Climate Of Lahore And Thermal Comfort Environmental Sciences Essay

Chapter 2

2.1 Introduction

Climate has great effect on building for its electricity consumption and building performance. Energy efficient design requires the consideration of the climate. The overall heating and cooling requirements in the buildings is controlled by climate during summer and winter season. As we have selected our site in Kala Shah Kaku, Punjab and kala shah kaku is located in Lahore region therefore the description of the climatic data of Lahore region is given below.

2.2 Climatic classification:

Climate can be classified in terms building design in which the country will be divided into five zones in such a way that the difference of climate from zone to zone can be seen in the building design, gauranting some special provision for each zone. Based on these criteria, there are five major climatic zones,

Hot-dry

Hot-humid

Cold

Temperate

Composite

Types of climate Typical Characteristics

Cold Excessive heat loss for most of the year

Minimum Temperature: below -15 °C

Temperate Excessive heat lost for part of year

Inadequate heat loss for part of year

Temperature changes: -30 °C to 30 °C

Hot/Dry Overheating for most part of the year

Dry air allow evaporation

Temperature ranges: -10 °C to 45 °C

High radiation

Strong Winds

Hot/Humid Overheating for most of year

Humid air inhibits cooling

Temperature often above 20 C

Mean relative humidity around 80 %

High rainfall in certain months

Table 2.1 typical characteristics of different types of climates

2.3 Climatic zones of Pakistan:

Pakistan has a wide range of climatic conditions. The Northern and North- Western parts of Pakistan are cooler, the centre of the Punjab is extremely hot and semi-arid, and the Baluchistan is cool and arid whereas the coastal strip is warm and humid. In cold regions, during the winter temperature drops well below the zero. In the desert regions, the maximum temperature remains above 40°C during most of the summer period; with a peak often approaching 50°C.The climatic regions have been divided based on various criteria.

Fig 2.1 map of climatic zones of Pakistan

2.4 Climate of Lahore Region:

Lahore lies 217 meters above sea level and it lies between 31°15′ and 31°45′ North latitude and 74°1′ and 74°39′ East longitude. On the North and West Lahore is bounded by Sheikhupura district, Wagah surrounds Lahore on east, and on the south of Lahore there is Kasur District. The river Ravi flows on the North of Lahore. Lahore city has a total land area of 404km2.

The Lahore region comes under the normal composite zone, according to climatic zone map. It lies near the hot dry zone and in summer the wind direction is such that the wind blows northeast towards southwest of the city affecting the climate of Lahore region. The effect of hot dry zone starts from March and remains till September as the wind direction is southwest which means from hot dry zone towards Lahore region. The main features of this season are that the winds blown are hot and full of dust during the daytime but are cooler in night. Throughout the day very strong wind blows. The climate of Lahore region can be divided into the following broad categories:

2.4.1 Hot and dry:

Hot and dry weather is found in the months of March and April. In these months the air is dry with the clear sky and no sea breeze to check the excessive heat as the Lahore region is located more than 1000 km from the nearest sea, the temperature rise is sharp. In the mid-March the maximum temperature reaches up to 90 F and till the middle of April temperatures reaches up to 100 F. Due to clear sky, nights are cooler having the average temperature of nearly 65 F (18 °C). The rainfall is very less in this duration.

2.4.2 Very hot and dry:

In the months of May and June the weather of Lahore is very hot and dry and the heat is at its peak in these months. In the second week of May, the temperature reaches up to 110 F (43 °C).The heat is oppressive, humidity level is low with very hot and dry wind which is called loo in local language blowing most of the time. The month of June still remains hotter with the temperature sometimes reaching to 119 F (48 °C). However in June due to intense low pressure created by the extreme heat, sometimes some moisture from Arabian Sea finds its way to this region and causes rain which for the brings the temperature down temporarily.

2.4.3 Hot and wet:

The months of July to September are rainy. Monson clouds travel from the Bay of Bengal and after traveling about 100 km over India reaches Pakistan and enter Lahore from the South- East. This rain system in these months called Monsoons. This is a regular characteristic of the Punjab, in this country. The weather is cool and pleasant when it rains, otherwise hot and humid. Although the temperature decreases significantly, from nearly 106 F ( 41 °C ) just before the monsoons starts in Lahore to about 95 F ( 35 °C ) but as the monsoon arrives there is the increase in the relative humidity which increases from less than 25% to more than 65%. The high temperature along with the excessive humidity makes the weather very exhausting. Moreover sometimes the rain falls with such an avalanche that it drowns the low lying area and people have to undergo great hardships so this weather is a mixed blessing.

2.4.4 Warm and dry:

Warm and dry weather remains in the months of October and November. Since the monsoon stops at the end of September, the weather again becomes dry again but the temperature is considerably less hot this time. The dry continental air remains in the region. The temperature is moderate to warm during days whereas the weather is cooler in nights. The average high temperature is nearly 85 F (29 °C) and at night due to dry air temperatures it can decrease to 53 F (12 °C) especially at the end of October or early November. There is no rain in this season and this season is the driest season as compared to other seasons. This extreme dryness creates a lot of dust and haziness which results in pollution in the environment.

2.4.5 Cool and dry:

The most pleasant months of all are December January February in Lahore. This is the most perfect weather of Lahore. The peak high temperature is nearly 68 F (20 °C) and the peak low temperature is nearly 41 F (6 °C). The climate remains cool, refreshing and sunny. However sometimes, a rain bearing system known as western disturbance which has its source in Mediterranean Sea affects the Lahore region and causes rain. But the rain is not so heavy neither it remains for a long duration. So the weather most of the times remains sunny, dry and refreshing.

In fact, the climate of Lahore is hot and dry for the maximum period of the year with a short winter period with very refreshing weather.

2.5 Climatic indicators of Lahore:

The table below displays average monthly climate indicators in Lahore which is based on 8 years of historical weather readings.

Temperature in: Fahrenheit

Table 2.2 average monthly climate indicators in Lahore

2.6 Hot dry climate:

The hot dry equatorial land lying between the two mean annual isotherms of 68°F (20°C) has regions where the temperature during the summer season may reach 43.3°C or may exceed and the vapor pressure is below 25millibars.

2.6.1 Elements of hot dry climate:

1 Seasonal variation

2 Air temperature

3 Humidity

4 Precipitation

5 Wind

6 Sky conditions and solar radiation

7 Dust storm

2.6.1.1 Seasonal variation:

In these regions there are two major seasons;

hot season

winter

Fig 3.2 Monthly averages of the minimum and maximum daily temperatures.

°C/°F

2.6.1.2 Air temperature:

The main features of the climate are temperature remains high during the day 80-130°F (26.67-54.44°C) in the summer months along with strong sunlight reflecting pitilessly from light dry and parched earth.

There is however a big difference between summer and winter temperatures due to the varying altitude of sun. During winter the day temperature is usually as high as 90°F (32.22°C) but the temperature is quite low at nights ,whereas during summer the temperature goes down to 50°F(10°C) compared with 70F (21.11C) at night.

2.6.1.3 Humidity:

Humidity remains low, Relative humidity varies with the air temperature. It can go below 20% in evening to over 40% at night. V.P fairly steady changing with location and season from about 500-1500 N/m2

2.6.1.4 Precipitation:

The amount of rainfall or precipitation is generally noted as the average mm/day or mm/month which provides a beneficial indication regarding seasonal changes.

Usually, average maximum rainfall in 24 hours or the average number of days in which a certain amount (in cm.) of rainfall happened is also given. This data is beneficial as an indicator of the rain intensity, which will affect the design of windows, roofs and sewerage etc.

July and August are the months in which most rainfall occur, whereas June and September also have some rainy days. For the rest of the year, hardly any rain fall occurs in Lahore. Based on temperatures and precipitation data, during the hot and rainy season it is not advised to go between June and September. The rest of the year is dry with temperate daytime temperatures.

Fig 3.3 Precipitation is any kind of water that falls from the sky as part of the weather.

2.6.1.5 Sky conditions and solar radiations:

The skies are without clouds for the most of the period of the year where as frequent dust haze and storms occur mostly in the afternoon. Direct solar radiation is severe and is amplified by radiation reflected from the barren and light colored landscape.

Fig 3.4 Monthly average numbers of hours of sunshine per day.

2.6.1.6 Wind:

Winds are mostly native; they are generally low in the morning however increasing towards afternoon and are maximum in the afternoon, most commonly accompanied by whirlwinds of dust and sand.

2.6.1.7 Dust storms:

Dust storms mostly occurs on summer afternoons which is the most intolerable part of the day , when routine winds blow at 15-220 miles per hour. These dust storms also occurs in other parts of the day i.e. in the evening and nights.

2.7 Climatic design of building:

Climate has a great effect on building performance and electricity consumption of the building. The process of recognizing, deducing and controlling climatic effects at the building is perhaps the most difficult part of building design. The main objectives of climatic design of building should include:

To minimize electricity cost of a building.

To take benefit of maximum of natural energy instead of mechanical system and electricity.

To provide comfortable and healthy environment for inhabitants.

2.8 Factors affecting climatic design:

The native micro-climate and site factors influence the actual environmental conditions of the building. Some important factors related to site should be included while making the climatic analysis:

Topography- elevation, slopes, hills and valleys, ground surface conditions.

Vegetations – height, mass, silhouette, texture, location, growth patterns.

Built forms – nearby buildings, surface conditions

Important thermal design factors which should be considered include: solar heat gain, conduction heat flow and ventilation heat flow. The design variables in architectural terms which are important to be considered are:

Shape – surface-to-volume ratio; orientation; building height.

Building fabric – materials and construction; thermal insulation; surface qualities; shading and sun control.

Fenestration – the size, position and orientation of windows; window glass materials; external and internal shading devices

Ventilation – air-tightness; outdoor fresh air; cross ventilation and natural ventilation.

2.9 Thermal comfort:

There is no approved standard for thermal comfort. This is not astonishing, as people can and do live in a wide range of climates from the equator to high latitudes. An Internationally- accepted definition of thermal comfort, used by ASHRAE, is “that condition of mind which expresses satisfaction with the thermal environment” (ISO7330).

Perceptions of this environment are influenced by air temperature, radiant temperature, relative humidity, air velocity, activity and clothing.

Fig 3.5 achieving comfort in buildings

2.9.1 The comfort zone:

The Comfort Zone means the range of temperature conditions of air movement, humidity and exposure to direct radiations, under which a normally clothed person feels comfortable. This will differ for Indoor conditions as compared to Outdoor conditions. This will also vary for different cultures and regions and varying climate conditions.

As an Architects we use our buildings to not only create comfortable inside environments, but also aesthetical and beneficial spaces outside our buildings.

2.9.2 Thermal comfort index:

A thermal comfort index serves the objective to evaluate the effect of environmental factor on the thermal feeling of inhabitants. Number of methods have been used in the previously to evaluate the thermal comfort level. However it is not possible to gratify every person in a building and a small number of persons are always discontented with the thermal environment due to individual variations. Several efforts have been made to produce a subjective and quantitative way of representing the thermal comfort.

Thermal comfort condition in tropical summer conditions is given by Sharma and All at the Central Building Research Institute, Roorkee, India. The ranges and optimum values are given in table.

Thermal sensation

Range °C

Optimum value °C

Slightly cool

19-25

22

Comfortable

25-30

27.5

Slightly warm

30-34

32

Table 3.3: temperature ranges in different thermal zones

2.9.3 Optimal settings for indoor temperature:

Researchers have carried out much research on the most suitable comfort conditions, and have come up with general comfort results like effective warmth, the recent comfort standards set by ASHRAE (American society of heating, refrigerating and air condition engineers) for an individual performing light work are:

Thermal comfort conditions:

Air temperature 73-77 F Relative humidity below 60 %

Mean radiant temp equal to air temperature

Air velocity 10-45 ft/min