and had a refractive index and cloud drop effective radius which eliminated all of the proposed cloud types except sulfuric acid. Kiyoshi Kawabata and Hansen expanded upon this work by looking at the variation of polarization on Venus. They found that the visible clouds are a diffuse haze rather than a thick cloud, confirming the same results obtained from transits across the sun. The Pioneer Venus project was launched in May 1978 and reached Venus late that same year. Hansen collaborated with Larry Travis and other colleagues in a 1979 Science article that reported on the development and variability of clouds in the ultraviolet spectrum. They concluded that there are at least three different cloud materials that contribute to the images: a thin haze layer, sulfuric acid clouds and an unknown ultraviolet absorber below the sulfuric acid cloud layer. The linear polarization data obtained from the same mission confirmed that the low- and mid-level clouds were sulfuric acid with radius of about 1 micrometer. Above the cloud layer was a layer of submicrometre haze. Global temperature analysis The first NASA Goddard Institute for Space Studies (GISS) global temperature analysis was published in 1981. Hansen and his co-author analyzed the surface air temperature at meteorological stations focusing on the years from 1880 to 1985. Temperatures for stations closer together than 1000 kilometers were shown to be highly correlated, especially in the mid-latitudes, providing a way to combine the station data to provide accurate long-term variations. They concluded that global mean temperatures can be determined even though meteorological stations are typically in the Northern hemisphere and confined to continental regions. Warming in the past century was found to be , with warming similar in both hemispheres. When the analysis was updated in 1988, the four warmest years on record were all in the 1980s. The two warmest years were 1981 and 1987. During a senate meeting on June 23, 1988, Hansen reported that he was ninety-nine percent certain the earth was warmer then than it had ever been measured to be, there was a clear cause and effect relationship with the greenhouse effect and lastly that due to global warming, the likelihood of freak weather was steadily increasing. With the 1991 eruption of Mount Pinatubo, 1992 saw a cooling in global temperatures. There was speculation that this would cause the next couple of years to be cooler because of the large serial