The pillar of fire

Exodus speaks of the pillar of cloud and pillar of fire. What is the reader supposed to visualize? Since the Bible records real events, it's useful for readers to enter into the accounts and imagine what the observers saw. 

At one level, this is a theophany. A visible symbol of God's presence and power. At another level, theophanies can be natural phenomena–like coincidence miracles.  

One question is whether the pillar(s) of cloud and fire represent two distinct phenomena or one. A fiery manifestation would be less luminous in sunlight. 

One the face of it, their descriptive names and functions suggest whirlwinds; specifically: a dust devil for the pillar of cloud and fire devil for the pillar of fire. Dust devils would be familiar sights to desert inhabitants. Fire devils would be rarer. These whirlwinds have a columnar appearance. They are mobile. A dust devil is darker while a fire devil is brighter–due to their respective composition. 

Normally, dust devils and fire devils are small, weak, momentary, aimless vortices. However, they can vary in size and intensity, sometimes rivaling tornadoes. 

In the case of the pillar(s) of cloud and fire, these are guided, durable phenomena. Unlike mindless, inanimate whirlwinds, they lead the Israelites in the trackless desert: 

And the Lord went before them by day in a pillar of cloud to lead them along the way, and by night in a pillar of fire to give them light, that they might travel by day and by night (Exod 13:21).

They can also assume a protective role:

19 Then the angel of God who was going before the host of Israel moved and went behind them, and the pillar of cloud moved from before them and stood behind them, 20 coming between the host of Egypt and the host of Israel. And there was the cloud and the darkness. And it lit up the night without one coming near the other all night (Exod 14:19-20).

This is a complex phenomenon. Since it's not a purely natural phenomenon, that's understandable. On the one hand, the "dark side" of the cloud might serve to conceal the Israelites from the Egyptian army. If, on the other hand, this is a tornadic fire devil, it would pose an impenetrable barrier–a wall of fire–shielding the Israelites from the Egyptian army. 

The fire devil identification may seem less suitable for the "cloud" that fills the tabernacle and the temple. For one thing, it would incinerate worshippers. In context, this is probably not a "pillar of cloud." A columnar shape seems less apt for filling rectilinear space. Also, it seems to emit light rather than heat.

Since this is not a purely natural phenomena, we'd expect that flexibility. God is manipulating natural forces. Bending nature to his will. 

At the same time, there may be something physically dangerous about the "cloud." Notice that the presence of the "cloud" is incompatible with human presence in temple dedication:

10 And when the priests came out of the Holy Place, a cloud filled the house of the Lord, 11 so that the priests could not stand to minister because of the cloud, for the glory of the Lord filled the house of the Lord (1 Kgs 8:10-11).

The "cloud" is inhospitable to human life. That, of course, reflects the holiness of God–where direct contact with sinners may be fatal.