California Climate Change Glossary

Letter W

Water Vapor. The most abundant greenhouse gas, it is the water present in the atmosphere in gaseous form. Water vapor is an important part of the natural greenhouse effect. While humans are not significantly increasing its concentration, it contributes to the enhanced greenhouse effect because the warming influence of greenhouse gases leads to a positive water vapor feedback. In addition to its role as a natural greenhouse gas, water vapor plays an important role in regulating the temperature of the planet because clouds form when excess water vapor in the atmosphere condenses to form ice and water droplets and precipitation. (EPA)

Weather. Weather is the specific condition of the atmosphere at a particular place and time. It is measured in terms of such things as wind, temperature, humidity, atmospheric pressure, cloudiness, and precipitation. In most places, weather can change from hour-to-hour, day-to-day, and season-to-season. Climate is the average of weather over time and space. A simple way of remembering the difference is that 'climate' is what you expect (e.g., cold winters) and 'weather' is what you get (e.g., a blizzard). (EPA)

Wedge Theory. The theory of opportunities for limiting or diminishing the greenhouse effect and global warming. Wedges include: decarbonised electricity, decarbonised fuels, fuel displacement by alternative energy sources (e.g. solar energy, nuclear energy), methane management, and natural carbon sinks (e.g. forests). (Lenn)

Wet/Dry Deposition. The removal of a substance from the atmosphere either through being washed out as rain (wet deposition) or through deposition on a dry surface (dry deposition). (IPCC)

WGII LESS Scenario. Scenarios developed for the SAR WGII to assess low CO₂ emitting supply systems for the world. The scenarios are referred to as LESS: Low-Emissions Supply System. (IPCC)