What is Semi-Diurnal Variation Of Atmospheric Pressure?

When comparing a number of barograms of mid-latitude observations. It is particularly noticeable that there is neither a regular pattern nor a mean pressure value evident. However. in low latitudes a more regular pattern is usually evident. During a 24 hour period there are two maximum (at1000 and 2200 LMT) and two minimum pressure values (at 0400 and 1600 LMT) hence termed the semi-diurnal variation of atmospheric pressure.  The maximum pressure values occur at 0500 and 1700 GMT.