The sun has set. The sky is not black yet. It glows a deep, saturated blue, and city lights are coming on. That is the blue hour, a short window of twilight when the light is cool, soft, and even. It happens twice a day, just before sunrise and just after sunset, and it lasts anywhere from twenty minutes to over an hour depending on latitude and season.

Blue hour is a favorite for cityscapes. The sky is bright enough to hold detail, and artificial lights are bright enough to register. The contrast between cool blue sky and warm tungsten windows is what makes the images work. The light is diffuse, so shadows are soft and colors are muted. Tripods are usually needed, because exposures run into seconds. White balance matters. Setting the camera to daylight or tungsten shifts the color of the sky. Many photographers leave it on auto and adjust in post.

What to shoot during blue hour

• Cityscapes. Skylines with lit windows and blue sky.
• Waterfronts. Reflections of city lights on calm water.
• Landmarks. Buildings and bridges with architectural lighting.
• Portraits. Soft, cool light for moody portraits.

The blue hour is short. Plan the shot before it starts, and be ready to shoot when the light peaks.