Utilizing only that portion of a dataset that satisfies a certain criterion. For example, in the boundary layer a convective thermal could be defined as a flow structure that has an upward velocity exceeding some value for some minimum time period. If the average value of all temperature measurements that satisfy the above criterion is calculated, the result would give the average temperature in thermal updrafts. Such data analysis procedures allow investigators to study flow structures such as coherent structures that exist in the atmosphere.