Even if you find your cat's mood difficult to read, that may not necessarily be true. You just may not know exactly where to look. For example, whether a cat's hair is lying flat or shooting up into the air can express a lot about how she is currently feeling.
When your cat's hair points straight up and takes on a rather spiked appearance, it may be a sign of pure belligerence and anger, according to the Humane Society of the United States. Perhaps your kitty is upset that another household pet has intruded onto "her" turf, and is getting ready to attack physically. If a cat's fur is bristling out of anger, don't be surprised if she takes an aggressive turn and exposes her teeth or sharp claws, so be extremely careful.
Spiked hair also sometimes denotes a poor kitty that is simply overwhelmed by fear and nervousness -- the classic "scaredy-cat." Perhaps your pet is in her dreaded carrier on the way to an appointment with the veterinarian and is afraid of what is about to come next! Maybe she hears the massive and imposing-looking German Shepherd next door barking from outside of the window and doesn't like it one bit.
A prickly coat can also signify a cat that is presently focused solely on defending herself -- her mind is strictly on that. She is fully aware of the danger and risk surrounding her -- think perhaps a hostile fellow feline in the home -- and her body is in lock down mode as she prepares to protect herself. If this cat feels it necessary, she probably won't hesitate for a second to scratch or bite to get out of the perilous circumstances.
When you're observing your cat's coat for signs of emotion, always make a point to pay attention to the fur on the tail, as it can often be very telling. If the hair on the tail is bristled and spiking into the air, the cat is probably feeling very hostile, even if you notice that the rest of her coat appears "normal" and flat. Another mood clue that often goes along with a spiked tail is a conspicuously arched back, so take note.