This is regarded as proof that humans have been creating and modifying clouds for generations through the combustion of fossil fuels. Although ship tracks can sometimes be visible, researchers usually scan the near-infrared light coming off the clouds. At this wavelength many ship tracks appear as bright lines that can be distinguished from the surrounding, uncontaminated clouds. On average, polluted clouds reflect more sunlight than their unaffected counterparts. When compared to normal clouds, the number of water droplets per volume of air in ship tracks is more than doubled, the radius of the drops is decreased by roughly six percent and the total volume of liquid water per volume of air is increased twofold. In other words, this excessive cloud seeding from the ship causes the clouds to retain more water. Normally, rain forms when cloud drops coagulate and reach a size at which gravity can pull them to the ground. Yet, in ship tracks, the cloud seeding makes the drops so small that they can no longer easily merge to reach the size needed to escape. Since no drizzle comes out of the seeded clouds, the liquid water just keeps building in the cloud. This makes the cloud brighter and more reflective to incoming sunlight, especially in the near-infrared part of the spectrum. Discoveries from satellite observations of several hundred ship tracks from 2006–2009 revealed that approximately 25% had a lower cloud albedo (reflectivity) than the surrounding unpolluted clouds. These dimmer ship tracks tended to have significantly less water despite the strong suppression of precipitation by the aerosol plume. Dimmer ship tracks are hypothesized to occur when the air above the cloud tops is sufficiently dry. Smaller droplets in polluted clouds enhance cloud-top evaporation and entrainment. The polluted clouds thus engulf more of the overlying dry air causing them to rigorously evaporate and thin under dry meteorological conditions. Under a moist/unstable atmosphere the entrainment effect is smaller and the pollutants from the ship cause the clouds to thicken and the albedo to increase. As part of a strategy to reduce ship emissions, the International Maritime Organization implemented a global standard in 2020 requiring a fuel sulfur content reduction of 86%. A study by NASA found that the frequency of ship tracks was reduced to its lowest level in decades due largely to the 2020 global standards, and to a lesser degree by trade disruptions related to the COVID-19 pandemic.