## 2 Establishing the Aircraft Bay Humidity Model
According to Equation (5), the humidity at the thermometer shelter is  
\[h_{0j}=h_{00}+q_{0}\left(T_{0j}-T_{00}\right) \tag{8}\]  
Therefore, Equation (7) can also be rewritten as  
\[h_{ij}=\left(h_{00}+p_{i}q_{0}m_{i}\right)+p_{i}s_{i}\left(h_{0j}-h_{00}\right) \tag{9}\]  
In real-world applications, \(p_{i}q_{0}\) in Equation (9) can be approximated as \(-3\), then the above formula becomes  
\[h_{ij}=\left(h_{00}-3m_{i}\right)+p_{i}s_{i}\left(h_{0j}-h_{00}\right) \tag{10}\]  
Both Equation (7) (model 1) and Equation (10) (model 2) can be used as predictive models for the local humidity of the bay. Equation (10) is simpler and there is no need to calculate \(q_{0}\) for each variation of temperature and humidity measured at the thermometer shelter. Therefore, Equation (10) is suitable for estimating the local humidity of the bay. Equation (7) is more accurate, the computational requirements are much higher, and dedicated computer software is needed. Classification of Aircraft Bay Structures Based on Local Humidity Characteristics and Ranges of Model Parameters  
Based on the bay temperature model, described by Equation (6), the bay humidity characteristic coefficient \(p_{i}\), a new parameter, is introduced into the bay humidity model. This parameter reflects the sensitivity of the bay humidity to temperature changes and is a quantity related to the bay type. Therefore, it is feasible to classify the aircraft bay structures according to differences in the bay HCC \(p_{i}\). The \(p_{i}\) values of different bays are provided in Table 1 and the bays are classified into three types according to the local humidity characteristics: Type I (no. 2, 3, 8, and 10), type II (no. 13, 5, 6, 11, and 4), and type III (no. 9, 1, 12, 14, and 7). The key model coefficient values for each bay type are listed in Table 2.  
It can be seen that the bay structure classification based on humidity characteristics is related to the open/closed state of the bay. The results of the humidity-based bay classification differ from the results of the temperature-based bay classification previously reported [23]. This is largely because the local temperature and local humidity of the same bay vary in a different way in response to the changing conditions. The HCC \(p_{i}\) of the type I bays is in the range of 0.409-0.694. Type I bays are closed with good sealing and large spaces. The HCC \(p_{i}\) values are larger for type I bays with relatively poor sealing and smaller spaces. The HCC \(p_{i}\) of type III bays is in the range of 0.943-0.994. These bays are characterized by good ventilation and lower bay temperatures. Bays located in the lower part of the aircraft, and therefore the least influenced by sunlight, are type III. The HCC \(p_{i}\) values are higher for type III bays with relatively good ventilation and lower temperatures. The rest of the bays belong to the type II group, with a HCC \(p_{i}\) ranging from 0.772-0.913. Generally, the main characteristic of these bays is that temperatures are moderate. Some open or semi-open bays have a relatively high temperature, and closed bays with poor sealing or small spaces fall into this category. The HCC \(p_{i}\) values are larger for type II bays with relatively good ventilation, low temperatures, and large spaces. The bay structure coefficient \(m_{i}\) and illumination coefficient \(s_{i}\) are related to the bay temperature characteristics. The structure coefficient \(m_{i}\) of the bays whose outer surface is exposed to direct sunlight ranges from \(-0.8\) to \(0.5\), and sunshine coefficient \(s_{i}\) ranges from \(1.56\) to \(1.86\). Bays located in the inner