Dear Dan, what is the difference between aridity and desolation?
DESOLATION. A temporary darkening of the mind and disturbance of the will and emotions, permitted by God to purify the souls of his followers. It may be caused by the evil spirit or brought on by a variety of other causes, but it is always purposeful, namely to withdraw a person’s affections from dwelling on creatures and bring them closer to the Creator.
ARIDITY. In ascetical theology, the state of a soul devoid of sensible consolation, which makes it very difficult to pray. It may be caused by something physical, such as illness, or voluntary self-indulgence, or an act of God, who is leading a person through trial to contemplation.
The key similarity between them is that both can be rooted in sin, the enemy, and/or in God’s desire to draw us into a deeper relationship with Him. Aridity is typically spoken of more in relation to prayer whereas desolation is an element of St. Ignatius observations about the movements of our soul in a more general sense. As you can see, they can almost be used interchangeably in the midst of real life circumstance.
PS: Both of these definitions are provided by courtesy of Catholic Culture.org