The Lord Sorcier by Olivia Atwater

The year is 1812, and Mr Albert Lowe is the only soldier in his regiment with a decent pair of boots. As the English withdraw to Portugal for the winter, a dark pall lies upon the troops, who cannot seem to decide which will kill them first-the lack of supplies, the bitter winter weather, or Napoleon's dread Lord Sorcier. The lack of ammunition does seem pressing-but what are muskets to the French when they have on their side a magician capable of summoning up a hurricane?
As a surgeon, Albert can fix gunshot wounds more readily than hurricanes. But when he discovers by chance a peculiar, low-born magician among the English ranks, Albert begins to hope that they may all yet survive to return home.