几个月一次（大概在非常低压天期间） 。
Not at all, about once in several months (most probably during very low pressure days).
通过分析认为，这是一次发生于纬向环流中的沙尘暴天气，主要影响系统是蒙古西部热低压和地面冷锋。
This is a sandstorm weather that happened on zonal circulation and affected by heat low and surface cold front in west Mongolia.
在成功地设计出低压预混一次注射rtm机的基础上，又研制了新型低压预混rtm注射机。
On the basis of successful design, a new low pressure premixing RTM machine has been developed.
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