In physics and meteorology, which SI unit is commonly used to measure air pressure at the Earth surface and in the atmosphere?

Introduction / Context:
Air pressure is a fundamental concept in physics, meteorology and engineering. It tells us how much force the air exerts per unit area on surfaces such as our skin, buildings and the ground. The SI system of units provides a standard way to express this quantity. This question checks whether you can correctly identify the Pascal as the SI unit of pressure, including air pressure.

Given Data / Assumptions:

• Pressure is defined as force per unit area.
• The SI unit of force is the Newton (N).
• The SI unit of area is the square metre (m²).
• Options include steradian, kg/m², Newton and Pascal.

Concept / Approach:
By definition, pressure P is given by P = F / A, where F is the normal force applied on a surface and A is the area. In SI units, force is measured in Newtons and area in square metres, so pressure is measured in Newtons per square metre (N/m²). This compound unit is given the special name Pascal (Pa) in honour of Blaise Pascal. Other units such as bar, atmosphere and millibar are also used in meteorology, but the fundamental SI unit is the Pascal. A steradian, by contrast, is a unit of solid angle, and Newton is a unit of force, not pressure.

Step-by-Step Solution:
Step 1: Recall that pressure P equals force F divided by area A, so P = F / A.Step 2: Identify that in the SI system, F is measured in Newtons (N) and A in square metres (m²).Step 3: Combine these units to get the unit of pressure as N/m².Step 4: Remember that 1 Pascal (Pa) is defined as 1 N/m².Step 5: Recognise that air pressure, water pressure and many other pressures are expressed in Pascals or multiples such as kilopascals.Step 6: Conclude that Pascal is the correct SI unit of air pressure.

Verification / Alternative check:
Weather reports and scientific publications often express atmospheric pressure in hectopascals (hPa) or kilopascals (kPa), which are multiples of the Pascal. Engineering handbooks also specify design pressures in Pascals or bars, where 1 bar equals 100,000 Pa. The fact that these are standard SI-based units confirms that Pascal is the correct base unit for air pressure.

Why Other Options Are Wrong:
Steradian is a unit of solid angle used in three-dimensional angular measurements and has nothing to do with pressure. A quantity like kg/m² without including acceleration due to gravity is not the correct SI unit for pressure, because pressure must be expressed as force per area, not mass per area. Newton is the unit of force, and while it appears in the definition of pressure, it is not itself a unit of pressure.

Common Pitfalls:
Students sometimes confuse Newton and Pascal because both appear in mechanics and may think that any quantity involving forces must use Newton as its unit. Others might recall non SI units like atmosphere and bar and forget the formal SI name. To avoid confusion, always remember that pressure is measured in Pascals, defined as Newton per square metre.

Final Answer:
Air pressure in the SI system is measured in Pascals, which equal Newtons per square metre.