2 Design Conditions In principle, the heating and cooling loads are calculated to maintain the indoor design conditions when the outdoor weather data do not exceed the design values.   2.1 Outdoor Design Conditions It is not economical to choose either the annual maximum or annual minimum values of the outdoor weather data in determining the outdoor conditions. The outdoor design data is usually determined according to the statistical analysis of the weather data so that 1 to 5% of the total possible operating hours is equalled or exceeded the outdoor design values.   2.1.1 Summer Design Condition The recommended summer design and coincident wet bulb temperature, when chosen as being equalled to or exceeded by 2.5% of the total number of hours (i.e. 2928 hours) in June, July, August and September, are (i) 23 oC dry bulb temperature, and (ii) 28 oC wet bulb temperature Figure 2 shows the outdoor dry bulb temperature and wet bulb temperature curves for a typically hot summer day in Hong Kong. Usually the maximum temperature of 33 oC occurs at 2 p.m. and the minimum temperature of 28 oC occurs just before sunrise. The daily range of dry bulb temperature is about 5 to 6 oC, and the daily mean dry bulb temperature is 30.5 oC.   2.1.2 Winter Design Condition The recommended winter design and coincident relative humidity, when chosen as being equalled to or exceeded by 1% or 2.5% of the total number of hours (i.e. 2160 hours) in December, January and February, are (i) 9 oC dry bulb temperature, and (ii) 50% relative humidity Minimum temperature occurs at 6 a.m. or 7 a.m. before sunrise and the daily range is about 6 to 8 oC during very cold winter days.   2.2 Indoor Design Conditions For most of the comfort air-conditioning systems used in the commercial and public buildings, the recommended indoor temperature and relative humidity are as follows: (i) Summer: 23.5 - 25.5 oC dry bulb temperature, 40 - 60 % relative humidity (ii) Winter: 21 - 23.5 oC dry bulb temperature, 20 - 30 % relative humidity