Abstract: A method and apparatus for measuring the velocity of a gas flow that does not require the seeding of the flow is disclosed. The method and apparatus produce a population of stable molecules within the flow. This population of stable molecules is produced within the flow at a first time point. A population of excited molecules is produced from the stable molecules at a second time point by exciting the stable molecules with a laser beam. This laser beam also induces fluorescence in the population of excited chemical species at the second time point. Typically, the interval between the first and second time points is known and the velocity of the flow is calculated by dividing the distance travelled by the stable molecules in the flow by the known time interval. The preferred example of the stable molecule is ozone. Oxygen molecules are fluoresced by a 193 nm laser beam at a first time point, producing oxygen atoms.