If the six conditions, we’ve covered in previous chapter, are met, the tropical cyclone appears. Let's now learn what phases there are to grow from the tropical disturbance of the atmosphere to a huge tropical cyclone.

It starts with a tropical disturbance

At the beginning of every tropical cyclone, there is a tropical disturbance or a wave that can be caused for example by thunderstorm going from a land above an ocean. In this phase, previously mentioned six conditions play a big role to start the engine of development.

What is a tropical depression

In the next phase, the wind rises and starts to circle a centre of the disturbance due to the Coriolis force. Now it’s called tropical depression and it is a measurable and observable object in the atmosphere with clouds slightly rotating around causing low precipitation.

Almost a Hurricane

When the wind speed reaches 39 mph (63 km/h), it is called a tropical storm. Tropical storm looks like a little hurricane, but it has no eye in its centre. It is all covered in clouds spinning around an invisible centre.

In this phase, the storm gets a name. It can stop growing here and the name will be already used for only tropical storm and not for a cyclone (like was a case with recent tropical storm Ivo that weakened to tropical depression). More on tropical storm naming in following chapters. Anyway, even tropical storms can make huge damage.

When Tropical storm becomes a Hurricane

Finally, tropical cyclone is a phase with wind speed above 74 mph (119 km/h). It has a fully developed eye in the centre and its structure is quite stable above an ocean.

There are different names for tropical cyclones based on location, they are called hurricanes in the Atlantic Ocean and north-eastern Pacific Ocean, typhoons in the north-western Pacific Ocean and simply cyclones in South Pacific and Indian Ocean.

Before we get to more details in the upcoming chapters, we’ll uncover the difference between tropical and extratropical cyclone in the next one. See you next chapter.

Štěpán Šubík

Prague based student of meteorology eager for science, who likes to learn, but also wants to live to the fullest every day. Follow me on Twitter.

Next chapters:

• What is the difference between tropical and extratropical cyclone
• What’s the difference between hurricane, typhoon, cyclone

Previous chapters:

• What is a cyclone
• What is a tropical cyclone

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A tropical cyclone is a low-pressure place in the atmosphere located near the equator. It can be recognized by its structure and origin.

Tropical cyclones have an unquestionable impact on the lives of people living in the tropics. But why do they appear only in certain places and how are the tropical cyclones formed in the first place?

How tropical cyclones are formed

Tropical cyclones can be formed only in the tropics because they need a huge amount of energy, which can be present only in the tropics above an ocean when six conditions are satisfied:

1. Temperature – the temperature of the ocean must be above 26 °C (79 °F). This condition leads to strong evaporation of the oceanic water and the water vapour is the most important source of energy for the hurricanes through its condensation.

2. Distance from the equator – the hurricane cannot be created right on the equator. Hurricanes need some force to start it's spinning and that force is the Coriolis force. Coriolis force is zero on the equator and rises when moving towards the poles. That is why hurricanes can’t be present within 5 degrees latitude.

3. Saturated lapse rate gradient near its centre – for the efficiency of using energy. When water vapour is present in the centre of the cyclone, it condensates and releases latent heat at a maximum rate due to a saturated lapse rate.

4. High relative humidity – for evaporated water and avoiding trade wind inversion. Trade wind inversion means subsidence of air mass which is present mainly in anticyclones. That destroys cyclones. And evaporated water must be present, but with low humidity, the ocean evaporates more and its temperature decreases and can drop below the condition one temperature.

5. Low vertical wind shear – a low difference of wind speed and direction vertically. If there is a strong wind in the upper atmosphere or in the opposite direction, the tropical cyclones won’t form in the upper levels and will lose energy quickly.

6. Final condition is the presence of a tropical wave. It can be any disturbance in the atmosphere, for example, a thunderstorm that moves off the coast of Africa to the east.

When all these conditions are met, a tropical cyclone is created.

Moreover, we can distinguish between two types of cyclones - tropical and extratropical. A subtropical cyclone is something in between.

Tropical cyclones are created a few degrees from the equator and are quite small on the horizontal scale. They are deeper, which means that there is a bigger gradient of pressure. They cannot have any frontal system and they move from east to west because they are pushed by the trade winds flowing close to the equator.

In their centre, tropical cyclones are warm because they collect energy from condensing water droplets and the process of condensation means warms the surrounding air.

Tropical cyclones are dangerous because of its huge energy hidden in water vapour that can release its potential by condensation and the heat can be transformed to lower pressure causing really strong winds capable of taking even cars and houses.

When a tropical cyclone is fully developed, with its eye, eyewall, wind speed above 74 mph (119 km/h), it is classified depending mainly on the wind speed and its location as a hurricane, typhoon or simply cyclone.

Before we get to hurricanes and typhoons, we’ll examine the anatomy of the tropical cyclone. See you in the next chapter soon.

Next chapters:

• Four stages of Tropical Cyclone. From Tropical Disturbance to a Hurricane
• What is the difference between tropical and extratropical cyclone
• What’s the difference between hurricane, typhoon, cyclone

Previous chapters:

• What is a cyclone

In simple words, a cyclone is a type of storm with winds spiralling inwards, or more precisely - like a meteorologist - would tell you, the cyclone is low-pressure structure in the atmosphere.

Pressure in the atmosphere is not the same in every place and it even changes in time. There are places with lower pressure called cyclones and places with higher pressure named anticyclones.

In cyclones, there is the pressure between 900 and 1020 hPa (in extremes a few hPa below 900 hPa) which is lower than the pressure around, so the air tries to get to its centre. The winds would aim straight to the centre of a cyclone if there was no Coriolis force.

In the large scale weather systems, we can distinguish between three main types of surface-based cyclones: Extratropical cyclones, Subtropical cyclones and Tropical cyclones. In the mesoscale weather systems category, we can find cyclones too. Tornado, waterspout, dust devil or firenado are just a few examples of them.

Now let’s get back to large-scale weather systems.

What direction cyclone spins

Coriolis force makes every particle which is moving on the Earth to slightly turn from its direction. In the Northern hemisphere, it is to the right and in the Southern hemisphere, it is to the left. That is why the wind turns around a cyclone anticlockwise in the Northern hemisphere and clockwise in the South.

To be continued… Next chapters:

• What is a tropical cyclone
• The anatomy of the tropical cyclone
• What is the difference between tropical and extratropical cyclone
• What’s the difference between hurricane, typhoon, cyclone

As we wrote about Altocumulus and its species, the rarest is Altocumulus volutus. It is very lucky to see this species above our heads.

Altocumulus volutus is quite like Stratocumulus volutus. That means a long, horizontal, detached, tube-shaped cloud mass which often appears to roll slowly about a horizontal axis. Unlike Stratocumulus, Altocumulus volutus is usually a single line and, like it is with the mid-level clouds, it appears smaller.

Photos of Altocumulus volutus are very rare even though it is a beautiful cloud that is not connected with precipitation. If you have seen one, send us your photos!

Among all of the Altocumulus cloud species, Altocumulus castellanus has the greatest impact on the weather forecast. So how to recognize it?

Altocumulus castellanus (Ac cas) is a mid-level cloud with its base from 2 km (6,500 ft) up to 6 km (20,000 ft). They are very similar to Stratocumulus castellanus and the main difference is that they look smaller and there is no precipitation connected to them.

But Altocumulus castellanus is a big help in predictions of incoming heavy showers or thunderstorms. Ac cas is created when a mid-level instability appears. If surface-based convection can connect to the mid-level instability, Ac cas can transform into Cumulonimbus clouds.

Have you ever seen this phenomenon? Send us your photos!

Sucho trápí Česko už několik let v řadě. Pro naše území jsou typické letní srážky, které jsou četnější i intenzivnější než zimní. Několik let v řadě však téměř chyběly.

Pro zemědělství je dostatek vláhy důležitým předpokladem pro dobrou úrody. Se snižujícími se zásobami podzemních vod roste ohrožení úrody. O víkendu proto přijdou vítané srážky, které by měli pomoct rostlinám v jejich nejnáročnějším období.

V pátek po řadě slunných dní, kdy se Česko nacházelo v teplém sektoru frontálního systému, začne naše území přecházet zvlněná studená fronta, která sníží teplotu v průměru o 10°C a s sebou přinese nejen déšť, ale i bouřky, hlavně na západě našeho území.

Srážky budou na hraničních horách narůstat a dostanou se v průběhu soboty nad celé Čechy, za Českomoravskou vrchovinou se budou vyskytovat jen výjimečně. Na obrázku je oranžově vyznačena oblast největšího deficitu využitelné vody v profilu lehké půdy pod travním porostem, tedy oblast, kde zásoba vody tvoří méně než 25% průměru z let 1961 až 2010, počítáno v pondělí 22. 4. 2019.

V sobotu se budou srážky vyskytovat hlavně na horách na západě a na Vysočině, ojediněle v nížinách. Tyto situace model často nadhodnocuje a proto ne v každém modře vyznačeném území bude pršet. Srážky se vzácně mohou dostat na Olomoucko přes Orlické hory.

Koncem víkendu bude srážek ubývat, mohou se však začít objevovat v Beskydech. Původcem srážek za studenou frontou bude vyplňující se tlaková níže na západě Německa, která přináší vítr ze západu až jihozápadu.

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Počátkem příštího týdne je zmíněná níže již zcela zaplněna a níže nad Polskem od nás odsune zbytek srážek ze zaniklé níže.

V úterý se nad naše území dostane opět déšť a můžeme jen doufat, že jej bude dostatek.

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