Cold wave: A rapid fall in temperature within 24 hours to a level requiring substantially increased protection to agriculture, industry, commerce, and social activities.

Derecho: “a widespread, long-lived, straight-line windstorm (no rotation like a tornado)” that is associated with a “band of rapidly moving showers or thunderstorms”.

The name comes from the Spanish word ‘la derecha’ which means ‘straight’.

It is a warm-weather phenomenon that generally occurs in June and July.

Analysis period of 1971-2010:  Cold wave/ Severe cold wave spells are 1-2 days, but in some cases, it lasted for 10 days. There are more than an average of 6 cold wave days in the period.

• India’s ‘core cold wave zone’ covers Punjab, Himachal Pradesh, Uttarakhand, Delhi, Haryana, Rajasthan, Uttar Pradesh, Gujarat, Madhya Pradesh, Chhattisgarh, Bihar, Jharkhand, West Bengal, Odisha and Telangana.

Nodal Ministry: Min. of Agriculture and Farmers Welfare (MAFW)

Cold Wave Types

1. Advection Cold Wave: This occurs when cold air moves into an area, lowering temperatures. An example is the Siberian cold wave in Europe during winter.
2. Radiation Cold Wave: This happens under clear skies with calm winds, allowing heat to radiate away at night. An example is a clear winter night in the northern United States.
3. Frontal Cold Wave: Results from the passage of a cold front, causing a sudden drop in temperature. Example: A cold front moving through during a thunderstorm.

Criteria to Declare:

If the Wind chill factor (a measure of the cooling effect of the wind on the temperature of the air)

is 10°C or less, then only the conditions for cold waves are considered.

• For the plains: When the minimum temperature is 10 degrees Celsius or below and is 4.5 to 5.5 degrees Celsius (C) less than normal for two consecutive days. A severe cold wave is declared when the temperature deviation from normal is -6 to -7 degrees Celsius. When the Wind chill factor is 0°C or less, the Cold Wave should be declared irrespectively.
• For coastal stations: The threshold value of a minimum temperature of 10 degrees Celsius is rarely reached. However, the local people feel discomfort due to the wind chill factor which reduces the minimum temperature by a few degrees depending upon the wind speed.

Causes:

• Cold wave:
  1. Absence of cloud cover in the region: Clouds trap some of the outgoing infrared radiation and radiate it back downward, warming the ground.
  2. Snowfall in the upper Himalayas has blown cold winds towards the region.
  3. Subsidence of cold air over the region: Subsidence is the downward movement of cold and dry air closer to the surface.
  4. Prevailing weak La Nina conditions in the Pacific Ocean.
  5. La Nina is the abnormally cooler sea surface temperature reported along the equatorial Pacific Ocean and it is known to favor cold waves.
  6. During La Nina years, the severity of cold conditions becomes intense. The frequency and area covered under the grip of a cold wave become larger.
• Derechos:
  1. Heat and Moisture: Associated with warm, moist air masses, often develop in the warm sector of a rapidly moving low-pressure system.
  2. Atmospheric Instability: Derechos require atmospheric instability, which is characterized by rising warm air and falling cool air.
  3. Squall Line: Derechos are a type of severe windstorm associated with a long-lived squall line of severe thunderstorms.
  4. Downbursts: Within the squall line, downbursts are powerful & can create straight-line wind damage over a wide area.
  5. High Wind Gusts: Derechos are known for their destructive straight-line wind gusts, which can exceed 58 miles per hour (93 kilometers per hour) and are often associated with widespread wind damage.

Mitigation:

1. Shelter and Relief Camps: Setting up temporary shelters and relief camps to provide warmth, food, and medical assistance.
2. Winter Clothing Distribution: Distributing warm clothing, blankets, and winter essentials to communities in need.
3. Weather Forecasting and Early Warning Systems: Improving weather forecasting capabilities and disseminating timely cold wave warnings to the public.
4. Community Awareness Programs: Educate people about the risks of cold waves and the importance of staying warm, along with tips for winter safety.
5. Heating Facilities: Installing heating facilities in public spaces, like community centers, schools, and hospitals.
6. Power Supply Maintenance: Ensuring the reliability of power supply during cold waves to avoid disruptions in heating systems and essential services.
7. Social Support Programs: Providing financial assistance and support to vulnerable populations.
8. Coordination with NGOs and International Agencies: Collaborating with non-governmental organizations (NGOs) and international agencies to enhance the reach and effectiveness of cold wave mitigation efforts.

Way Forward:

Cold Wave: Improve winter preparedness, provide shelter for vulnerable populations, and ensure access to heating resources in affected areas. Address climate change to reduce the frequency and severity of cold waves.

Derecho: Enhance early warning systems for severe thunderstorms, strengthen infrastructure against high winds, and educate the public on derecho risks. Foster regional cooperation for effective response to widespread derecho events.