Try this mental model:
Imagine 3 columns of air that start off identical. The surface pressure is the weight of air (per unit area) of the entire column pressing down on the surface. When the middle part of the surface heats up, the column of air above it heats up and expands upwards. If the pressure at the surface stayed the same, the column would therefore be higher pressure at upper levels, and thus the air spills out into the neighbouring columns. Thus the surface pressure in the hot column decreases.
Code:
|+++|+++|+++|
| | | |
|+++|+++|+++|
|+++|
|+++| |+++|
| | | |
|+++|HOT|+++|
<-|+++|-> spills out
|+++| |+++|
| | | |
|+++|+++|+++|
|+++|+++|+++|
| | | |
| HI|LOW| HI|