As a structure fire becomes ventilation limited it can often hide the fires location from the exterior. Understanding the concept of a neutral plane can aid in determining the floor the fire is located on.
The neutral plane marks the level in a structure fire where below it air will be drawn into the structure and above it combustion gasses will be exhausted. This plane can be identified only after an opening is made. Reading the smoke after the opening is made and identifying the location of the neutral plane will provide an indication as to the fires location.
If the fire is located on the first floor and the front door is opened smoke will exhaust out the top of the door and fresh air will be drawn in the lower portion. This is known as a bi-directional vent which indicates the opening was created on the same level the fire is on.
If the fire is located in the basement, making an opening anywhere above the neutral plane such as the front door would result in a unidirectional flow out or exhaust. This is because the neutral plane and the fire are located below the vent opened.
Conversely if the opening made is below the neutral plane you may not see any outflow of smoke from the new vent. If the structure is fully charged with smoke, there may be an initial outflow which is short lived as the smoke level raises up above the vents location. This is an indication that neutral plane and the fire are located above the level at which the opening was created.