The irregularity in the amount of rainfall in different parts of north Indian plains is mainly due to :

The irregularity in rainfall across the North Indian plains is primarily attributed to the variation in the location of the axis of the monsoon trough [c1][t5]. This trough is an elongated area of low pressure extending from northwest India to the Bay of Bengal. Its axis is not stationary; it oscillates northward and southward depending on meteorological conditions [t3][t5]. When the axis lies over the plains, rainfall is abundant in those regions [c1]. However, when the axis shifts closer to the Himalayan foothills, the plains experience 'breaks' or dry spells, while the mountainous catchment areas receive heavy precipitation [c1][t3]. Additionally, the frequency and intensity of tropical depressions, which follow this axis, further determine the spatial and temporal distribution of monsoon rains [c5]. While the intensity of the heat low in the northwest influences the overall monsoon strength, the specific spatial irregularity within the plains is governed by the trough's migration [t3][t5].

Sources

1. [1] CONTEMPORARY INDIA-I ,Geography, Class IX . NCERT(Revised ed 2025) > Chapter 4: Climate > Advancing Monsoon (The Rainy Season) > p. 30
2. [2] https://mausam.imd.gov.in/imd_latest/monsoonfaq.pdf
3. [3] CONTEMPORARY INDIA-I ,Geography, Class IX . NCERT(Revised ed 2025) > Chapter 4: Climate > Advancing Monsoon (The Rainy Season) > p. 32