learning a new word: lour, ophidian, water strider

'Lour' is a variant of 'lower'. In this case, it was being used in the noun sense of 'a threatening or sullen look' or 'a dark and ominous look'. I wouldn't have applied it to summer *heat*, myself, although I can understand applying it to clouds during such weather. (Credit to The American Heritage Dictionary for explaining to me what it meant.)

'Like the man dreaming, who feels even in sleep the refreshing storm break the lour of summer heat...'
- chapter 1 of The Silent Tower by Barbara Hambly

As for 'ophidian', it means 'snakelike'. Nothing to do with obsidian at all.

(As for 'water strider', that's a kind of insect that appears to walk on water. Yes, they're the ones also known as 'Jesus bugs'. The chapter that uses this term is being told from the viewpoint of a character who has never heard of Christianity, hence the term being used.)