Clouds of the Day - Spectacular Crepuscular Rays - Friday, September 29, 2023
/We woke up to Crepuscular Rays this morning. Shadows being cast by clouds create the dark banding across the sky. The lighter bands are sunlight being scattered by the air and small particles of dust or other matter in the air. The dark bands have less sunlight in them so there is less light to be scattered. The dark and light bands appear to radiate from the Sun but that is an optical illusion. The bands are really parallel. If you look down a road into the distance or down railroad tracks, the tracks appear to converge. The same is true of crepuscular rays.
Photos Copyright 2023 by Craig Johnson, Weather Briefing LC
Photo copyright by Craig Johnson, Weather Briefing LC, September 2023
Photo copyright by Craig Johnson, Weather Briefing LC, September 2023
The photo below shows Crepuscular Rays and a wider view of the cloud formation associated with them. The distinct cloud base is the condensation level. That is where rising motion has cooled the air enough to cause condensation. The condensation level was very distinct and at a uniform level.
Notice the individual cumulus cells along the cloud band. Condensation releases the latent heat of evaporation that evaporated the water in the first place. The process of evaporation and condensation are part of the Earth’s temperature regulation system. As water evaporates the heat that causes evaporation is stored in the water vapor, moving with the wind and cloud motions. Water vapor is invisible and it can be carried for hundreds or thousands of miles before the heat is released by condensation.
The stored heat does not raise the temperature of the air. It is called the heat of evaporation. It becomes the heat of condensation when it condenses and becomes a cloud. The cloud shape, form, and texture is determined by the air motion. In this photo the heat from condensation raised the temperature of the air causing it to rise in the shape of the cells that we can see. The cells are areas of stronger rising motion where the air accelerates upward because the air mass becomes unstable.
It is common for clouds that form in Iowa to come from moisture that evaporates from the Gulf of Mexico. It can remain invisible until it condenses here. This type of scenario is played out everywhere on Earth as heat is moved and released a long way from its source.
Photo copyright by Craig Johnson, Weather Briefing LC, September 2023
Photo copyright by Craig Johnson, Weather Briefing LC, September 2023
This color enhanced closeup photo shows a little more detail in the cumulus towers. It was one of the last photos taken when the Sun had moved slightly further into the southern sky. That allowed more shadows to be visible in the sunlit cumulus cloud tops. The enhanced color gave the cloud base a more distinct almost razor edge look.