long period variation meaning in English

长期变化

As for frequency domain , tirls includes mainly 3 frequency bands : ( 1 ) high frequency band with period less than 1 year , which is relative to weather ; ( 2 ) mid frequency band with predominant period 1 year , relative to sun , plant and season ; ( 3 ) low frequency band with period more than 1 year , relative to the crustal activity and long period variation of atmosphere temperature
从频率域看，地表热红外辐射主要存在3种频段： ( l )高频，周期小于1年的天气变化； ( 2 )中频，以1年周期为主的太阳、植被与季节变化； ( 3 )低频，大于1年周期的大气变化的长周期成份，地壳缓慢的运动引起的热辐射变化亦属于长周期变化。