Female cats in heat show a variety of distinct behavioral symptoms, including raising their tails. When your cat parades around with her tail held high during her heat cycle, she may have more than one reason for doing so. This is because of both her natural instincts and the physical changes that she experiences while her body goes through heat.
Welcoming Male Attention
The most significant reason for her elevated tail is her desire to invite a mate. With her rear end lifted and her tail held up in the air, she is in the natural mating position -- taking this position shows any males in the vicinity that she is receptive to their advances, and essentially ready to mate when they are. It is the clearest invitation she can send to a male, and she will do so even if she has already mated during the heat cycle.
Intact, a cat may spray urine throughout the year to mark her territory, but a cat in heat does so with a more specific intent. A female cat in heat habitually sprays urine on vertical surfaces not so much to mark them as hers but to advertise her availability as a potential mate. The pheromones in her urine change while she's in heat, so a cat investigating the mark can identify that it was made by a female looking for a partner. By advertising her reproductive status like this, she hopes to attract a male, and she has to keep her tail elevated to effectively spray her urine.
A female cat in heat is notably more affectionate not only to other cats but to her owners, as well. When you respond to that attention with affection of your own, it can trigger an amorous reaction in your cat. This is biological more than psychological. Because your cat is so receptive to mates during heat, petting her can cause her instinct to kick in and send her into the mating position. Stroking her back, in particular, triggers a reaction that includes lifting her hindquarters and elevating her tail.
Length of Cycle
Your cat's tail-lifting behavior depends on the stage of her heat cycle. The first part of her cycle, proestrus, lasts only a day or two. While she may urine-mark during this period, she is not ready for the act of mating. The second stage, typically known as estrus, or being in heat, is when she's sexually receptive and will walk around with her tail held up. This period lasts between four and six days, during which time she will be more affectionate, more vocal and more restless than usual.