What is a Cold Wave?

The winter season starts to set in after the autumnal solstice (23rd September) in the northern hemisphere. The months of December and January are the coldest months of winter in India. We often hear the news of cold waves. The reporters warn us of the different alerts issued by the Indian Meteorological Department (IMD). However, very few of the citizens understand, What is a Cold Wave?

Definition of Cold Wave

Generally speaking, cold waves are the continuous prevalence of low air temperature for one or more days.

• Qualitatively, cold waves are the condition of low air temperature which becomes detrimental to human health when exposed.
• Quantitatively, the cold wave is defined for different regions based on the departure of the observed temperature from the normal temperature. Please note that the departure of the observed temperature should be lower than the normal temperature.

Important Terminologies

Before proceeding to the criteria for cold waves, it is important to understand the following terms.

1. Normal Minimum Temperature: It is the 30 years average of observed minimum temperature. We calculate the normal minimum temperature on a daily, weekly, monthly and yearly basis from data observed over 30 years.
2. Observed Minimum Temperature: It is the data which has been recorded by the weather station in real time.
3. Departure from Normal Minimum: When the temperature is higher or lower than the Normal Minimum Temperature. The question of the cold wave only arises when the temperature falls lower than the normal minimum

Criteria for Cold Wave

IMD uses the following criteria for classification and declaring cold waves and cold days.

Types of Cold Waves

There are three bases on which IMD classifies and declares the cold waves.

1. Based on the Departure of Minimum Temperature

When the minimum temperature of a weather station is ≤10°C for plains and ≤0°C for hilly regions, IMD declares the cold wave as follows.

1. Cold Wave: Departure from the minimum normal temperature is -4.5°C and -6.4° C. It means if the normal minimum temperature of a weather station in a plain region is 10°C and if the observed minimum temperature is 4°C then the departure from the normal minimum temperature is 10-4= 6°C. Therefore, IMD will declare it as a cold wave. Similarly, if a hilly station has a minimum normal temperature as 0°C and the actual observed temperature is -5°C, then the departure from the normal minimum temperature is 0-5= -5°C. Thus, a cold wave will be declared.
2. Severe Cold Wave: Departure from the minimum normal temperature is ≥-6.5°C. It means that if the normal minimum temperature of a weather station in plain regions is 10°C and if the observed minimum temperature is 2°C then the departure from the normal temperature is 10-2= 8°C. Hence, the IMD will declare it as severe cold wave. Similarly, if a hilly station has a minimum normal temperature as 0°C and the actual observed temperature is -7°C, then the departure from the normal minimum temperature is 0-7= -7°C. Thus, a severe cold wave will be declared.

2. Based on Actual Minimum Temperature (Plains Only)

This method of classification is valid for the plains only. According to this criteria, there are two categories of cold waves.

1. Cold Wave: When the minimum temperature is ≤4°C, it is a cold wave.
2. Severe Cold Wave: When the minimum temperature is ≤2°C, it is a severe cold wave.

Please be clear that when the temperature in plains fall to such a low level, we have to consider only the absolute temperature. There is no need to calculate the departure from normal.

3. For Coastal Stations

• When the observed minimum temperature is ≤15°C and the departure from the normal minimum temperature is -4.5°C.
• This method of classification is valid for coastal stations only.
• For coastal regions, there is a provision of declaring a cold wave only. There are no criteria for declaring severe cold waves because the temperature does not plummet greatly in the coastal regions.

Types of Cold Days

IMD has also set criteria for declaring a cold day based on maximum temperature departure from the normal. It is applied to plain and hilly regions with a normal minimum temperatures of ≤10°C and ≤0°C, respectively.

1. Cold Day: When the maximum temperature departure from normal is between -4.5°C and -6.4° C.
2. Severe Cold Day: When the maximum temperature departure from normal is ≥-6.5° C.

The Heat Waves are completely opposite of the cold waves/days.

Cold Index and Cold Discomfort

Apart from the above criteria, we can use two methods to understand the cold weather problems.

Cold Index

Cold index conveys the impact of cold air temperature in combination with wind speed. Usually, the wind increases the rate of removal of heat from the human body. Thus, humans feel much greater discomfort in cold weather when winds are blowing. Contrarily, calm but cold weather is rather tolerable.

Cold Discomfort

It is an overall measure of the impact of cold weather on society at large. Normally, humans can manage any type of cold weather with the help of technology, housing, clothing etc. However, some people are marginalized and lack basic amenities such as food, shelter and clothing. So, cold discomfort conveys the impact of cold temperature, wind speed and sunshine in the lack or presence of food, shelter and clothing among different sections of society. 

Cold Wave Alerts

The IMD issues four types of alerts based on the persistence of cold waves and their impacts.

• Green Alert: The deviation from the normal minimum temperature is negligible, therefore no precautionary measures are required.
• Yellow Alert: When cold wave conditions persist in isolated areas for two days. Cold is tolerable but it can cause mild discomfort to infants, sick and old people.
• Orange Alert: Severe cold wave persists for two days. If not the severe cold wave conditions persist for four or more days. People become more prone to colds and flu. The cold may aggravate existing sickness. The long exposure to cold may cause severe frostbite and hypothermia.
• Red Alert: Severe cold wave persists for more than two days. Otherwise, the total number of cold wave days and severe cold wave days exceeds 8 or more days.

Causes of Cold Waves

• Cold waves are natural phenomena which occur due to the reduction of insolation during winter months i.e. December-January in the northern hemisphere.
• Lack of insolation and loss of heat through earth radiations makes the earth’s surface very cold. Subsequently, the cold earth’s surface makes the air cold. Hence, a high pressure system or anticyclone is established over northwest India.
• The Jet Streams are the high velocity winds which flow in the upper troposphere and lower stratosphere from west to east. These winds along with the northwestern anticyclone prevent the warm and moist winds from the Indian ocean from entering the Northern Plains of India. Therefore, cooling intensifies from the middle of December till the middle of January.
• The Jet Streams also bring moisture from the Mediterranean sea to northwest India resulting in winter rainfall or snow. The winds from the snowy Himalayas further cool the plains.

In sum, there are many factors which contribute to cold waves.

Conclusion

In short, cold waves occur as a result of net heat loss during winter months. These waves become stronger with the help of western disturbances and jet streams. IMD usually declares the cold waves when the departure of minimum temperature is greater than equal to 4.5°C below the normal minimum temperature (30 years average). IMD also declares cold waves when the observed minimum temperature is below 4°C in the case of plains. In the case of coastal areas, a departure of -4.5°C from the normal minimum temperature and a temperature lower than 15°C is sufficient to declare cold waves.