Mount Everest, the highest point on Earth, has weather conditions consistently considered the most hostile on the planet. Its sheer altitude of nearly 29,000 feet fundamentally alters atmospheric dynamics, creating a frozen environment profoundly different from conditions at sea level. The summit exists within the upper troposphere, where air is thin and high-altitude forces rule the weather. This extreme elevation drives a perpetual state of cold, wind, and low oxygen that challenges human survival.
Defining the Core Extremes
Temperatures at the summit remain below freezing year-round. During the warmest month of July, the average temperature is approximately -19°C (-2°F). January, the coldest month, sees averages drop to around -36°C (-33°F) and sometimes as low as -60°C (-76°F). These low air temperatures are compounded by the wind factor, as the summit is routinely battered by hurricane-force winds often exceeding 160 km/h (100 mph).
The combination of extreme cold and high wind produces a devastating wind chill effect, dropping the perceived temperature to around -70°C (-90°F) during windiest periods. At these temperatures, exposed skin can suffer instantaneous frostbite. Precipitation is minimal, as the intense cold and high winds scour the peak, with most moisture falling as ice crystals rather than heavy snow.
The Impact of Seasonal Cycles
Climbing feasibility is dictated by the seasonal movements of large-scale weather systems. The year is divided into four distinct periods:
Pre-Monsoon Season (April–May)
This period offers the primary window for summit attempts. The weather becomes relatively more stable and wind speeds temporarily decrease, creating a brief opportunity before the summer rains arrive.
Monsoon Season (June–September)
The monsoon brings warm, moist air and frequent, heavy snowfall. This period is characterized by poor visibility and a significantly increased risk of large-scale avalanches, making climbing nearly impossible.
Post-Monsoon Season (September–October)
This offers a secondary, smaller climbing window. While colder and less reliable than the spring, it provides another chance for stable weather before winter begins.
Winter Months (November–March)
Winter is defined by relentless cold and extreme winds. The summit is directly exposed to the full force of high-altitude wind currents. The severe conditions, including plummeting temperatures and sustained hurricane-force winds, render the mountain practically unclimbable.
Unique Atmospheric Drivers
The primary cause of Everest’s extreme conditions is the powerful, high-altitude wind system known as the Jet Stream. For much of the year, this fast-moving river of air sits directly over the summit, generating winds that often exceed 285 km/h (177 mph). The brief climbing windows occur only when the Jet Stream temporarily shifts its path northward, moving away from the peak and allowing for a short lull in the hurricane-force winds.
The mountain’s extreme height also results in dramatically reduced atmospheric pressure. At the summit, the air pressure is only about one-third of what it is at sea level. This thin air reduces available oxygen and impacts weather systems. The low pressure is directly linked to the rapid onset of cold-related injuries, as it affects the body’s ability to retain heat.
Consequences for Human Safety and Survival
The combination of low temperature, high winds, and reduced atmospheric pressure creates a deadly environment for climbers. Frostbite is an immediate and constant threat, with exposure times for facial skin to freeze sometimes measured in mere minutes during severe conditions. The intense cold and wind rapidly draw heat from the body, leading to the swift onset of hypothermia.
The low barometric pressure exacerbates high-altitude illnesses. Cold stress can trigger or worsen conditions like High-Altitude Pulmonary Edema (HAPE) and High-Altitude Cerebral Edema (HACE), which involve fluid accumulation in the lungs and brain, respectively. The unpredictability of the weather, especially the sudden arrival of strong winds or whiteouts, contributes significantly to the mountain’s fatality rate. Climbers must time their ascents precisely to catch narrow, stable “weather windows.”