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.
Reducing attack value modifications for HE's CAs "Frost Flare" and "Raging Nimbus" are added with each other multiplicatively (layerwise). For example, 120 CA level "Frost Flare" and 105 CA level "Raging Nimbus", used on the same enemy, will reduce target attack value by 66% at first and by 61% secondly. The final target attack value = x*0.34*0.39 = 0.13*x Due to the peculiarities of a chance to hit calculation this means, that HE will be hit by enemies 3-4 times less. To effect stacking you must use CAs with a difference of not less than 0.1 CA level.