rain_modify
rain_modify is a point entity available in the following 
GoldSrc games or engine branches:
Paranoia and 
Cry of Fear. Which is used to directly turn on and off the rain.
Important:It is possible to simply set constant rain on the map, without switching on and off - for this, do not specify the rain_modify name.
Keyvalues
- Fading time
(m_flTime)<integer> - The time it takes for rain to acquire the specified settings. Set in seconds. This is to smoothly turn the rain on and off.
Tip:A minute or two is fine (anyway, in action games always develop faster than in real life, because the genre obliges)
- Drips per second
(m_iDripsPerSecond)<integer> - The number of drops that appear per second.
Tip:Normal such rain - somewhere around 2000 drops.
- Wind X
(m_flWindX)<integer> - Wind deflection
- Wind Y
(m_flWindY)<integer> - Wind deflection
Tip:100, 200 - already a decent "oblique" rain.
- Rand X
(m_flRandX)<integer> - Wind deflection spread for each drop.
- Rand Y
(m_flRandY)<integer> - Wind deflection spread for each drop.
Example:if the wind is worth 200 and the spread is 40, then for each drop the deviation will vary from 160 to 240.