Raging Nimbus reduces the movement speed of all opponents who at least once passed through its area of effect for the entire duration of the spell. The slowing effect does not disappear even if the opponent leaves the area of the blizzard.
The Roar modification reduces the opponents' attack value in pulses - for one second per pulse. It means that the effect itself lasts for only a second and is renewed every second for every opponent in the area of effect. Therefore, all enemies who stay in the blizzard have their attack values reduced for the duration of the spell, while on those who leave the area the effect dissipates in a second.
The Roar mod uses a particular formula to calculate the attack value reduction: 1-1/(1+(500+20*CA lvl)/1000). For level 1, this gives 1-1/(1+(500+20*1)/1000) = 34.2% reduction, level 10 gives 1-1/(1+(500+20*10)/1000) = 41.2% reduction, level 50 gives 1-1/(1+(500+20*50)/1000) = 60% reduction.