Ambient, barometric and atmospheric pressure are three terms used to describe different types of pressure that can affect both short and long term weather patterns. How these different types of pressure can affect the weather varies, however.
Atmospheric pressure is also called air pressure or barometric pressure. This pressure can show strong variation from region to region. Differences in air pressure readings are caused mostly by temperature changes and differences in altitude.
Barometric pressure is defined as the force that is exerted on objects by the weight of the atmosphere. According to the National Aeronautics and Space Administration (NASA), essentially, this force is created by the weight of the surrounding gas in the atmosphere. Put another way, barometric pressure is measure of how much air pressure is present. Molecules of air in the atmosphere have empty space between them which can expand or contract depending on various factors such as altitude and temperature.
Ambient pressure, on the other hand is defined as the pressure of the surrounding air that comes into contact with a measuring apparatus. In any given area, the ambient pressure is typically dependent on altitude. Put another way, ambient pressure is simply a measure of the density of the air molecules in the air. Because it is entirely dependent on an unchanging factor, altitude, ambient pressure should remain the same in a given region. Ambient pressure is often compared to sea-level pressure when it is recorded.
When looking a barometric pressure, the reading must be compared to the ambient pressure in order to make a prediction about local weather patterns. Remember that ambient pressure is always a constant and barometric pressure can fluctuate based on temperature and atmospheric conditions. Warm temperatures cause molecules to become active, which creates higher pressure in the atmosphere. As the temperature drops, however, the air molecules slow down, creating lower pressure. Since ambient pressure is determined by altitude, however, the ambient pressure of an area must be compared to the barometric pressure in order to determine is a pressure rise or fall is significant enough to foretell a change in the weather.
Because of this, a region that is experiencing high barometric air pressure will typically have clear days with little to no precipitation. On the other hand, an area experiencing low barometric air pressure may see rain, sleet or snow. This is due to the fact that precipitation develops as air molecules cool and release their moisture content. The cooling of the air will lower barometric pressure.