The M in M04 and Why Dewpoint Spread Matters

Near the end of a METAR, just before the pressure, sits a small two-number group like 02/M01 or 24/19. M04 means minus 4 degrees Celsius, and a narrow temperature-dewpoint spread warns of fog or low ceilings forming. The two numbers are the temperature and the dewpoint, both in whole degrees Celsius, and wxdecoder splits them into labeled lines and computes the spread for you. The gap between them is one of the most useful single numbers in the whole report.

This guide covers the T/D format, the M-for-minus prefix, what the spread tells you about humidity and fog, the cold-weather icing setups, and where to find the temperature to a tenth of a degree. It is part of the METAR field guide.

The format: whole degrees Celsius, slash-separated

The temperature and dewpoint group follows a fixed template:

T/D              both values in whole degrees Celsius
24/19            temperature 24C, dewpoint 19C

The number before the slash is the air temperature. The number after the slash is the dewpoint, the temperature to which the air would have to cool for the moisture in it to start condensing. Both are reported in whole degrees Celsius, never Fahrenheit, and never with a decimal point in this group.

So 15/08 is a temperature of 15C with a dewpoint of 8C, a 7-degree spread. The two values can never cross: the dewpoint is always equal to or lower than the temperature, because air cannot hold less moisture than it already contains.

M means minus

Below zero, the METAR cannot use a negative sign without confusing the parser, so it uses the letter M as a prefix instead. M stands for minus.

• 02/M01 is a temperature of 2C and a dewpoint of minus 1C.
• M04/M07 is a temperature of minus 4C and a dewpoint of minus 7C.
• M01/M01 is minus 1C with the dewpoint right on top of it, fully saturated air.

This is the single most common misread in the group. M04 is not 4 below the dewpoint or some coded value: it is simply minus 4 degrees Celsius. When you paste the report into the decoder, each value is shown with its sign spelled out so there is nothing to misinterpret.

The spread is the humidity story

Subtract the dewpoint from the temperature and you get the temperature-dewpoint spread. That single number is a proxy for relative humidity:

• A wide spread (say 10C or more) means dry air, far from saturation.
• A narrow spread (1 to 2C) means the air is nearly saturated. Relative humidity is high, and only a small amount of cooling is needed to push it to 100 percent.

When the temperature and dewpoint meet, the air is saturated and moisture condenses out as fog, mist, or cloud. That is why a tight spread is a forecasting clue you can read straight off the surface observation.

A narrow spread plus cooling is a fog and low-ceiling setup

The spread matters most when you know which way the temperature is heading. A 2-degree spread at midday with the air warming is unremarkable. The same 2-degree spread near sunset, with the surface cooling overnight, is a classic radiation-fog setup. As the temperature falls toward the dewpoint, the gap closes and fog or low stratus can form quickly.

This is where the group connects to the rest of the report. Forming fog and lowering cloud can drop visibility and ceiling fast, and that is exactly what moves a field through the VFR, MVFR, IFR, LIFR thresholds. A field reporting 12/11 at dusk is telling you the margin is thin: a single degree of cooling could mean the difference between a clear evening and an indefinite ceiling reported as vertical visibility. Watching the spread tighten across successive hourly reports is one of the earliest signs that conditions are about to deteriorate.

Near-freezing temperatures and precipitation mean icing

The temperature group also flags cold-weather hazards. When the temperature is at or near 0C and precipitation is present or expected, the setup favors icing. Freezing rain and freezing drizzle, coded with the FZ descriptor as in FZRA, freeze on contact with the airframe and the runway. The temperature group is your first confirmation that the air is cold enough for that to happen.

A report like M02/M03 FZRA is unambiguous: the air is below freezing and freezing rain is falling. For how the present-weather codes describe freezing and frozen precipitation, see decoding present weather such as FZRA and snow. The temperature group and the weather group are read together: the weather tells you what is falling, and the temperature tells you whether it will freeze on your aircraft.

Worked examples

The same T/D group implies very different situations depending on the spread and the temperature:

Read across the table and the pattern is clear: the smaller the spread, the closer the air is to producing fog or cloud, and the lower the temperature, the more any precipitation tilts toward an icing concern.

Where the precise value lives

The main group rounds to whole degrees, which is fine for situational awareness but hides borderline cases. The exact temperature and dewpoint to a tenth of a degree are encoded in the remarks, in the T group. There, the leading digit of each block is the sign (0 for plus, 1 for minus) and the remaining digits are the value in tenths. For the full decode of that group and the rest of the coded remarks, see inside METAR remarks. When the field temperature is sitting right at freezing, that extra tenth of a degree can be the detail that settles whether precipitation falls as rain or as ice.

Read the spread at a glance

The group is short but it carries two of the most operationally useful numbers in the report: how cold the air is, and how close it is to saturation. Remember that M means minus, that both values are whole degrees Celsius, and that the gap between them is your fog, low-ceiling, and humidity warning. Paste any raw report into wxdecoder to see the temperature, dewpoint, and spread broken out and labeled, then read the flight categories to see how a closing spread can move a field toward IFR. wxdecoder is for situational awareness, not an official briefing.

Last updated June 29, 2026