Does Dim Light created by an effect override Bright Light in an area?

Question

Two examples.

A silvery beam of pale light shines down in a 5-foot-radius, 40-foot-high cylinder centered on a point within range. Until the spell ends, dim light fills the cylinder.

—Moonbeam, Player's Handbook, pg. 261

You create up to four torch-sized lights within range, [...] each light sheds dim light in a 10-foot radius.

—Dancing Lights, Player's Handbook, pg. 230

Now consider that each of these spells are cast on a bright day, obviously covered by Bright Light, as the game understands it.

1. Do these spells change the Bright Light within the area affected by them into Dim Light?
2. Is the behavior different between Moonbeam ("Dim Light fills the cylinder") and Dancing Lights ("each light sheds dim light"), or do both of them behave the same?

Answer 1

Spells override Bright Light with Dim Light if the spell description says that it does.

Spells do what they say.

Dancing Lights states that "you create up to four torch-sized lights within range."

Moonbeam states that "until the spell ends, dim light fills the cylinder."

The light sources shed dim light, but nothing says this is a special type of dim light, or that the light is part of the spell's effects.

Moonbeam specifically indicates that the area of the spell is filled with dim light.

If the light from Moonbeam were overridden by the area being normally filled with bright light, this would make the spell's area invisible which seems counter to the intent.

So to answer the questions about your examples directly:

1. Do these spells change the Bright Light within the area affected by them into Dim Light?

Moonbeam does, Dancing Lights does not.

2. Is the behavior different between Moonbeam ("Dim Light fills the cylinder") and Dancing Lights ("each light sheds dim light"), or do both of them behave the same?

The behavior is different between the two spells.

Answer 2

Unless the spell, item, ability, etc mentions it specifically changing the level of light in a area/radius, then I would say no, it does not change it. Just think about turning on a dim flashlight outside during the day. It is still emitting light, but it doesn't make a difference. Now take that same light and go into a tunnel or cave and the dim light matters.

Here is an example of an ability changing the level of light:

The Oathbreaker paladin's 20th-level Dread Lord feature:

At 20th level, the paladin can, as an action, surround himself or herself with an aura of gloom that lasts for 1 minute. The aura reduces any bright light in a 30-foot radius around the paladin to dim light.

And a spell:

Shadow of Moil:

The shadows turn dim light within 10 feet of you into darkness, and bright light in the same area to dim light.

To clarify, though: For some spells or effects, such as the Twilight Domain's Channel Divinity, it is possible to be in the area of dim light from a spell/effect and be affected by it even if you are in an area with bright light... Because magic in D&D does not follow our world's physics to a T.

Answer 3

Spells indicate if they reduce illumination

The illumination rules allow for multiple light sources, and one light source does not dim another.

Spells that reduce ambient light (as opposed to providing light) will say so. Examples:

• Darkness says: "nonmagical light can’t illuminate it"
• Tiny Hut says: "Until the spell ends, you can command the interior to become dimly lit or dark. The dome is opaque from the outside"

Moonbeam and Dancing Lights provide a light source, and do not say they reduce light. Spells like Holy Aura, Daylight and Sunbeam similarly state that they "shed dim light". These effects (and other light sources) are not 'invisible' in bright light, even if they don't alter local illumination levels.