<P> The study of these anomalies forms the basis of gravitational geophysics . The fluctuations are measured with highly sensitive gravimeters, the effect of topography and other known factors is subtracted, and from the resulting data conclusions are drawn . Such techniques are now used by prospectors to find oil and mineral deposits . Denser rocks (often containing mineral ores) cause higher than normal local gravitational fields on the Earth's surface . Less dense sedimentary rocks cause the opposite . </P> <P> In air, objects experience a supporting buoyancy force which reduces the apparent strength of gravity (as measured by an object's weight). The magnitude of the effect depends on air density (and hence air pressure); see Apparent weight for details . </P> <P> The gravitational effects of the Moon and the Sun (also the cause of the tides) have a very small effect on the apparent strength of Earth's gravity, depending on their relative positions; typical variations are 2 μm / s (0.2 mGal) over the course of a day . </P> <P> Tools exist for calculating the strength of gravity at various cities around the world . The effect of latitude can be clearly seen with gravity in high - latitude cities: Anchorage (9.826 m / s), Helsinki (9.825 m / s), being about 0.5% greater than that in cities near the equator: Kuala Lumpur (9.776 m / s), Manila (9.780 m / s). The effect of altitude can be seen in Mexico City (9.776 m / s; altitude 2,240 metres (7,350 ft)), and by comparing Denver (9.798 m / s; 1,616 metres (5,302 ft)) with Washington, D.C. (9.801 m / s; 30 metres (98 ft)), both of which are near 39 ° N. Measured values can be obtained from Physical and Mathematical Tables by T.M. Yarwood and F. Castle, Macmillan, revised edition 1970 . </P>

Acceleration due to gravity at different locations on earth