Weather

Weather is a feature introduced in the Fallout: New Vegas add-on Honest Hearts. During the gameplay, the player character can face rain during their travels around Zion Canyon. In the games preceding this the standard unchanging weather conditions were sunny or overcast, however, using weather as a feature in the gameplay itself was originally considered in Van Buren, the canceled Fallout 3 by Black Isle Studios, later becoming more immersive for Fallout 4 and Fallout 76.

Fallout
Some Adytum citizens comment on the likelihood of rain soon, others note how previous rainfalls have been "glowing."

Fallout 2
In 2241, the Northern California area is suffering through a drought, the worst dry season in years, following an unusually short rainy season. Despite being relatively sustainable communities, Modoc and Arroyo were hit particularly hard. This extreme meteorological occurrence, which would go on for seven years, dominated the fate of some wasteland inhabitants.

A random encounter with a weary fisherman reveals there is apparently a regular "season of storms," not seen during the games timeframe.

Fallout: New Vegas
The sound of thunder can be heard upon fast travelling to, or just standing around, Black Mountain.

Fallout 4
Fallout 4, compared to the previous games has a much more immersive real-time weather system. The world alternates between clear skies, assorted degrees of overcast, fog, and even rain. The length of these varies and progresses naturally as the day goes on. There is a chance for thunder (not related to the radiation-emitting lightning from radiation storms) while it is raining at night. Rain is harmless in-game.

On occasion, a relatively more dangerous radiation storm may ensue. These storms are brought in from the Glowing Sea and temporarily go over the majority of the Commonwealth. During this time, the outside world will gain a green tinge, and occasional flashes of lightning will inflict radiation damage to the Sole Survivor. Although lightning strikes, it doesn't rain during radiation storms. Radiation storms last for about two in-game hours and are comparatively rare in terms of weather conditions.

Weather can be changed using weather ID console commands.

Fallout 76
Fallout 76 built upon and expanded the real-time weather system from Fallout 4. The introduction of an improved dynamic lighting model and subsurface scattering further enhances the appearance of weather effects compared to previous games.

In addition to overcast, fog, and rain, the game introduced ash storms and radioactive, post-nuclear dust clouds. Rain clouds and radstorms in Fallout 76 move across the map and can be seen from a distance. Each region of Appalachia has different chances of weather types occurring.

Ash storms occur within the Ash Heap, primarily near Mount Blair. A heavy grey haze blankets the area, notably lowering visibility. This effect is amplified when facing toward the sun and looking outside from interiors.

Post-nuke dust clouds are localized within blast zones, the player character-designated areas targeted by a nuclear missile. The area is enclosed in a yellow-orange haze with occasional flashes of lighting. The contaminated environment inflicts severe radiation damage without the proper protection, causing death within seconds. The area can be seen from a distance as a swirling light-brown dust cloud.

Van Buren
Boulder is subject to frequent bouts of acid rain, something for the player character to be aware of when traversing this area.

The Blackfoot tribe rely on the weather generated by the surrounding mountains for water, they collect rain and the winter snow.

The eastern fringes of Denver are regularly caught up in seasonal radioactive dust storms whipped up in the Midwest, making travel to certain parts of the wasteland inaccessible.

Project V13
Concept art featuring Seattle shows it, in what one would assume is winter, covered in snow suggesting regular snowfall.

Gallery

 * Fallout 76