You create them in Map.ini files ;)

This is for snow:

Weather
SnowEnabled = yes ; enable/disable snow on the map.
SnowTexture = ExSnowFlake.tga ; texture used for each snow particle.
SnowBoxDimensions = 100 ; width/height of box around camera containing snow.
SnowBoxDensity = 1 ; amount of snow particles per world-unit. Raise to increase particle count.
SnowFrequencyScaleX = 0.0533 ; speed of side-to-side wave movement.
SnowFrequencyScaleY = 0.0275 ; speed of side-to-side wave movement.
SnowAmplitude = 4.0 ; amount of side-to-side movement.
SnowVelocity = 3.0 ; speed at which snow falls
SnowPointSize = .16 ; scale the size of snow particles.
SnowMaxPointSize = 10.0 ; maximum pixel size of point sprite particles (min-spec requires <= 64).
SnowMinPointSize = 0.0 ; minimum pixel size of point sprite particles.

;Some video cards can't do hardware accelerated particles. Set this
;setting to "no" in order see what emulation will look like. Use the
;SnowQuadSize setting to make it look similar to PointSprite version.

SnowPointSprites = yes ;set to "no" for point-sprite emulation.
SnowQuadSize = 0.5 ;scale the size of snow particles when using point-sprite emulation.
End


This is for rain! ;)

Weather
  SnowEnabled = yes             ;enable/disable snow on the map.
  SnowTexture = EXRainDrop.tga  ;texture used for each snow particle.
  
  SnowBoxDimensions = 100       ;width/height of box around camera containing snow.
  SnowBoxDensity = 1            ;amount of snow particles per world-unit. Raise to increase particle count.
 
  SnowFrequencyScaleX = 1       ;speed of side-to-side wave movement.
  SnowFrequencyScaleY = 1       ;speed of side-to-side wave movement. 
 
  SnowAmplitude = .1            ;amount of side-to-side movement. 
  SnowVelocity = 25.0           ;speed at which snow falls
  SnowPointSize = .3            ;scale the size of snow particles.
 
  ;Some video cards can't do hardware accelerated particles. Set this
  ;setting to "no" in order see what emulation will look like. Use the
  ;SnowQuadSize setting to make it look similar to PointSprite version.;;;

  SnowPointSprites = yes   ;set to "no" for point-sprite emulation.
  SnowQuadSize = .1        ;scale the size of snow particles when using point-sprite emulation.
End

Originally Posted by: Zero Hour Mad Map Maker