Tropospheric Humidity Variations at Brownsville, Texas and Great Falls, Montana, 1958-80

Abstract

As a preliminary step in evaluating the feasibility of determining meaningful tropospheric humidity trends on a hemispheric or global scale using a sparse radiosonde network, radiosonde data at the earth's surface and at 850, 700 and 500 mb mandatory pressure surfaces, and significant levels between, have been examined for the interval 1958-80 at Brownsville, Texas and Great Falls, Montana. Adjustments had to be applied to the data prior to 1966 because at this earlier time dry observations (?motorboating?) were not reported. In general, the relative humidity at these two stations decreased or remained constant between 1958 and about 1970, and increased between about 1970 and 1980, but over the full record, it decreased at Brownsville and increased at Great Falls. Mixing ratio and precipitable water decreased during the earlier interval and increased during the later interval, similar to the variation in Northern Hemisphere temperature, although this may well be coincidence. On the seasonal and yearly time scale the relative humidity has tended to vary inversely with station temperature, and mixing ratio directly with this temperature, but these two stations do not define the relation among long-term trends in temperature, relative humidity and mixing ratio. It is concluded that to establish hemispheric or global trends in humidity will require use of a fairly extensive radiosonde network, as well as knowledge of instrumental changes and changes in measurement technique at individual stations within this network.