What Is a "Trace" of Precipitation?

In meteorology, the word "trace" is used to describe a very small amount of precipitation that results in no measurable accumulation. In other words, a 'trace' is when you can observe that some amount of rain or snow fell, but it was not enough to be measured using a rain gauge, snow stick, or any other weather instrument.

Since trace precipitation falls as very light and brief sprinkles or flurries, you often won't know it unless you happen to be outdoors and see or feel it falling. 

• Trace amounts of precipitation are abbreviated by the capital letter "T", often placed in parenthesis (T).
• If you must convert a trace to a numerical amount, it would equal 0.00.

Rain Sprinkles and Drizzle

When it comes to liquid precipitation (rainfall), meteorologists don't measure anything under 0.01 inch (one hundredth of an inch). Since a trace is anything less than can be measured, anything less than 0.01 inch of rain is reported as a trace of rain.

Sprinkles and drizzle are the most frequent types of rain that result in immeasurable amounts. If you've ever seen a few random raindrops dampen the pavement, your car windshield, or felt one or two dampen your skin, but a rain shower never materializes — these, too, would be considered trace rainfall.

Snow Flurries, Light Snow Showers

Frozen precipitation (including snow, sleet, and freezing rain) has a lower water content than rain. That means that it takes more snow or ice to equal the same amount of liquid water that falls as rain. This is why frozen precipitation is measured to the nearest 0.1 inch (one tenth of an inch). A trace of snowfall or ice, then, is anything less than this.

A trace of snow is commonly called a dusting. 

Snow flurries are the most common cause of trace precipitation in winter. If flurries or light snow showers fall and it doesn't accumulate, but continuously melts as it reaches the ground, this would also be considered trace snowfall.

Does Moisture From Dew or Frost Count as a Trace?

Although fog, dew, and frost also leave behind light moisture, surprisingly none of these are considered examples of trace precipitation. Since each result from the process of condensation, none are technically precipitation (liquid or frozen particles that fall to the ground). 

Does a Trace Ever Add up to a Measurable Amount?

It's logical to think that if you add up enough tiny amounts of water you will eventually end up with a measurable amount. This is not so with precipitation. No matter how many traces you add together, the sum will never be more than a trace.