Thirty years of daily 1000–500 mb thickness data from 25–90°N latitude at 383 grid points are used to obtain charts of mean temperature change in the lower troposphere between successive 5-year periods from 1949–53 to 1974–78 [1000–500 mb thickness is proportional to the mean (virtual) temperature of the layer]. These charts are then intersected in succession to determine areas of negative or positive trends lasting 25 years or more. A negative trend lasting 25 years is found in four areas and a positive trend in one, while an unbroken negative trend is found in two of the four areas.
The layer mean temperature changes between 5-year periods are averaged over all grid points within four latitude belts. Warming is found between the 1954–58 and 1959–63 periods and a stronger cooling between the 1959–63 and 1964–68 periods in all four belts. The analysis also shows that cooling is accompanied by a significant increase in the number of negative layer mean temperature changes at individual grid points (indicating an increase in the total area experiencing temperature decrease).
Time series of 5-year, annual and seasonal mean 1000–500 mb thickness values for the areas of cooling or warming are analyzed to determine the characteristics of the trends within each area. Areas containing only one grid point are increased in size to at least 106 km2 by the inclusion of surrounding grid points to reflect regional rather than merely local influences. A negative trend extending over the 30-year period is found in the east Asia area in summer. A positive trend of 25 years is found in north Iran in the fall.
Five-year mean 1000–500 mb thickness values over the 30-year period in each area are examined for evidence of climatic change using criteria proposed by Rosini. Evidence of a change to a cooler regime in the lower troposphere is found in East Asia in summer, and in eastern North America in winter. Thirty years of data are found of insufficient length to determine whether the cooling constitutes a climatic change under the given criteria. No evidence of climatic warming is found.