ThisSmall Business Innovation Research ( SBIR) Phase II research project will develop, test fly, and inter-compare a balloon-based sensor for measuring atmospheric water vapor and methane. The chemical sonde is based upon low power vertical cavity lasers, compact optical cells, and noise-lowering data analysis algorithms. Water vapor is the most important radiative gas in the atmosphere, but accurate measurements of it in the upper troposphere and lower stratsophere are limited to custom, one-of-a-kind instruments. Methane is the second most important anthropogenic greenhouse gas, photochemically breaks down into water vapor in the stratosphere, and is a useful tracer for troposphere-stratosphere exchange. In combination, the water vapor and methane balloon based sensor offers more accurate insight into atmospheric chemistry (e.g. recovery of the ozone layer), atmospheric dynamics, and the Earth's radiative budget. <br/><br/>Improved data on water vapor and methane in the upper troposphere and lower stratosphere will help to better understand and predict how climate will change in the future. The costs of action and inaction on climate change are expected to be large, and it is imperative that society implement policies that maximize environmental protection while minimizing economic costs. More accurate assessments of climate change will indirectly benefit the economy by giving society time to prepare and adapt to potential changes in future climate.