造成这样现象的原因一方面是在夏季人们逐渐增多了外出活动的频率，另一方面还因这段时期是雷电暴雨频繁出现时间。
This is thought to be due to an increase in outdoor summertime activities coupled with storms peaking during this period.
华北汛期降水量的多寡主要受其汛期大雨的频率和暴雨的贡献率影响。
The rainfall over North China during rainy seasons is mainly influenced by the heavy rain frequency and the excessively heavy rainfall contribution.
城市设计暴雨，具有历时短、设计标准较低的特点，与常用的频率计算方法不尽相同。
For urban design storm, there are some features which are shorter duration and lower design level.
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