Weather and Climate

1. Time Duration: Weather refers to atmospheric conditions over a very short period (hours, days, or a week); Climate represents the synthesized average weather conditions observed over a long period (30 to 35 years).
   2. Geographical Area: Weather applies to a small, localized territory (e.g., weather of Mumbai today); Climate applies to a vast regional or national zone (e.g., Tropical Monsoon climate of South Asia).
   3. Stability: Weather is dynamic, erratic, and changes frequently; Climate is relatively stable and permanent.

• Mechanism: Moisture-laden winds blowing from an ocean encounter a tall mountain barrier across their path. Forced to rise up the Windward Slope, the air expands and cools adiabatically. When it cools below its dew point, water vapor condenses into clouds, dropping heavy, torrential orographic rain on the windward side.
• Rain-Shadow Area: After crossing the mountain crest, the dry air descends down the Leeward Slope. As it descends, atmospheric compression warms the air, increasing its moisture-holding capacity and preventing condensation. Consequently, the leeward side receives little to no rainfall, creating a dry Rain-Shadow Area.
• Indian Example: Mahabaleshwar on the windward side of the Western Ghats receives over $600\text{ cm}$ of monsoon rain, while Pune, situated just $65\text{ km}$ east in the rain shadow, receives only about $70\text{ cm}$.

• Ferrel's Law: Formulated by American meteorologist William Ferrel, it states that any moving body or wind deflects toward its RIGHT in the Northern Hemisphere, and toward its LEFT in the Southern Hemisphere.
• Cause: This deflection is caused by the Coriolis Force (or Coriolis Effect), a fictitious deflecting force generated by the eastward axial rotation of the Earth.

• Doldrums: The Equatorial Low Pressure Belt situated between $5^\circ\text{ N}$ and $5^\circ\text{ S}$ of the Equator.
• Why Calm Winds: Intense, year-round vertical solar heating warms the air, causing it to expand and rise vertically in powerful convection currents. Because the air movement is primarily vertical upward, there is virtually zero horizontal wind movement, creating notorious navigational calms for sailing ships.
• Afternoon Thunderstorms: The rapidly rising humid air cools quickly, forming towering cumulonimbus clouds that unleash daily torrential downpours known as the "4 o'clock rain".

• Mumbai (Maritime Climate): Located directly along the Arabian Sea coast. Land heats up and cools down much faster than water. During the day, cool sea breezes blow onshore, moderating summer heat; during the night, land breezes blow offshore, preventing severe winter cold. This produces a moderate, equable climate with little annual temperature variation.
• Delhi (Continental Climate): Located over $1,000\text{ km}$ inland, completely isolated from maritime sea breezes. The land heats up intensely in summer ($>45^\circ\text{C}$) and loses heat rapidly in winter ($<4^\circ\text{C}$), resulting in an extreme climate with vast seasonal temperature swings.

• Relative Humidity: The ratio of the actual amount of water vapor present in a given volume of air at a specific temperature to the maximum amount of water vapor that same air could hold to become fully saturated at that temperature, expressed as a percentage:

$$\text{RH} = \frac{\text{Actual Water Vapor Content}}{\text{Saturation Capacity}} \times 100\%$$

• Measurement: Measured using a Wet and Dry Bulb Hygrometer (Psychrometer). The difference in temperature between the dry and wet bulb thermometers is used alongside hygrometric tables to determine relative humidity.

• Working Principle: An Aneroid Barometer contains a sealed, partially evacuated metallic capsule with a corrugated corrugated metal face. When atmospheric pressure increases, the capsule is compressed; when pressure drops, it expands. This mechanical flexing moves a pointer across a calibrated dial via delicate levers.
• Why More Convenient: A mercury barometer contains fragile glass tubes and toxic, heavy liquid mercury that spills easily during transport. The aneroid barometer contains no liquid whatsoever ("aneroid" means without liquid), making it compact, rugged, and easily portable for mountaineers and aviators.

• Trade Winds: Permanent planetary winds that blow steadily from the Subtropical High Pressure Belts ($30^\circ\text{ N/S}$) toward the Equatorial Low Pressure Belt ($0^\circ$).
• Direction (via Ferrel's Law):
1. In the Northern Hemisphere, they deflect to the right, blowing from the northeast as the North-East Trade Winds.
2. In the Southern Hemisphere, they deflect to the left, blowing from the southeast as the South-East Trade Winds.
• They were named "Trade Winds" from the old Saxon word tred (meaning steady track), as sailing merchant ships relied on them for transatlantic commerce.