If you're cooking, it's the flavor bombs (don't call it grease) hitting the coals. When you said "thick" that threw me off. But it will definately be white.
Here is one of my cookin buddies engulfed in a wave of smoke about 7AM one morning after an all night cook. Had his neighbors not been invited to the party later that day, there most likely would have been shots fired.
KCBS - CBJ
Big Green Eggs, Big Drum Smokers, Big Chiefs, Weber Smokey Joe "Custom Tall Boy"
Oil Patch Horizontal
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Barbecue is not rocket surgery