If the scientists who monitor Earth's atmosphere are correct, most of the volcanic particles that caused one of the coolest summers of the 20th century won't be noticeably affecting the world's weather by the end of 1993.
That means the so-called "Pinatubo Effect" will be chilly history.The PE, as some meteorologists and climatologists call it, began in June 1991 with the eruption of Mount Pinatubo, a volcano on Luzon, the largest island in the Philippines.
The explosion hurled 20 million to 30 million tons of ash and sulfur-rich particles into the stratosphere, 10 to 15 miles above the Earth. Over the past 18 months, these particles have scattered and reflected some of the sun's incoming energy back into space.
As a result, temperatures in the Northern Hemisphere dropped 1 to 2 degrees, particularly last June, July and August when the United States had its third coldest summer in 98 years.
Federal scientists estimate that 60 percent to 85 percent of Pi-na-tu-bo's particles have now fallen back to Earth, many of them as the nuclei of raindrops. By the end of next year, 98 percent will be gone.
There's a chance the remaining Pinatubo particles may cool the Northern Hemisphere one- or two-tenths of a degree next summer, National Weather Service research meteorologist Vernon Kousky said. That's not enough to make a noticeable impact on the hemisphere's temperatures.
However, Pinatubo may continue to exert a tiny effect on the world's weather in 1994 and 1995 because its particles will have cooled ocean temperatures about 1 degree. The cool oceans modify all air masses that pass above them.
Scientists have been fascinated by Pinatubo's impact. To their surprise, they learned that volcanic particles spread more quickly during winter than during summer.
And they confirmed that a volcanic eruption's impact on the world's weather depends on several factors: the time of year, whether the eruption is in the tropics or middle latitudes, and its intensity.
Particles tossed into the atmosphere during the fall or winter spread quickly, Kousky noted, and their cooling effect occurs rapidly. In contrast, the particles from summer eruptions move slowly and their effects are fully felt for a year.
"If Pinatubo had occurred six months earlier we probably would have had cold summers in 1991 and 1992," he noted. "Instead, we had only one cold summer, 1992."
Kousky also noted volcanic eruptions in the tropics - like Pinatubo - spread their particles over the entire earth. In contrast, volcanoes that are in middle or upper latitudes spread their particles only over their hemispheres. That means an eruption in Alaska, for example, only gunks up the stratosphere above the Northern He-mi-sphere.
The size of a volcanic eruption obviously determines its impact on the weather. The bigger the boomer, the colder the weather. Big busters like Krakatau in 1883 and Tambora in 1815 (both in modern-day Indonesia) affected the weather for several years. Tambora clearly caused 1816's infamous "Year Without a Summer" in the United States.