.
instrument – vane anemometer for high speeds
kata thermometer for low speeds

C. RH (Relative Humidity) – amount of water in the air.
instrument – hygrometer (in %) or sling psychrometer
measured in 0F or 0C if WBT (wet-bulb temperature)
- early morning relative humidity (in %)
- early afternoon relative humidity (in %)

D. Precipitation – mainly rainfall but could also be dew.
instrument – rain gauge measured in inches or centimeters
- monthly total (in mm)

E. Sky – either cloud cover, measured in 1/8 or 1/10 or % of the sky covered, or it could be measured in hours of sunshine
Cloud cover - based on visual observation and expressed as a fraction of the sky hemisphere (tenths, or 'octas' = eights) covered by clouds.
Sunshine duration - the period of clear sunshine (when a sharp shadow is cast), measured by a sunshine recorder which burns a trace
on a paper strip, expressed as hours per day or month.

F. Solar Radiation - measured by a pyranometer, on an unobstructed horizontal surface and recorded either as the continuously varying irradiance (W/m2), or through an electronic integrator as irradiance over the hour or day.

Four environmental variables directly affecting thermal comfort are temperature, humidity, solar radiation and air movement, these are the four constituents of climate most important for the purposes of building design. Rainfall data may sometimes be needed, such as for designing drainage systems and assessing the level of precipitation.

1 comment:

  1. Thank you so much for giving a very knowledgeable informations, Archt. Gabitan...