I realized that this question seems to keep popping up. And that often a less effective setting is answered.
Maybe the issue is that the correction is too difficult to implement. Hence a step by step version.


To make snow:

Prop
Get a prop called Aircon. This prop is entire useless so great for things that are to be hidden and yet need a bit of moving about at first. Avoid dummy, it is near impossible to find or alter if some settings need correction.


Script Settings
Script: Attach Emitter.
Notice that below we follow the most logical sequence, not the sequence all settings are in.

Texture File:cum.png
Emitter Life: default (seems to have no measurable effect)


Particles per Second: 10. This initial setting prevent massive lag if other settings are wrong.

Shape of the snow
Start Scale: 0.1 (Scale of 0.05 is about the right size for rain bubbles. For Snow experiment with numbers between 0.3 and 0.05)
End Scale: 0.1
Enable Fade: UnChecked
Alpha Scale:1
Scale regulates how big the texture is going to be. Snow does not explode, it does not evaporate so we can save a lot of needless lag inducing action by fixing the same size and avoiding Fade.


Direction of the snow
Normal X: 0.05
Normal Y: -1.0
Normal Z: 0.05
The Normal is a geek word for vector or direction. The above means: Each second the snow will move a bit positive X ... a bit positive in Z and downward.
Snow in general falls with wind. If you are 'in the mountains' best use it to increase the feeling of height by having it fall more diagonal towards the valley below. This false horizon effect can be strengthened by slanting other objects too, like trees. Experiment with getting the fall direction correct by changing the X and Z.


Correct speed, height and lifetime of Particles

Start with fixing a nice speed. Rain looks natural between 600 to 1000 points/second. Snow gently floats between 50 and 200 points a second. Experiment till you have something that belongs to your zaby.
Speed: 100

Next thing to do is to figure out how long a snow particle will be visible. Since fade is off they jump into existence and should disappear just above the floor. Avoid getting snow through the floor since that creates a fair bit of extra work for the graphics card having to clip each snow particle. Best build in a buffer of about 100 points.

Example: Say your aircon is 3000 points above the floor. Say you used a speed of 100 points / second. In that case your snow should live for 29 seconds.

Example2: Say your floor is at 50000y. You like your snow to live for 20 seconds and float with 120 points/second. The height of the aircon should then be 50000 + 100 + (20 seconds x 120 points/sec) = 50100 + 2400 = 52500y

Adjust the above till you have the snow nicely falling as you like without getting through the floor.


Making the field
With the snow correct shaped and falling as intended we now can increase the density and size of the snow field.
Spawn Cube X: 10000
Spawn Cube Y: 0
Spawn Cube Z: 10000
Spawn cube deals with at what distance from the center snow can be generated. Notice Cube Y of zero is required to prevent snow falling through the floor.
Increase the Particles per Second up till 1000 or any amount that seems reasonable.


Integrating it all
Given that most likely you have some snow cabin in the mountains or so, you might need 4 basic fields. Best place them as follows











If your hut is not rectangle you might try a more advanced method with two types of snowfields. One nearby the house. This should fall more vertical, and possible live longer. By making snow larger scale you also benefit of an increased depth of field. The other could be as before.