What is the sea level pressure indicated by SLP270 in the METAR?

The designator "SLP" in a METAR report stands for "Sea Level Pressure," and it is typically reported in hectopascals (hPa). The following digits represent the pressure value, but they can be a bit misleading at first glance.

When you see "SLP270," the "270" indicates the pressure in a specific format. To convert this properly into a hPa value, you must consider that the pressure is indicated as a three-digit number plus 1000 hPa. This means you add 1000 hPa to the number represented by these last three digits (in this case, 270).

So, the calculation would go as follows: 1000 hPa + 270 hPa = 1270 hPa.

However, since standard sea-level pressure is typically expressed relative to atmospheric conditions at sea level, the focus shifts to recognize that "SLP" represents pressures below 1000 hPa, thus requiring a subtraction.

In this METAR format, when "SLP" is followed by digits exceeding 1000 (like the "270"), it reflects two full integers indicating hPa. Therefore: 1000 - 270 = 1027.0 hPa