Fields
A field is a volume or area of space within which a condition exists.
For example, 12(field,heat) represents a volume within which the heat rating is 12. If its a spherical volume with a radius of 5 metres, we express it as:
5(radius) 12(field,heat)
If we want to say that the volume is shaped into a spray or a cone, then we use:
15(spray) 12(field,heat)
10(cone) 12(field,heat)
The ambience and emissions discussed in senses are both field effects, as is the spray of buckshot from a shotgun.
If the field is small enough in volume that you could reach a boundary within a round (the boundary is closer than your move speed), and there is area outside the field available (you are not in a confined space), then you always get a sv(Agility) against the challenge of the effect to escape it for half damage.
Intensity of Fields
In the property description above:
5(radius) 12(field,heat)
The number 12 is the intensity of the field. This is generally a simple number representing the number of health points you would suffer if you spend part of a round within the effect area. Unless otherwise noted, the save challenge is equal to the field strength. So if you have movement enough to potentially escape the area of 5 metres radius in the above example, then the challenge is 12, and it would be 12(sv,Agility) to halve the effect damage to 6.
Shapes of Fields
The example of the spherical field presented above is one of several shapes that an effect can fill. Two other common shapes are sprays and cones.
Spray
A spray is typified by the fire from a flame thrower, the water from a fire hose, the buckshot from a shotgun, and many other similar effects. The spray occupies a quarter of a square immediately in front of the source of the effect, then expands to 1 metre at 1 metre distance and remains 1 metre until 10 metres distance, whereupon it expands to 2 metres for the next 10 metres, and so on.
The overall length or range of the effect is noted in the effect property:
15(spray) 12(field,heat)