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
Pulse pressure aorta wall. A pulse pressure is considered abnormally low if it is less than 25% of the systolic value. The most common cause of a low (narrow) pulse pressure is a drop in left ventricular stroke volume. In trauma, a low or narrow pulse pressure suggests significant blood loss (insufficient preload leading to reduced cardiac output). If the pulse pressure is extremely low, i.e. 25 mmHg or less, the cause may be low stroke volume, as in Congestive Heart Failure and/or cardiogenic shock. A narrow pulse pressure is also caused by aortic valve stenosis and cardiac tamponade. Usually, the resting