When entering a structure or initiating fire attack, knowing the wind direction is a critical fireground factor. Winds greater than 5 mph will directly affect the rate at which the fire develops and potentially jeopardize the safety of the advancing crew(s). Knowing whether the wind is at your back or if you are going against the wind is a key size-up factor. Entering into the wind is like fighting your way down a chimney. As the winds increase, the fire growth and speed of spread will increase, too.