What does pressure have to do with storm strength?

I recently was asked why a 960 mb storm means that the storm is strong. Before fully answering the question, we need to understand the definition of air pressure. Air pressure is the weight per unit area of the air in the column above our head. Therefore, as we go up in elevation, less air is above our heads, and hence, we see a corresponding decrease in air pressure. At the surface, low pressure indicates inclement weather whereas high pressure is more commonly associated with fair weather.

Next, we need to understand why a low pressure would be a favorable location for inclement weather. In order to understand this, we need to know something about the pressure gradient force. The pressure gradient force is the force that acts from high to low pressure. In other words, air flows from regions of high surface pressure to regions of low surface pressure. Many of us have seen the pressure gradient force in action when we fly. When you open your water bottle to take a sip when flying, you fill the bottle with lower air pressure. Once the plane lands, we are at ground level where pressure is higher than it was in the cabin when the plane was aloft. We notice that the bottle appears to be crunched together. This crunch occured because the pressure inside the bottle is lower than the pressure outside the bottle, and effectively squeezes int he bottle. The same thing happens in the atmosphere. As air flow in from all sides of the low pressure region, it converges. Because air cannot enter the ground, it is forced to rise. This rising motion leads to stormy weather.

So when we ask what pressure has to do with storm strength, the simple answer is that a lower surface pressure causes stronger convergence, which means we have more vigorous upward motion. The more vigorous upward motion creates stormier weather. Of course, this is a simple explanation to a more intricate phenomenon.