Atmospheric Pressure

Atmospheric pressure and relative humidity

Formation of precipitation requires a 4-stage process:

1. Saturation of air by cooling to reach dew point temperature
2. Condensation of water vapor to form ice crystals or droplets
3. Growth of droplets or crystals into raindrops, snowflakes, or hailstones, to a size large enough to fall to the ground before evaporating
4. Continuous input of water vapor to sustain the process (depending on vertical wind and humidity)

 

Relative humidity = Actual water vapor pressure / maximum water vapor pressure at a specific air temperature X 100

 

Uplifting mechanisms

Precipitation originates from different uplifting mechanisms:

1. Convective
   • Results from strong heating of air near ground (unstable) that expands, becomes lighter, and rises
   • Causes adiabatic cooling
   • Cells typically 10s of km apart
   • Precipitation is intense, heavy, and short-lived
   • Associated with thunderstorms and hail
   • Highly patched rainfall patterns; might not be detected by rain gauge
2. Convergent (frontal or non-frontal)
   • Frontal:
     • Warm front
       • Warm air is less dense than cold air and easily slides up gradually pushing the cold air out of the way.
       • In BC, comes from the ocean
       • precipitation Covers large areas, is not intense, and longer lasting
     • Cold front
       • Cold air is denser than the warm air mass it replaces. As the cold air moves into the warm air, it forces it to rise quickly.
       • Results in deeper clouds and heavier precipitation than in warm fronts
       • Precipitation covers a smaller area and is more intense
3. Orographic
   • Air moving horizontally hits a topographic barrier
   • Hills and mountains deflect air upwards

Errors in ground-based measurements

• Wind: increased wind results in decrease in catch
• Evaporation and wetting: Handling traces is problematic
• Splashback: Gauges are usually mounted 1.5 - 2 m above ground the minimize splash but mounting closer to ground reduces wind effects
• Location of gauge: observe angle and distance to obstructions

Spatial interpolation

Process of using points with known values to estimate values at other points