What does "SLP 987" represent?

The notation "SLP 987" refers to sea level pressure measured in hectopascals (hPa) or millibars (mb), and it indicates a specific atmospheric pressure value. To decode "SLP 987," it is essential to recognize that the number provided (987) indicates the pressure in tenths of millibars.

When interpreting this value, the way it is expressed can lead to the actual sea level pressure being calculated. The formula used is that you take the value of "SLP" and add 1000 mb. Therefore, for "SLP 987," the computation is as follows:

1000 mb + (987 / 10) = 1000 mb + 98.7 mb = 1098.7 mb.

However, when the formatted number is less than 1000, it reflects the pressure below 1000 mb. In this case, "SLP 987" indicates a pressure of 998.7 mb, as the actual conversion requires you to use the value as tenths relative to 1000 mb, thus leading to a value of 998.7 mb.

This understanding confirms that the indicated sea level pressure in this instance is indeed 998.