The 540 Line: A Rule Of Thumb Meteorologists Use To Determine The Snow Versus Rain Line

Fall is full of firsts, including the first frost, freeze and snowfall for some parts of the country.

But what I look for is the first signs of the so-called 540 line appearing on our forecast model map views in the U.S. and Canada, as we are seeing this week.

The 540 line is sometimes a good first estimate of where the rain versus snow line will set up in a storm system. It refers to the distance of 5,400 meters between two pre-determined pressure levels in the atmosphere, one near the Earth’s surface, or about the 1,000 millibar pressure height level, and another some 16,000-20,000 feet above the ground, or about the 500 millibar pressure height level.

The distance, or thickness, between those pressure levels varies, and is smaller in colder weather and larger in warmer weather. You can see this in the diagram below.

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In general, when the distance is 5,400 meters or less, then snow will occur. You can see this in the generic setup below, with the 540 line located north of the low-pressure system's warm front and behind its cold front.

O​f course, the 540 line can be located near or south of your location, making it theoretically cold enough for snow, but if there is no moisture source from a storm system, then it just remains dry.

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It’s not a perfect rule of thumb because of other influences. This week is a good example.

T​he model forecast map below shows the 540 line draped just south of the Great Lakes on the morning of Monday, Oct. 14. But in most of those areas outside of higher terrain, rain instead of snow was falling.

T​hat's because despite the thickness of the atmosphere being ripe for snow well above the ground by hundreds or thousands of feet, the air located near the surface where we live was too mild, or warmer than 32 degrees, meaning the snowflakes melt into raindrops. This can often happen both early in the fall and later in spring when the source of cold air is modest, but we even see this in midwinter on occasion.

S​o while the 540 line is a good first stab at determining the rain versus snow line, meteorologists don't stop there. They often look at several other model forecasting charts to further hone in on where snow versus rain will occur.

Chris Dolce has been a senior meteorologist with weather.com for over 10 years after beginning his career with The Weather Channel in the early 2000s.