Low-clouds and fog moderate the diurnal temperature range (DTR) through radiative effects. Consequently, frequent foggy events make montane cloud forests (MCFs) stable and unique. However, observations in the understory of the forest are rare. To investigate the DTR variation in elevations, we surveyed the Central Cross-Island Highway in central Taiwan transects with MCFs. The results from paired weather stations revealed that the DTR increases significantly with altitude in open fields but not in the forest’s understory. Furthermore, the continuous observations in altitude across non-cloud forest and MCFs indicate that DTR decreases in both the open field and understory of MCFs. The DTR discontinuity highlights the indispensability of MCF for the mountain ecosystem. Further simulating the integrative effect of the climate and land-use change on fog is crucial for the ecoclimate in mountainous regions.