## 2 Methods
### EHTC Simplified Calculation Model
#### 2.3.2 Simplified Processing Method  
A moist envelope surface is the precondition for evaporation heat transfer, and the wet condition of an envelope surface is largely influenced by local rainfall. Similar to solar radiation, rainfall takes on discontinuity. However, it also has its own unique characteristics, such as randomness and uncertain intensity, causing the determination method of EHTC to be more complex than solar radiation. The main problematic point is that the occurrence of rainfall and evaporation do not correspond exactly in real time, namely, when rainfall intensity is particularly small over a certain period of time, evaporative heat transfer is probably very weak or non-existent. However, if quite heavy rainfall occurs at a certain time, evaporation heat transfer may last for a long time. For a rainy day, we can obtain hourly air temperature, relative humidity, and precipitation according to existing weather data in the tropical island region. By considering the complexity of evaporation heat transfer, this study puts forward a simplified method to obtain EHTC. We stipulate that if the rainfall precipitation within an hour is more than zero, the envelope surface is wet, and water on the surface transfers heat by evaporation in this hour all the time. Then, EHTC can be obtained by determining \(T_{a}\), \(\varphi_{a}\), \(T_{w}\), and \(\varphi_{w}\) in this hour. Among these parameters, \(T_{a}\) and \(\varphi_{a}\) can be acquired directly from existing weather data in the tropical island region, and \(\varphi_{w}\) on wet surfaces is regarded as 100%. Considering that the clouds are thicker on rainy days and can strongly weaken solar radiation, the temperature difference between the envelope surface and air is actually small. Additionally, based on the temperature ranges of the envelope surfaces and air on rainy days derived from reference [32], we determined the \(T_{w}\) by adding 3 K to \(T_{a}\). Therefore, when there is rainfall precipitation within an hour, according to the determination of hourly CHTC, \(T_{a}\), \(\varphi_{a}\), \(T_{w}\), and \(\varphi_{w}\), the daily average EHTC can be obtained. It should be pointed out that several ideal assumptions are proposed referring to the previous work [32] to simplify the problem above, and they are as follows: (a) If there is precipitation within an hour, the envelope surface is wet and water on the surface transfers heat by evaporation in this hour all the time; (b) prolonged evaporation heat transfer caused by intense rainfall moments is ignored; and (c) the \(\varphi_{w}\) on wet surfaces is regarded as 100%. Here, we need to clarify that the assessment of EHTC in this paper is based on the approach that was formulated for the case of evaporation of liquids from free surfaces, and the research necessity of this part stems from the special climate of high wind speed and strong rainfall in the tropical island region. There is actually another type of evaporation heat transfer, for which the wall structure is made from porous material and permeated by moisture, and the evaporation process mechanism is different from that in this paper and even more complex. Considering that the exterior surfaces of buildings in this area are often in a humid state under the combined action of high wind speed and strong rainfall, there must be liquid water on surfaces to transfer heat by evaporation, and so this study mainly discussed the case of evaporation of liquids from free surfaces. It should be noted that there are several issues in the assessment method: (a) The impact of building external surface characteristics on EHTC was not considered in this assessment method, and (b) the assumptions above were determined on the basis of theoretical analysis or reference material, instead of actual measurements. Additionally, wet and rainy weather is the prerequisite for discussing the EHTC, and it is unnecessary to analyze the EHTC for the dry areas with little rain.