The Seasonal Weather Patterns and Geography of Asia

Asia is an area of geographical diversity and varying seasonal patterns. It contains the tallest mountain chain, some of the most dramatic seasonal shifts, and some of the driest places on the planet. It is bordered to the south and east by large oceans, yet its landmass stretches nearly half the globe. However, much of that diversity can be explained by large-scale seasonal patterns, such as the temperature/pressure shifts that cause intense directional monsoon winds. It is when these seasonal patterns meet the equally immense geographical features that we see the consistent year-round dryness of the Arab peninsula and inland Asia. It is a combination of these two factors that produce most of the variation in weather patterns across the continent.

The most obvious observation that can be drawn from the precipitation charts below is the large differences in seasonal rainfall along the Asian coast from India up into eastern China. July in Mumbai, for example, saw the heaviest rainfall out of all the cities surveyed, yet in January it saw no rainfall at all. A similar pattern of heavy summer rains and dry winter months can be seen in Seoul, Honk Kong, Ho Chi Minh City, and even Tokyo and Beijing to a lesser extent. All of these cities lie on the coast, and all are outside of or at the northern end of the tropical zone, which means at least some variance in seasonal temperatures, and many of them are famous for seasonal monsoons.

With the above information, the cause the seasonal precipitation patterns are easier to see. The summer months in India, SE Asia, and eastern China, experience a spike in temperature due to more direct sun angles. This causes heating of both the land and the ocean that abuts these areas, which means evaporation and moist air over water. However, the ability of the land to absorb heat is far less that of water, which causes a significant imbalance of atmospheric pressure between the two. As the air above the land heats and expands, a low-pressure area is created while high atmospheric pressure is maintained over water, which absorbs much of the heat. The resulting disparity in pressure causes a directional shift in the wind pattern, with strong, moist air rushing in towards the coastline toward the areas of low pressure. As the air rises in altitude over land, it cools, and condensation occurs, which causes rain to fall over the land. The exact opposite effect occurs in the winter as water continues to act as a moderating force in terms of temperature. Therefore, as the dry wind rushes out to the ocean in search of lower atmospheric pressure and higher temperatures, the same cities that experienced heavy rainfall in the summer are relatively dry.

The degree of change from summer to winter is generally determined by the exposure of a given city to direct winds and large stretches of ocean, as well as the geography around the city, specifically altitude change. For example, Mumbai averages 34.2 inches of rain in July, while Hong Kong averages 14.9. However, Mumbai is directly exposed to thousands of miles of Indian Ocean, with little landmass for the monsoon winds to unload moisture upon. Similarly, it is on the northern end of the tropical zone and therefore experiences extreme humidity from the evaporation of the Indian Ocean. Hong Kong, on the other hand, receives its summer monsoon winds from Pacific air to the south, which passes through much of SE Asia and Indonesia before reaching the Chinese coastline. Moreover, northern India is ringed by the massive Himalayan mountain range, and as air is pushed higher and cools quickly, it condenses and is pushed back down onto the Indian subcontinent, dumping its moisture as it goes. The area around Hong Kong shows some altitude change, but it is not nearly as dramatic, and therefore the precipitation is spread more evenly throughout the area, reaching all the way into Chengdu.

A similar logic of prevailing winds and topography can be used to explain the weather patterns in more consistent areas of the Asian continent. For example, Singapore shows little seasonal changes, with steady rainfall throughout the year. Located deep in the tropical zone, Singapore experiences little seasonal change and direct sun year-round. Therefore, atmospheric pressure and winds move in relatively consistent patterns. These winds almost always carry a large amount of humidity from consistently hot temperatures, which results in rainfall whenever they reach the relatively mountainous islands of Indonesia. Likewise, the inland region of Asia becomes consistently drier as the distance from the coastline increases, with cities such as Almaty or Urumqi experiencing little rainfall. Even though these areas experience dramatic seasonal shifts in temperature, a topographical analysis shows dramatic altitude gains in the Himalayas and the Central Asian Plateau, which effectively blocks moisture from reaching inland Asia and causes extremely arid landscapes as dry winds blast the land. Even the Arabian Peninsula, Iran, and Afghanistan, which border the ocean, remains extremely dry. This desertification is caused partially by changes in the tilt of the Earth (pushing monsoon winds further south), but it is still governed largely by immense wind patterns, which originate mostly from the African continent to the west, therefore losing humidity long before it reaches cities such as Riyadh or Tehran. There are more nuanced factors that can be attributed to these regional weather differences, but they are generally governed by similar factors, just at different strengths.

The dramatic weather patterns in Asia can generally be attributed to large-scale changes in temperature, atmospheric pressure, and directional winds. However, the more consistent weather across the inland continent shows the massive effect that the geography has in disrupting those patterns, causing large areas with drier climates. Therefore, while Asia’s weather patterns are not necessarily consistent, there is a consistency to the logic that governs them.