Low-pressure area area of wind divergence aloft occurs ahead of embedded shortwave troughs, which are of smaller wavelength. Diverging winds aloft ahead of these troughs cause atmospheric lift within the troposphere below, which lowers surface pressures as upward motion partially counteracts the force of gravity. Thermal lows form due to localized heating caused by greater sunshine over deserts and other land masses. Since localized areas of warm air are less dense than their surroundings, this warmer air rises, which lowers atmospheric pressure near that portion of the Earth's surface. Large-scale thermal lows over continents help drive monsoon circulations. Low-pressure areas can also
Pressure carburetor fairly simple to operate compared to float carbureted engines. The four chambers in the pressure carburetor are all in a row and are referred to by letters. Chamber A contains impact air pressure at the carburetor inlet. Chamber B contains the lower air pressure from the throat of the venturi. The difference in pressure between the two air chambers creates what is known as the "air metering force", which acts to open the servo valve. Chamber C contains metered fuel, and chamber D contains unmetered fuel. The difference in pressure between the two fuel chambers creates the "fuel metering force",