 To understand the relationship between the temperature of air columns and the height of constant pressure surfaces, let's take a virtual balloon ride. One balloon is located in a warm air column, while the other is in a cold air column, and at the surface their pressure gauges read the same value. As the balloons rise, pressure decreases. We can see the pressure readings on their gauges decrease. But if we check our pressure gauge on the balloon in the cold air column, we find that it reads lower than that in the balloon in the warm column. And that's true regardless of altitude, because a cold air column tends to be deflated vertically compared to a warm air column. In other words, pressure decreases faster with increasing height in a cold air column than it does in a warm air column. So at any given altitude, the balloon in the warm air column has a higher pressure than the balloon in the cold air column. However, if we wanted the balloons to be at the same pressure, one balloon must change its altitude. The balloon in the colder air column sinks so that it's now located at a level with an equal pressure to that of the balloon in the warmer column. This dashed line represents the constant pressure surface common to the two balloons. Over the warmer air column, the constant pressure surface is located at a higher altitude than over the colder air column. Furthermore, at the altitude of the balloon in the warm column, the pressure is less in the cold air column. So cold air can be identified by either lower heights on a constant pressure surface or lower pressures on a constant height surface. Warm air columns have just the opposite, high heights on a constant pressure surface and high pressure on a constant height surface. So to reiterate the fact that pressure decreases faster with increasing height in a cold air column, then it does in a warm air column leads us to the important conclusion that cold air columns mark regions of lower heights on a constant pressure surface while warm air columns mark regions of higher heights on constant pressure surfaces.