The Redness of the Sky Before Sunrise

Before the sun is visible, the sky is dark. This darkness is known as night. Between 24 and 0 minutes before sunrise, also called civil twilight, it becomes a bit more bearable, as you can still see without artificial lights. However, between 24 and 48 minutes before sunrise, things shift to a period called nautical twilight. Here, you can barely discern landmarks without using lights. By 48 minutes, the position of the horizon becomes uncertain for sailors, adding an extra layer of complexity to navigation.

Rayleigh Scatter

The redness of the sky before sunrise is largely due to Rayleigh scatter, a phenomenon named after the nineteenth-century British physicist Lord Rayleigh. This occurs when sunlight interacts with particles in the atmosphere that are much smaller than the wavelength of light. As the sun approaches the horizon, its light must pass through more of the Earth's atmosphere, which causes shorter wavelengths (like blue and green) to scatter more. This leaves only the longer wavelengths, such as red and orange, to reach our eyes, creating the familiar red sky at sunset and before sunrise.

Auroras and Precipitation

The redness of the sky before sunrise can also be attributed to other atmospheric phenomena, such as auroras. A red sunrise might indicate a high pressure system that has already passed, potentially suggesting that a low-pressure system, often accompanied by storms, may be moving towards the east. Additionally, a deep fiery red morning sky can indicate high water content in the atmosphere, signaling that rain could be on its way.

Twilight Defined and Its Legal Implications

Civilian twilight is defined as 30 minutes before and 30 minutes after sunrise or sunset for traffic purposes in the U.S., where headlights must be turned on. This is measured at a 6-degree angle from the horizon. Astronomical twilight, on the other hand, occurs when the sun is 18 degrees below the horizon, which translates to approximately one hour and 12 minutes if the sun were to travel straight down. However, due to the curvature of the Earth and the tilt of the axis, the time required in the "summer" hemisphere is longer.

Air Travel Regulations

Aircraft operators in the U.S. can log flights as happening in night conditions based on civilian twilight. However, passenger-carrying flights must comply with strict regulations, requiring pilots to have completed at least three full flights in true night conditions (at least 60 minutes after sunset) within the last 90 days. This ensures that pilots are prepared for nighttime operations, especially in low-light conditions.