Trace (precipitation)
In meteorology, a trace denotes an amount of precipitation, such as rain or snow, that is greater than zero, but is too small to be measured by standard units or methods of measurement. The designation of a trace rather than zero is used to indicate that precipitation did fall, but not enough to be measured reliably. This is important for both weather forecasting and climatological purposes, because even precipitation amounts too small to be measured can have significant societal impacts.
Definitions
The term "trace" is used in two different but related contexts. The first is in weather forecasting and record-keeping of rain, snow, and other precipitation, where a trace denotes an amount of precipitation that is greater than zero, but is too small to be measured by standard units or methods of measurement. This can be as little as just a few raindrops or snowflakes, or be enough to wet or coat the ground, but will not be enough to register via standard measurements with a rain gauge or other measuring device. The second is in the context of snowpack depth, or the amount of snow on the ground at a given time. If less than a measurable amount is present on the ground, or if less than half of the ground is covered with snow, this can be denoted by a trace.A trace is usually indicated by a capital letter "T" or the word "trace" in place of a numerical amount of accumulation. A trace measurement is not usually considered equivalent to any numerical value, and so adding together several trace amounts will still be considered equal to a trace in most cases.
For frozen precipitation, a trace can indicate a very light accumulation, or it can indicate a larger amount of snowfall, ice pellets, or other frozen precipitation that is continuously melting as it hits the ground. A trace of snow is sometimes referred to as a "dusting".