What Causes low pressure and why is it always accompanied by rain?

[skinsdomination09]

my chemistry teacher acted as if this was impossible to figure out so I'm confused if I'm wrong or what haha.

The quesiton is what causes low pressure (like what causes it to occur) and why is it always accompanied with rain.

I say

"Low pressure areas form when atmospheric circulations of air up and down remove a small amount of atmosphere from a region. This usually happens along the boundary between warm and cold air masses by air flows "trying" to reduce that temperature contrast. The air flows that develop around the low pressure system then help to accomplish that reduction of contrast in temperature, with the colder air flowing under the warmer air mass, and the warmer air flowing over the colder air mass."

[tmagan]

my chemistry teacher acted as if this was impossible to figure out so I'm confused if I'm wrong or what haha.

The quesiton is what causes low pressure (like what causes it to occur) and why is it always accompanied with rain.

I say

"Low pressure areas form when atmospheric circulations of air up and down remove a small amount of atmosphere from a region. This usually happens along the boundary between warm and cold air masses by air flows "trying" to reduce that temperature contrast. The air flows that develop around the low pressure system then help to accomplish that reduction of contrast in temperature, with the colder air flowing under the warmer air mass, and the warmer air flowing over the colder air mass."

Divergence aloft promotes convergence at the surface and vice versa. Look up I believe it is called 'Dynes Principle'. When you lift air, to make a long story short, it cools and condenses to form a cloud which eventually creates precip.

[isohume]

The quesiton is what causes low pressure (like what causes it to occur) and why is it always accompanied with rain.

There are various mechanisms which produce sfc lows and precipitation is not always associated with them.

There are too many details to go into here, but mid-latitude sfc lows can form in response to large scale lift along thermal boundaries, where vertical motions are enhanced by the change in vorticity or thermal gradients with height.

Dynamics play a key role in the development of lows as well with the constant attempt of balancing forces aloft (ie: pressure gradient forces, coriolis forces related to ulvl div, conv, jet interactions, etc)...this produces direct and indirect circulations either enhancing or weakening llvl thermal gradients and llvl cyclonic conv...thus leading to a maintained and deepening low or perhaps a weaking cyclone.

Other mechanisms include thermal lows, which are common in the deserts. Basically the llvls become so warm and buoyant that mass leaves (lifts) faster than it can be replaced at the sfc and thus the pressure becomes lower and the winds turn cyclonic. Lee side trofs often produce thermal meso-lows based on the config and degree of downslope warming where the localized thermal difference is the greatest.

Large scale, seasonal, semi-permanent lows like the Aleutian low and Hudson Bay low are direct products of occluded systems (stacked or blocked) related to large scale airmass differences. ie: Polar/Artic air stalled due to strong downstream airmass ridging...like an NAO displacement.

Generally weak wake lows are formed behind departing strong dynamical systems and large tropical cyclones. This is akin to sweeping your hand under the surface of water and seeing the whirlpool and lower water heights in the wake.

Of course barotropic processes (deep convection, ulvl div, oceanic latent heat fluxes, etc) are yet another mechanism which develop tropical lows and cyclones.

There are other mechanisms I'm sure.

[salbers]

A simple way I think of it is that air speeds up as it travels from a trough into a ridge to maintain geostrophic balance (given the changing curvature of the flow). As the acceleration occurs it creates divergence in the column and the pressure falls.