Female cats in heat
Female cats are seasonal polyestrous of induced ovulation. This means that female cats exposed to natural light won't be in heat during winter months. However, if a female cat is living indoors under artificial light and a heating system, it might be in heat the entire year. Induced ovulation means that they do not ovulate until copulation happens. This ensures pregnancy after copulating with a male cat.
The way of knowing if our female cat is in heat is through its behaviour. Two days before, during proestrus, they are overly affectionate. They rub themselves against everything and they begin vocalizing differently.
During the period of oestrus, which lasts for 6 days as a maximum or until the moment of copulation, this behaviour is enhanced. Some owners can't live with such a high pitch of continuous vocalizing from the female cat. If there hasn't been any copulation, this process will be repeated every 21 days, as long as it is the correct time of the year.