What’s with all this rain?

Mbona mvua zimezidi mwaka huu?

(Scroll down for swahili)

Let’s break it down and start with the simple water cycle. Water evaporates from the ocean or a lake, it rises, condenses and turns into clouds, gets blown with the wind over the land and then when the conditions are right turns into rain. The same thing happens over forests & woodlands which is technically called evapotranspiration. Based on this cycle, three things that are going to influence the amount of rain we get are going to be 1- amount of evaporation, 2- direction of the wind, and 3- conditions for making the water in clouds turn into rain.

Let’s start with the amount of evaporation. Evaporation is caused by water heating, so in our case evaporation is going to increase when the heat from the sun is at its maximum or the oceans are the warmest, so let’s remind ourselves about how the location of maximum heat from the sun changes during the year.

a) Wakati wa equinox jua inagonga kwenye ikweta kwa mwelekeo wa nyusi 90. b) Mwezi wa sita, jua inalenga kwenye tropik ya kaskazini. Maeneo ya ikweta na kusini mwake ni kiangazi. c) Mwezi wa tatu, juna inalenga kwenye tropic ya kusini na kuleta mvua ya masika.
(Diagram from The Biology of African Savannahs, by Bryan Shorrocks, 2007, pg 23)

The Earth spins which is what causes day & night. In East Africa, the amount of daylight doesn’t change much throughout the year, but you’ll notice the where the sun rises and sets on the horizon does. This is because of the tilt of the earth and is usually used to explain the extreme seasons of the north and south (like in the video below), but it also explains the seasons in East Africa. In September and March, the sun’s rays are hitting the ocean at 90 degrees at the equator and so they have the greatest warming (and low pressure). If the ocean is getting warmer, then evaporation will increase and so we expect more rain. Because it takes time to heat up and cool down, in reality there’s about a month delay which is why the first rains in East Africa are at the end of October. It also explains why southern Tanzania and Zambia will receive rain at the end of November and only have one wet season. The technical name for this band of maximum evaporation & low pressure is called the Inter tropical convergence zone or ITCZ for short. If you are close enough to the equator that the zone crosses twice you will get two wet seasons, but if you are closer to the tropic like Zambia or southern Tanzania, you will only experience one wet season.

Here is a great short video to watch by the California Academy of Sciences.

The other factor that has a major effect on evaporation in the ocean has to do with ocean currents that naturally effect the surface temperature of the ocean. In the Pacific Ocean the extremes are called El Niño and La Niña, but the same things are happening in the Indian Ocean, something called the Indian Ocean Dipole or IOD for short. In a year like last year, water towards Africa in the eastern Indian ocean were warmer than usual. Wind blowing across the Indian ocean pushed more warm water towards Africa. The warm water increased evaporation rates and thus caused the excessive early rains in October, through until now. When this happens, the opposite weather happens in Australia and as we know 2019 was dry and very hot in Australia. If you’d like to watch an explanation from the Australian point of view, check this video out here.

If you’d like to watch some more weather influence related videos. The following are short and do a good job of explaining something quite complex.

Another look at global weather cells.

Understanding the ENSO which causes El Nino.

Why does the Earth have deserts.

Swahili version

Tuanze kwa kuangalia mzunguko wa maji hewani. Maji yanayeyuka kutoka kwenye bahari au ziwa, yanapanda hewani, na baada ya hapo yanatoneshwa kuwa mawingu na kusukumwa na upepo angani juu ya nchi kavu halafu hali ya hewa ikiwa nzuri maji haya yanabadilika kuwa mvua. Mzunguko huohuo unatokea juu ya misitu na unaitwa ‘evapotranspiration’.  Kutegemeana na mzunguko huo, vitu vitatu vivwatayo vinasababisha kiasi cha mvua tunayopata: 1- kiasi cha maji kuyeyuka, 2- mwelekeo wa upepo, na 3- hali ya kusababisha mawingu kubadilika kuwa mvua.

Tuanze na kiasi cha maji kuyeyuka. Kiasi hicho kinasababishwa na maji kuongezeka joto, na kwa mfano wetu, kiasi hicho kitaongezeka kulingana na joto la jua kali linapokuwa juu angani au maji ya bahari kuwa na moto zaidi. Kwa hivyo, tujikumbushe jinsi joto wa jua linanavyobadilika  kulingana na mwelekeo wa dunia kwa majira tofauti katika vipindi vya mwaka.

Dunia inazunguka na hiyo mzunguko unasababisha kuwa na mchana na usiku. Hapa Afrika Mashariki, kiasi cha muda wa mchana na usiku hakibadiliki kwa majira ya mwaka, lakini utaona tofauti ya mahali jua linapopambazuka na kuzama kwenye upeo wa macho. Hii ni kwa sababu ya mwelekeo wa dunia ambayo unabadilika na kusababisha mabadiliko ya misimu huko nchi za kaskazini na kusini, lakini pia  inasababisha mabadiliko ya misimu hapa Afrika Mashariki. Mwezi wa tatu na mwezi wa tisa mwanga kutoka kwenye jua unagonga kwenye ikweta kwa mwelekeo wa nyusi 90, na kusababisha kupasha joto zaidi na pia kupunguza presha ya hewa. Kama bahari inapashwa joto zaidi, tunategema kupokea mvua zaidi. Kwa vile maji ya bahari yanachukua muda kupashwa na kupoozwa, msimu wa mvua za vuli ambayo ungeanza mwezi wa tisa huchelewa mwezi mmoja na ndiyo maana mvua huanza kunyesha mwisho wa mwezi wa kumi hapa Afrika mashariki. Inasababisha pia mvua kuanza mwisho wa mwezi wa kumi na moja kusini mwa Tanzania na Zambia kwa msimu mmoja tu, mvua za masika. Jina la kisayansi kwa tokeo hili ni ‘Intertropical convergence zone’ au ITCZ kwa ufupi. Kama upo karibu na equator kupitiwa mara mbili na hili ITCZ utapata misimu miwili ya mvua, lakini kama uko karibu na ‘tropic’ kwa mfano Zambia au kusina mwa Tanzania, utapata msimu mmoja tu wa mvua.

Hapa kuna video fupi inayoonyesha vizuri ITCZ.

Sababu nyingine kubwa ya kuongeza kuyeyuka kwa maji baharini inatokana na mizunguko ya maji kwenye bahari yenyewe na namna mizunguko hubadilisha joto la uso wa vilindi vya bahari. Joto likitokea kwenye bahari ya Pacific, mabadiliko kubwa yanaitwa El Nińo na La Nińa, lakini mabadiliko haya yanatokea kwenye bahari ya Hindi pia, na inaitwa ‘Indian Ocean Dipole’ au ‘IOD’ kwa kifupi. Mzunguko hutokea kila baada ya miaka kadhaa; mwaka jana (2019), maji kwenye bahari ya Hindi yaliongezeka joto karibu na Bara la Africa kuliko sehemu ya bahari karibu na Australia. Upepo ulipuliza kuelekea Bara la Afrika ukapita juu ya bahari ya Hindi na kuongeza maji vuguvugu kuja juu. Maji vuguvugu haya yaliyeyuka zaidi na kusababisha ongezeko la mvua mwanzoni mwa mwezi wa kumi, na hadi sasa. Hali hii ikitokea, hali ya hewa inakua kinyume huko Australia, na tunavyofahamu, mwaka jana kulikuwa na ukame mkali na hewa ya joto sana huko. Kama unapenda kuona maelezo ya upande wa Australia icheki video hii.

Kuna video zingine za kutazama.

Angalia namna hewa kuzunguka angani wa dunia.

Kuelewa ENSO kinacho sababisha El Nino.

Sababu ya Dunia kua na jangwa.

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