This paper summarizes observations with an Owen's dust counter furnished to the Weather Bureau by the Bureau of the Meteorological Section of the International Union of Geodesy and Geophysics, to promote international cooperation in a study of the dust content of the atmosphere. A similar instrument has been furnished to eleven other countries affiliated with the Union.

Most of the measurements by the Weather Bureau have been made at the American University, a thinly settled suburb to the northwest of the city of Washington. Some have been made at the central office of the Weather Bureau, and at the base and top of the Washington Monument, within the city. A few have been obtained in other cities, and interesting series have been obtained during airplane flights.

A summary of the results shows at the American University an average of about 850 particles per cubic centimeter in winter, and about 400 during the summer months. The extremes vary between about 4,000, on an unusually smoky day in January, 1923, and about 100 on unusually clear days throughout the year. A comparison of December, January, and February, 1922–23 with the same months in 1923–24 shows an excess of over 75 per cent in the number of dust particles in 1922–23, which is attributed to a general use of bituminous coal for heating purposes at that time on account of a shortage in the supply of anthracite.

During January, 1924, counts at the central office of the Weather Bureau gave more than double the number of particles found at the university, with a maximum of 6,046 particles per cubic centimeter. During February, probably on account of increased wind movement the excess at the Weather Bureau was only 26 per cent.

A few measurements made in Chicago between September 24 and October 11, 1923, under average conditions, gave at the Federal Building a maximum of 7,180 particles per cubic centimeter, and at the University of Chicago a maximum of 4,800 particles.

The records obtained during airplane flights show a marked decrease in the dust content of the atmosphere above au altitude of 6,000 feet in August and 3,000 feet in October and November. Also a general decrease at all levels up to 7,000 feet after a rain-storm. There is also a slight excess at high levels on clear days, as compared with cloudy days due to the dust carried upward by convection currents.

A close relation is shown between the dust content of the atmosphere and the visibility both at the surface and from the altitude at which airplanes usually fly.