Four climbers died over the weekend as a rush of adventurers tried to reach the top of Mount Everest, creating what Nepali mountaineering official Gyanendra Shrestha described as a "traffic jam." With so many people crowding the world's highest peak, many have to spend more time at high altitudes than they should, forcing them to use up their oxygen, and increasing the chances that the notoriously perilous climb will prove deadly. And yet, this weekend, another crowd is expected to exploit a narrow window of good weather in the prime May climbing season as they scramble for the summit. Here, a look at the risks of scaling Everest, by the numbers:

Climbers who died last weekend: Ha Wenyi of China, Eberhard Schaaf of Germany, Nepal-born Canadian Shriya Shah, and South Korean Song Won-bin. They were apparently killed from exhaustion and altitude sickness as they descended from the summit. Another climber, a Nepalese sherpa guide, had been reported missing, but reached base camp safely on Monday.

Nepalese guides who died on the mountain last month

Deaths on Everest in 1996, the most deadly climbing season on record

Deaths on May 10, 1996, the deadliest day since Edmund Hillary and Tenzing Norgay became the first climbers to reach the mountain's summit in 1953. Climbers started too late in the day, making them vulnerable to a late-afternoon snowstorm. That's why wait times caused by this month's crowds are so dangerous, says one Sherpa guide. "Having to wait for extra hours... makes the situation worse."

Climber who dies for every 10 successful ascents to Everest's summit

People who have died trying to climb the 29,035-foot Everest

Deaths above the 26,247-foot mark between 1921 and 2006, according to the British Medical Journal

Climbers who tried to reach the top of Everest last weekend

Climbers who will try this weekend

Age of the oldest woman ever to reach the summit. Tamae Watanabe — a retired office worker who lives at the foot of Japan's tallest mountain, Mount Fuji — reached the top last weekend, breaking a record she set at age 63 in 2002.

Wind speed, in miles per hour, recorded at the summit in February 2004. That was the highest ever, but winds of more than 100 mph are common throughout the year. When the Asian Monsoon is beginning and ending (in May and September) the winds die down, giving climbers seven- to 10-day windows when they can climb Everest without being hit by hurricane-force winds that can kill them in minutes, or simply hurl them off the mountain.

Minimum wind speed, in miles per hour, for a category 5 hurricane

Low end of the average temperature range, in degrees Fahrenheit, at the top of Everest. The high end of the range is -4 degrees.

Sources: Associated Press, BBC News, Daily News, Popular Mechanics