A warm front is the arrival of relatively warmer conditions, along with moisture and humidity, which is replacing the cold air as it moves around the planet. That begins the sequence of events when sees the cold front replace the warm front with the opposite conditions, namely colder, drier air, with good visibility, and often sunny, or at least clear air at night-time.
In between the two, you get what’s called the warm sector, which is often dull, damp, drizzly air, with poor visibility. It varies depending on the strength of the front. For example, it’s very rare for a warm front to bring blue skies, though it’s more so in summertime when the front is weaker. When there’s hot, sunny weather, there’s no front involved, you’re just under high pressure, or between systems, so there is no frontal activity. But high pressure doesn’t always mean high pressure, it can give you anti-cyclonic gloom, especially in wintertime. In other words, slate-grey cloud that goes nowhere fast, and changes will be slow.
The warm-cold cycle can get a bit complicated with what’s called an occluded front, when cold air and warm air are caught up together, where one has over-ridden the other and lifted it off the ground, and it’s just a big mass of grey, when it’s hard to know what’s going on, except it will be dull, and often wet. When the occluded front is slow to change, it’s called a quasi-stationary front, nothing moves it on, because there’s not enough wind in the upper levels of the atmosphere, or there are wind forces pushing in opposite directions.