You mention a few different issues which I shall try to pull together:
snow, Zone system, spot meter. There are different phases of the whole Zone System too involved to discus here, now. Briefly, the Zone system is a process to reduce the subject luminance range to fit within the density (reflectance) range of a photographic print (reproduction).
Note also that the resulting Zone system negatives are used to make a "straight print;" one that requires little if any manipulation (masking, dodging, or burning-in detail). It is exposed, and processed according to a standardized procedure according to an initial calibration phase to determine actual film speed, standardized processing, and printing procedure along with a myriad of other variables.
Snow is not a trivial material to photograph. It is high luminance but has a narrow luminance range. It is crystalline and highly prone to specular high-lights. It can be dead-white line across a line of distant mountain peaks or it can be depicted as a full range of tones from deep texture to paper white You must determine how it will be rendered.
It is a fragile material in a dynamic natural harsh environment.
Conditions often change from the time you make readings and when you press the shutter release.
Pre-visualize your subject to determine what you want represented in your print.
Meter your scene, making notes of values for later calculation of the necessary processing for contrast control. (Normal, compression, or expansion.)
The Zone system assigns ten zones of subject luminance values to be represented as print values from:
Zone 0: Photo paper black; maximum black representing emptiness; nothing; openings into unlit rooms. Seems solid rather than mysterious.
Zone 1: Nearly black, the beginnings of a sense of empty space. Unlit rooms; forests; depths; shadows in dim light.
Zone 2: The first sense of an emergence of texture, Othe the border line between visible and invisible texture—mysterious.
Zone 3: TEXTURED SHADOW. Texture and detail are firm and full of the sense of substance in dark materials such as black garments leather. Visible creases and folds encourage the sense of touch.
Zone 4: Average dark foliage, dark stone, shadow in landscape or buildings. Recommended shadow zone for portraits in sunlight or floodlight.
Zone 5: The pivotal zone for both paper and film. Middle gray; 18% neutral test card. Dark or sunburned skin in sunlight; clear North sky on panchromatic film; average weathered wood or stone, light foliage.
Zone 6: Caucasian skin with sunlight, but not glare or highlights. Snow in shadow when both sun and shadow are in the same picture. Clear North sky with orthochromatic film. Poured concrete buildings in overcast light.
Zone 7: TEXTURED BRIGHTS. Light skin entirely in diffuse light. Average snow in raking sun; light gray concrete bright colours. "Whites" with textures and delicate values. Sense of substance remains tactile.
Zone 8: Last vestige of texture. Glaring surfaces; snow in flat light; whites without texture.
Zone 9: Threshhold gray into pure white of the paper. Tends to feel solid representing only itself. Loss of sense of substance. Represents highlights (as opposed to brights) such as chrome trim, or specular reflections.
Use the above "scale" to identify and visualize subject values in zones. Snow is mentioned specifically, here. Photographically, the difference from one value to the next IS EQUAL TO ONE FULL STOP DIFFERENCE IN EXPOSURE. This is fundamental to the understanding and using the zone system. (I checked my notes.)
Metering for the Zone system. (Are you batteries good - remember it's cold and you rely on the meter's accuracy.)
- When taking a luminance reading, whether it be an overall average or a closeup of a shadow area, cheekbone, or the highlight of a snowdrift, always point the meter along the lens-to-subject axis. Different angles of view may include reflection and brightnesses different from those seen from the point of view of the camera lens.
- Take every precaution to prevent the shadow of the meter, your hand, or your body from falling on the subject while the reading is being made, or a false reading will be obtained. With back-lighted subjects, be careful that your body does not act as a reflector filling in the shadow area as you meter it. Light sources should not be in included in the field of view of the meter.
- Consider carefully the acceptance angle and coverage. Spot meters allow as little as 1° of light to enter, and make readings of small or distant areas quite simple.
- Determining luminance values in fog or heavy mist is most accurate when the subject is metered from the camera position. The light falling on the subject (incident light) has been scattered by the fog. The luminance reflected from the subject is then further scattered and diffused as it travels to the camera position. A exposure reading made close to the subject will read he immendiate amount of diffuse light reflected from the subjct, but not the quantity that finally gets to the camera.
- A luminance meter is an instrument that gives a physical measurement of eth quantity of light flux reflected from various areas of a scene. The luminance measured is purely objective. How you decide to represent the luminances of a scene as print values is subjective.