Weather, climate and the elements of weather

Detail

Weather: the atmosphere right now

• Weather is the state of the atmosphere at one place and at one time.
• It changes from hour to hour and from day to day.

• A clear morning in Delhi can turn into a thunderstorm by evening. That is a change in weather.

• Weather is described by measuring a set of elements (see below) at a given moment.

• Organised weather observation in India: the India Meteorological Department (IMD) was set up in 1875. H. F. Blanford was its first Meteorological Reporter. [6]
• Today IMD watches the weather with ground stations, automatic weather stations (AWS) (stations that record and send readings without a human observer), radars and satellites.

Climate: the long-term average

• Climate is the average weather of a large area over a long period. The period is usually 30 years or more.

• Climate also includes the extremes and the usual range of change, not just the average.

• Climate normal: the 30-year average that scientists use as a baseline. Present-day temperature or rainfall is compared with it to decide whether a season is "above normal" or "below normal". [2]

• WMO rule since 2015: the 17th World Meteorological Congress (2015) decided that a Climatological Standard Normal is the latest 30-year period that ends in a year ending in zero, e.g. 1981–2010, then 1991–2020. [3]
• Before this, normals were fixed blocks of 30 years that did not overlap (e.g. 1961–1990).

• WMO now advises updating the normal every decade, because the climate is changing fast. [2][3]

• Current standard normal: 1991–2020. In 2021 WMO recommended that this baseline be adopted globally. It replaced 1981–2010. [2]

• 1961–1990 is still kept as a fixed reference period for tracking long-term global climate change, so that warming can be measured against the same base over time. [2]
• Exam trap: "1991–2020" is the baseline for everyday climate use. "1961–1990" is the fixed baseline for measuring long-term climate change. Both are in use.

Weather vs climate

Basis	Weather	Climate
Time scale	Hours to days	30 years or more (normal: 1991–2020)
Area	One place or a small area	A large area or region
How fast it changes	Fast, often within hours	Very slowly, over decades
How it is measured	Readings at one time (thermometer, barometer, rain gauge, AWS, satellite)	30-year averages, extremes and ranges of the same readings
Branch of science	Meteorology	Climatology
Example	"It is raining in Delhi today"	"Delhi has a hot semi-arid to humid subtropical climate"

• Seasons sit between weather and climate. A season is a few months with a fairly steady weather pattern.
• IMD recognises four seasons:

Season (IMD)	Months
Cold weather (winter)	December–February
Hot weather (summer, pre-monsoon)	March–May
South-west monsoon (advancing monsoon)	June–September
Retreating monsoon (post-monsoon)	October–November

• Details of the monsoon seasons are covered under Indian climate and monsoon.

Elements of weather and climate

• Elements of weather are the measurable parts of the atmosphere that together describe weather. Climate is built from the same elements, averaged over a long time.

Element	What it means	Instrument
Temperature	How hot or cold the air is	Thermometer; maximum–minimum thermometer
Pressure	The weight of the air column pressing on a surface	Barometer
Wind (speed and direction)	Air moving across the surface	Anemometer (speed); wind vane (direction)
Humidity	Water vapour in the air	Hygrometer (wet- and dry-bulb thermometer)
Clouds	Visible masses of tiny water drops or ice crystals	Observed by eye and from satellites
Precipitation	Water falling to the ground as rain, snow, sleet or hail	Rain gauge

• Exam trap on the count: Class 11 lists six elements and includes clouds. Classes 7 and 9 list five and leave clouds out.

One element often dominates

• At different times of the year, one element shapes the weather more than the others (NCERT Class 7):

When	Dominant element	Why
July	Rainfall	South-west monsoon is at its peak
May	Temperature	Peak summer heat in north India
December	Temperature	Winter cold in north India
When a cyclone moves	Pressure	A very low-pressure centre drives the storm
When the loo blows	Wind	Strong, hot, dry wind

• Loo: a hot, dry, dusty wind. It blows from the west and north-west over north India and north-west India on May–June afternoons. It can cause heat stroke.

Simple links between the elements (Class 9)

1. Sun → temperature

• The sun's rays fall straight down near the equator and at a slant near the poles.
• So insolation (incoming solar radiation: the sun's energy that reaches the earth) and temperature fall from the equator to the poles.

2. Temperature → pressure → weather

• Warm air expands, becomes light and rises → low pressure at the surface → rising air cools, its water vapour condenses → clouds and rain.
• Cold air is dense and sinks → high pressure at the surface → sinking air warms and dries → clear skies.
• Indian example: in summer the land of north-west India heats strongly → low pressure forms there → moist south-west winds are pulled in from the Indian Ocean → monsoon rain.

3. Pressure → wind

• Wind blows from high pressure to low pressure.
• A wind is named after the direction it comes from. A westerly blows from the west. The south-west monsoon blows from the south-west.
• Pressure belts, Coriolis force and planetary winds are covered under pressure, winds and circulation.

Measuring wind speed: a simplified Beaufort scale

• The Beaufort scale grades wind strength from the effects that can be seen, such as smoke, leaves, trees and waves.
• Class 9 gives a short version of it (Table 3.1):

Description	Speed (km/h)	What you see
Calm	0–1	Smoke rises straight up
Light breeze	6–11	Wind felt on the face; leaves rustle
Strong breeze	39–49	Large branches move; umbrellas are hard to use
Storm	103–117	Damage over a wide area

Heat and humidity together: wet-bulb temperature and heat index (beyond NCERT)

Wet-bulb temperature

• Wet-bulb temperature is the lowest temperature that air (or a wet object) can cool down to when water evaporates into it. [4]
• It is read on a thermometer whose bulb is wrapped in a wet cloth. Evaporation cools the bulb.
• Dry air: a lot of evaporation, so the wet-bulb reading is much lower than the air temperature.
• Humid air: little evaporation, so the wet-bulb reading is close to the air temperature.
• So it measures heat and humidity together. It shows how well the body can cool itself by sweating.

The survival limit

• About 35°C wet-bulb for about 6 hours was proposed as the upper limit of human survival in the open (Sherwood and Huber, 2010). [4]

• Cause: the air is so hot and humid → sweat cannot evaporate → the body cannot lose heat → core temperature rises → fatal hyperthermia (dangerous overheating of the body).

• Lower real-world limit: later lab tests on people (Vecellio et al., 2022) put the critical limit at about 31°C wet-bulb, especially in humid heat.

• Already happening: since 2005, wet-bulb temperatures above 35°C have been recorded nine times, in subtropical places such as Pakistan and the Persian Gulf. Readings of about 32–35°C (90–95°F) more than tripled over the 40 years studied. [4]
• Projected hotspots: by 2050, South Asia, the Persian Gulf and the Red Sea coasts. By 2070, eastern China, parts of South-east Asia and Brazil. [4]
• At-risk zones in India: humid coasts (Odisha, Andhra Pradesh, Tamil Nadu, the Gujarat and Konkan coasts) and the Indo-Gangetic plain, where pre-monsoon heat meets high humidity.

IMD Heat Index

• In 2023, IMD began issuing an experimental Heat Index for the whole country. [5]
• It gives a "feels-like" (apparent) temperature: how hot the body feels once humidity is added to air temperature. [5]
• It uses a heat-index equation like the one used by the US National Weather Service (NOAA). [5]

Colour	Heat Index (°C)
Green	Below 35
Yellow	36–45
Orange	46–55
Red	Above 55

[5]

Prelims Hooks

• Weather describes a short time at one place. Climate is the average over 30+ years across a large area. Meteorology studies weather; climatology studies climate.
• The current WMO Climatological Standard Normal is 1991–2020, updated every decade under a 2015 WMO Congress decision. 1961–1990 is still kept as the fixed baseline for long-term climate change monitoring. [2][3]
• IMD was founded in 1875, with H. F. Blanford as its first Meteorological Reporter. [6]
• Count of elements: six in Class 11 (temperature, pressure, wind, humidity, clouds, precipitation). Classes 7 and 9 list five and leave out clouds.
• Dominant element pairs: July – rainfall; May and December – temperature; moving cyclone – pressure; loo – wind.
• Warm rising air → low pressure → cloudy and wet. Cold sinking air → high pressure → clear. Wind blows from high to low pressure.
• Trap: a "westerly" blows from the west, not towards it. Winds are named after where they come from.
• Simplified Beaufort scale (Class 9): calm 0–1 km/h; light breeze 6–11; strong breeze 39–49; storm 103–117.
• Wet-bulb survival limit: about 35°C for 6 hours in theory (Sherwood and Huber, 2010), about 31°C in practice (Vecellio et al., 2022). Values above 35°C have already been seen in Pakistan and the Persian Gulf. [4]
• IMD experimental Heat Index (2023) colour codes: Green below 35°C, Yellow 36–45, Orange 46–55, Red above 55. [5]

Mains Points

• Why baselines matter (GS-I/III): WMO updates the 30-year normal every decade (now 1991–2020) because the climate is changing. Planning for water, energy and farming with an old baseline gives wrong results. Monsoon "normal" forecasts in India depend on which baseline is used. [2][3]
• Humid heat is a growing disaster risk (GS-III): dry-bulb temperature alone hides danger.
• High humidity on India's coasts and the Indo-Gangetic plain → sweat cannot evaporate → heat stress even at moderate air temperatures.
• South Asia is expected to cross survivable wet-bulb levels by 2050. [4]

• Most exposed: outdoor workers, the elderly and people in crowded cities.

• Policy response: IMD's experimental Heat Index (2023) adds humidity to heat warnings [5]. Further steps for an answer: city heat action plans, cooling shelters, changed work hours for outdoor workers, and early-warning systems linked to wet-bulb thresholds.

• Interlinked elements: sun → temperature → pressure → wind → humidity, clouds and rain. This chain explains Indian seasons, from the summer low over north-west India pulling in the south-west monsoon to the hot, dry loo of May–June. It is a useful framework for answers on the Indian monsoon and on regional climates.