Dust Storms And Thunderstorms
A thunderstorm is a local storm violent storm that develops from cumulonimbus clouds on a hot and humid afternoon and always accompanied by lightning and thunder.
All thunderstorms form due to rising of warm air from the ground, The air may rise due to
- ground heating which promotes vertical motions in the air,
- Supply of significant amount of moisture from near the ground, and
- sucking effect of air by winds high above causes the air to rise. This can happen because of divergence of jet stream like the one that happened during Kedarnath.
In the presence of these conditions, a triggering effect is required to initiate thunderstorm. This can be provided by the following—
(i) Heating from below producing warm air Convection.
(ii) Upliftment over mountains
(iii) or an upliftment along two different air masses having contrasting temperature and humidity.
|Thunderstorms form as the warm air rises, called as updrafts. This rising air causes water to evaporate and carry the moisture upwards to form clouds by condensation. The cloud then builds and grows up to 16kms. These clouds become dark and heavy. Cold heavy air from the upper part brings this moisture downward called as downdraft. The caused rains.||VIDEO 2|
Thunderstorms are full of energy and are a energy reservoir. They get their energy from the condensation in the clouds.
As clouds in the thunderstorm develop, excess negative and positive charges form and at certain point this built up of opposite charges lead to the transfer of electrons in the form of lightning. This transfer occurs so quickly that it breaks sound barrier that we hear as thunder.
In fair weather, the ground is negatively charged to balance the positively charged ionosphere. During a thunderstorm, however, the negative charges that form in the cloud repel the negative charges in the ground. The negative charges are pushed deep into the ground, which becomes positively charged as a result again producing a lightning flash.
There are four types of thunderstorms
Single-cell thunderstorms are small, isolated, brief, weak storms that grow and die within an hour or so. They are typically driven by heating on a summer afternoon. Single-cell storms may produce brief heavy rain and lightning.
A multicellular thunderstorm is a cluster is a thunderstorm that is composed of multiple cells, each being at a different stage in the life cycle of a thunderstorm.
A squall line is a group or collection of storms arranged in the form of a line, that can be hundreds of kms long but are typically only 10 or 20 kms wide. They are often accompanied by “squalls” of high wind and heavy rain.
A supercell is a long-lived (greater than 1 hour) and highly organized storm that is tilted and rotating. This rotating updraft – as large as 16 kms in diameter and up to 16kms tall – can be present as much as 20 to 60 minutes before a tornado forms. Scientists call this rotation a mesocyclone when it is detected by Doppler radar. Most large and violent tornadoes come from supercells.
The northern Indian plains have witnesses a series of thunderstorms and dust storms with differential severity. In one year 2018 there were series of thunderstroms, observed in India.
Reasons behind the phenomena in 2018
Causes of Generation of these thunderstorms
- Very hot conditions
Temperatures of over 40 degree Celsius are observed in northwest, central and east and north peninsular India. When Maximum temperature are raised upto 8 degrees celsius above normal as it happened in 2018, it can lead to an intense heat wave.
In such a situation, Interaction of hot air near the surface with colder winds from the western disturbances can give rise to intense and widespread storms
- Western Disturbances (WD)
These are extra tropical or temperate cyclones originating in the Mediterranean region that brings sudden winter rain to the northwestern parts of the Indian subcontinent
While the western disturbances normally peak between December and February, a greater number of active WD have been observed in spring and summer months
Instead of the normal 2-3 active WD seen during the month of April and May, the number of active WDs can be more as it happened in 2108.
WDs carry high-velocity winds that almost sucks the air from below and transports it further accentuating low pressure
- Existence of a Troughs
- A trough extends as an area of low pressure.
- This is where moisture laden winds from the Bay of Bengal met hot and dry air come from central and western India. These winds also came in contact with the cold front that develops due to active WDs.
- The confluence of these different winds culminated in intense and widespread storms across the Indo-Gangetic plain in 2018
- Similarly, a North-South trough was formed from Bihar to Northern Tamil Nadu along which stormy weather was observed in Telangana, Andhra Pradesh and some parts of Karnataka
- Movement of Easterly winds into India.
- Easterly winds from the Bay of Bengal carry moisture and is associated with pre-monsoon thunderstorms in the eastern coast. But this usually happens in the winter months. In 2018 the easterlies have continued well into May and have interacted with the WDs owing to the east-west trough. This fuelled intense activity over large parts in south india.
- Anomalies at sea surface
- The anomalies in sea surface temperatures (i.e 1-2 degree warmer waters) over the Bay of Bengal and the Arabian sea can spurr greater moisture transfer than usual by the easterly and westerly winds (respectively) causing the spate of storms.
Problems in forecasting?
Outdated forecasting models
- India uses the (obsolete) NowCast model.
- Switch to the contemporary ‘Unified Model’ is hamstrung by lack of data.
Inadequate Doppler radars
- IMD has installed only 25 doppler radars so far across the country
- These cover less than 15 % of India’s land area.
Poor maintenance and upkeep of instruments.
Some of the Doppler radars were not functioning when the storms hit.