The atmospheric pressure in a location (as measured by a barometer) is the amount of force per unit area applied on an object by the air particles randomly bumping into it. The pressure is highest at ground level and decreases with altitude as less particles exist, or they have less energy (for movement).

The atmospheric pressure is not constant, though the international standard atmospheric pressure is 1013.25 hPa (hectopascals) at mean sea level. You will notice when looking at any pressure chart that the pressure varies significantly from place to place, but these variations change over time, sometimes slowly (taking many days to change even by 1 hPa) or sometimes quickly (pressure changing every 30 minutes).

The pressure pattern dictates the wind, which is always attempting to remove any pressure differences, however due to the Coriolis effect of the spinning Earth, this never happens. The pressure patterns are therefore constantly changing, dependent on the winds (including jetstreams at high level), and weather systems in the region, though almost all of these variation happen originally because of temperature differences. Whilst the pressure changes are immensely complex, the original reason behind all of them is temperature variations, of the surface and the air mass, due to solar heating, cloud cover, and the thermal properties of the surface (rocks, sand, trees, water, etc.).

Therefore, in answer to this question (which is not as simple as it looks), we know that the Sun is a key influencer of atmospheric pressure changes, however, the variations are certainly irregular (not the same or working in cycles, etc.), because of the other influences (different surface types, Earth spin, and the weather systems that are already in place).

Going through the options:

• "due to weather systems and the daily heating effect of the Sun."
  • CORRECT --> As discussed above, the Sun is the biggest sole influencer of pressure patterns, but the weather systems also have an important role, both changing the pressure patterns, but also themselves being affected by the pressure pattern changes; they are very closely linked together.
• "irregular, due to frontal systems."
  • INCORRECT --> Whilst the pressure changes are irregular, it is not just frontal systems that cause them.
• "regular, due to the daily heating effect of the Sun."
  • INCORRECT --> Pressure changes are not regular at all, though they are highly influenced by the heating of the Sun.
• "affected only by the terrain."
  • INCORRECT --> The terrain does affect pressure changes but is not the sole reason behind their existence