Welcome to Weather Processes and Phenomena!

Hi there! This chapter is all about understanding how the air around us turns into clouds, fog, rain, or snow. It might seem like magic, but it’s pure science, and it’s fundamental to Core Physical Geography.
Don't worry if terms like 'sublimation' or 'orographic uplift' sound intimidating—we will break them down step-by-step using clear analogies. By the end, you'll be able to explain exactly why it rained today!

This topic forms part of the Core Physical Geography syllabus (Atmosphere and weather, section 2.3).

Section 1: Atmospheric Moisture Processes (The Six States of Water)

Water exists in three states: solid (ice), liquid (water), and gas (water vapour). Weather processes rely entirely on water moving between these states. This change of state requires either the absorption or release of latent heat (hidden heat).

1.1 Processes that Require Energy (Absorbing Latent Heat)

These processes cool the atmosphere or the surface because they are taking energy (heat) from the surroundings.

  1. Melting: Changing from solid (ice/snow) to liquid (water).
    Example: An ice cube left on a table starts to absorb heat from the air and melts.
  2. Evaporation: Changing from liquid (water) to gas (water vapour). This is the single largest source of atmospheric moisture.
    Example: Puddles disappearing after a rainstorm, or boiling water creating steam.
  3. Sublimation: Changing directly from solid (ice) to gas (water vapour), skipping the liquid phase entirely.
    Example: Frost on the ground slowly turning into vapour on a cold, sunny day without first turning into water.

1.2 Processes that Release Energy (Releasing Latent Heat)

When water vapour turns back into liquid or ice, it releases the heat it originally absorbed, slightly warming the surrounding air. This is crucial for keeping storms going!

  1. Freezing: Changing from liquid (water) to solid (ice).
    Example: Water turning into ice in a freezer.
  2. Condensation: Changing from gas (water vapour) to liquid (water droplets). This is the process that forms clouds, fog, and dew.
    Example: The tiny droplets forming on the outside of a cold glass of soda on a hot day.
  3. Deposition: Changing directly from gas (water vapour) to solid (ice/frost), skipping the liquid phase.
    Example: The formation of delicate frost patterns on windows when the air is very cold and moist.

Quick Memory Aid:
If the process involves Gaining Order (Gas -> Liquid -> Solid), heat is Released (Condensation/Freezing/Deposition).
If the process involves Losing Order (Solid -> Liquid -> Gas), heat is Required (Melting/Evaporation/Sublimation).

Key Takeaway for Section 1

The six atmospheric moisture processes are simply the state changes of water. They are critical because they either absorb or release latent heat, influencing atmospheric stability and temperature.

Section 2: The Causes of Precipitation

For precipitation (rain, snow, hail) to happen, two things must occur:

1. Air must be cooled to its dew point (the temperature at which the air becomes saturated).
2. This cooling causes condensation to form clouds (water droplets need tiny particles called condensation nuclei to condense onto, like dust or salt).
3. The tiny cloud droplets must grow large enough to fall as precipitation, overcoming air resistance.

The most common and effective way to cool air is to force it upwards. As air rises, it expands because the atmospheric pressure is lower higher up. This expansion uses up energy, causing the temperature to drop—this is called adiabatic cooling.

There are three main mechanisms that force air to rise and cause large-scale precipitation:

2.1 Convectional Uplift

How it works:

  1. The sun heats the ground intensely.
  2. The ground heats the air directly above it.
  3. The warm air becomes less dense (lighter) and rises rapidly in a strong column (a convection current).
  4. As the air rises, it cools adiabatically, reaching its dew point.
  5. Rapid condensation forms huge, vertical cumulonimbus clouds.
  6. This often leads to short, heavy bursts of rain, common in tropical regions or during summer afternoons (thunderstorms).
Analogy: It’s like boiling water—the hot air (or water) rushes upwards in a plume.

2.2 Frontal Uplift (or Cyclonic Uplift)

How it works:

This occurs when two air masses of different temperatures and densities meet—this meeting zone is called a front.

  1. A warm air mass (less dense) meets a cold air mass (more dense).
  2. The less dense warm air is forced to rise up and over the cold air mass.
  3. The uplifted warm air cools adiabatically, leading to widespread condensation.

This is the most common cause of precipitation in mid-latitude regions (like much of Europe and North America), often resulting in widespread, continuous rain or drizzle associated with depressions (low-pressure systems).

Did you know? The slope where the warm air rises over the cold air in a warm front is very gentle, which leads to layered (stratus) clouds and long periods of lighter rain.

2.3 Orographic Uplift

How it works:

  1. Moist air travelling across a landscape encounters a physical barrier, such as a mountain range.
  2. The air is forcibly pushed up the side of the mountain (the windward slope).
  3. As the air rises, it cools adiabatically, condenses, and forms clouds, leading to precipitation.
  4. Once the air passes over the peak and descends the other side (the leeward slope), it warms adiabatically (compression), creating a rain shadow region which is typically much drier.
Analogy: A cyclist hitting a ramp—the mountain forces the air upwards.

2.4 Radiation Cooling (Not a Cause of Uplift, but a Cause of Condensation)

While the three above cause precipitation via uplift, radiation cooling causes condensation (dew and fog) without rising air. This happens when the Earth's surface rapidly loses heat at night, cooling the lowest layer of air immediately in contact with it to the dew point. This commonly happens on clear, still nights.

Key Takeaway for Section 2

Most precipitation is caused by the adiabatic cooling of air that is forced upwards by one of three mechanisms: Convection (heat), Fronts (air mass collisions), or Orographic barriers (mountains).

Section 3: Types of Precipitation and Moisture Phenomena

Once condensation occurs, different forms of moisture phenomena and precipitation develop, depending on temperature and atmospheric processes.

3.1 Clouds

Clouds are visible masses of tiny water droplets or ice crystals suspended in the atmosphere. They form when saturated air cools below the dew point and moisture condenses onto condensation nuclei.

Quick Classification (The main two types):

  • Cumulus (Heaped): Clouds formed by vertical movement (convection). They look puffy and lumpy (e.g., Cumulonimbus, the huge storm clouds).
  • Stratus (Layered): Clouds formed by horizontal movement or gentle uplift (like in warm fronts). They are flat, layered, and often cover the whole sky.

3.2 Forms of Precipitation

Precipitation is any form of water falling from a cloud to the Earth's surface.

  1. Rain: Liquid water drops. If precipitation starts as snow/ice higher up, it must fall through a layer of air above freezing point to reach the ground as rain.
  2. Snow: Precipitation in the form of solid, hexagonal ice crystals. The air temperature from the cloud base down to the ground must be close to or below freezing \(0^\circ\text{C}\).
  3. Hail: Irregular lumps of ice. Hail forms inside large, powerful cumulonimbus (thunder) clouds where strong vertical air currents (updraughts and downdraughts) circulate frozen water pellets. Each circulation adds a new layer of ice, like building up layers of an onion.

Common Mistake to Avoid: Freezing rain is NOT hail. Freezing rain is liquid rain that freezes *on contact* with an extremely cold surface (like a road or tree branch).

3.3 Near-Surface Moisture Phenomena

These forms of moisture occur very close to the ground, usually caused by radiation cooling or the chilling effect of a cold surface.

  1. Dew: Liquid water that condenses directly onto surfaces (like grass or car roofs) when those surfaces cool below the dew point of the surrounding air, typically overnight.
  2. Fog: Essentially a cloud at ground level. Fog forms when the air near the surface cools to its dew point and condensation occurs.
Types of Fog Explained Simply
  • Radiation Fog: Forms overnight due to radiation cooling (surface loses heat rapidly). Common in valleys or low-lying areas on calm, clear nights.
  • Advection Fog: Forms when a mass of warm, moist air moves horizontally (advects) over a cold surface (like a cold ocean current or snow-covered land). The contact chills the air mass from below, causing widespread condensation.

Don't worry if this seems tricky at first—remember that all these types of moisture phenomena (clouds, rain, dew, fog) are just different manifestations of condensation, driven by cooling!

Key Takeaway for Section 3

The form of precipitation (rain, snow, or hail) depends mainly on the temperature profile of the air column. Near-surface phenomena like dew and fog are primarily caused by radiation cooling.