Key Climate Terms Explained

Climate Terms Latest News

• India is entering a summer marked by multiple climate extremes, including unseasonal rainfall from Western Disturbances, the possible impact of El Niño on monsoon patterns, and rising heatwaves intensified by humidity. 
• These weather conditions are making temperatures feel more severe than actual readings and are influencing rainfall, heat stress, and seasonal climate patterns across the country. 
• The article explains key climate terms shaping this summer’s weather and their practical implications for everyday life.

Western Disturbance

• The India Meteorological Department (IMD) has forecast a fresh Western Disturbance over North India between May 11 and May 13, 2026.

About Western Disturbance

• A Western Disturbance is an eastward-moving rain-bearing weather system that originates beyond Afghanistan and Iran and influences weather conditions in North India and neighbouring regions.
• These systems gather moisture from the Mediterranean Sea, Black Sea, Caspian Sea, and Arabian Sea.
• Western Disturbances are extra-tropical cyclones formed due to interactions between polar and tropical air masses, creating low-pressure systems that bring rain and snowfall.
• They are embedded within the subtropical westerly jet stream — a high-altitude, fast-moving air current flowing from west to east over the Himalayan and Tibetan regions.
• Western Disturbances are most common during the winter months from December to March, although they can also bring unseasonal rainfall during summer.
• These weather systems mainly affect: northwestern India, Pakistan, Afghanistan, and parts of Central Asia such as Tajikistan.

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What Does an El Niño Year Mean

• El Niño Southern Oscillation (ENSO) is a climate phenomenon involving changes in Pacific Ocean temperatures and atmospheric conditions that influence global weather patterns.
• ENSO has three phases:
  • El Niño – warm phase, 
  • La Niña – cool phase, and 
  • Neutral phase. 
    • These cycles generally occur every 2 to 7 years.
• Under neutral conditions, the eastern Pacific Ocean near South America remains cooler than the western Pacific near Indonesia because trade winds push warm surface water westward.

What Happens During El Niño

• During El Niño, the eastern Pacific Ocean becomes unusually warm, disrupting atmospheric circulation and weakening moisture-bearing winds reaching India.
• El Niño conditions often lead to:
  • weak or delayed monsoons, 
  • dry spells in agricultural regions, and 
  • more frequent and intense heatwaves across India.
• The U.S. Climate Prediction Center projected a 61% probability of El Niño developing between May and July 2026 and continuing through the year.

La Niña and Its Impact

• La Niña is the opposite phase, marked by cooler eastern Pacific waters. It generally strengthens India’s southwest monsoon by enhancing moisture-bearing winds.
• While La Niña often improves rainfall in India, stronger monsoons can also trigger flooding and crop damage during extreme conditions.

What is a Heat Wave

• A Heat wave is a prolonged period of unusually high temperatures significantly above the normal levels of a region.
• Heat waves can become more severe due to:
  • high humidity, 
  • strong winds, and 
  • longer duration of extreme heat conditions.
• The India Meteorological Department (IMD) considers heat wave conditions when temperatures reach 40°C or more in plains, or 30°C or more in hilly regions.
• A heat wave may be declared when temperatures are:
  • 4.5°C to 6.4°C above normal, while 
  • a severe heat wave is declared if temperatures exceed 6.4°C above normal.
• Regardless of normal temperatures:
  • a heat wave is declared at 45°C or above, and 
  • a severe heat wave is declared at 47°C or above.
• For coastal regions, where humidity is higher, heat wave conditions may be declared if temperatures reach at least 37°C and remain significantly above normal.
• Heat wave criteria must be met in at least two stations within a meteorological subdivision for two consecutive days, with the official declaration issued on the second day.

What is Wet Bulb Temperature

• The temperature usually measured by a normal thermometer is called Dry Bulb Temperature, which does not account for humidity. 
• Wet Bulb Temperature measures the lowest temperature achievable through evaporation and reflects the effect of humidity on cooling.
• Wet Bulb Temperature indicates how effectively the human body can cool itself through sweating and evaporation.
• High humidity reduces the evaporation of sweat, making it harder for the body to regulate temperature and increasing the risk of heat stress and dehydration.

Difference Between Dry and Wet Bulb Temperatures

• Wet Bulb Temperature is generally lower than Dry Bulb Temperature. The difference becomes larger when the air is drier and evaporation is more effective.

Health Risks

• According to the Intergovernmental Panel on Climate Change (IPCC):
  • sustained exposure above 35°C wet bulb temperature can be fatal, and 
  • exposure above 31°C can become dangerous during intense physical activity. 
• People with heart disease, diabetes, and outdoor labourers are especially vulnerable because humidity and dehydration increase physical stress during extreme heat.

What is “Feels Like” Temperature

• “Feels like” temperature, also known as apparent temperature, is different from Wet Bulb Temperature. 
• It estimates how hot or cold weather actually feels to the human body by considering humidity and wind conditions.
• Apparent temperature combines – actual air temperature, humidity levels, and wind conditions – to measure human thermal comfort.
• Apparent temperature helps people better understand the real physical stress caused by weather conditions, especially during heatwaves and humid summers.

Heat Index

• The “feels like” temperature is often expressed through the heat index, which combines temperature and relative humidity to estimate how hot conditions feel to the body.

Wind Chill Index

• Another type of apparent temperature is the wind chill index, which combines temperature and wind speed to show how cold weather feels when wind removes the body’s insulating warm air layer.

Source: IE