MSD Library

24/05/2022

Lightning: Information for Workers

Risk

Lightning strikes occur most often in those who work outside or engage in outdoor recreational activities. The following occupations have the highest risk:
• Construction and building maintenance
• Farming and field labor
• Logging
• Explosives handling or storage
• Heavy equipment operation
• Pipefitting or plumbing
• Telecommunications field repair
• Power utility field repair

Steps to Protect Yourself
If you work in high risk occupations, you can take steps to protect yourself.
• Check the forecast
Know the daily weather forecast so you are prepared and know what weather to expect during the day.
• Watch for signs
Pay attention to early weather signs of potential lightning strikes such as high winds, dark clouds, or distant thunder or lightning. When these occur, do not start any activity that you cannot quickly stop.
• Follow the program
Know your company’s lightning safety warning program, if it has one. These programs should include access to a safe location and danger warnings that can be issued in time for everyone to get to the safe location.
• Assess the threat
Although no place outside is safe during a storm, you can minimize your risk by assessing the lightning threat early and taking appropriate actions. For example, if you hear thunder, lightning is close enough to strike you. Stop what you are doing and seek safety in a building or metal-topped vehicle with the windows up.
• Avoid tall structures
Avoid anything tall or high, including rooftops, scaffolding, utility poles, ladders, trees, and large equipment such as bulldozers, cranes, and tractors.
• Avoid conductive materials
Do NOT touch materials or surfaces that conduct electricity, including metal scaffolding, metal equipment, utility lines, water, water pipes, or plumbing.
• Stay away from explosives
If you are in an area with explosives, leave immediately.

18/01/2022

KUDZIVIRIRA MHENI

Mheni inogona kufungidzirwa kuitika pakuongorora makore emvura paanenge achikura. Kana makore aya azara nemvura yekunaya, anotanga kusviba. Kusviba kwacho kunoreva kuti makore acho akakura uye mvura yacho inenge ichiperekedzwa nemhepo inovhuvhuta chaizvo. Chinguva chipfupi mvura iyi isati yatanga kunaya, kunze kunomboti zii kunyarara.
Ipapo

vanhu vano kuridzirwa kuti vabve pasi pemiti yakakurisa munzvimbo yavari.
2. Vanhu ngavabve panenzvimbo dzizere matombo kana simbi, pamusana pekuti izvi zvino fambirana nemheni nguva zhinji.
3. Avo vane magetsi mudzimba, vanokurudzirwa kuti vadzime midziyo inoshandisa magetsi. Ngavatobvisa tambo dzacho kumadziro, kuitira kuti mheni yarova munzvimbo iyoyo, inoshaya pekufambira nayo mumba.
4. Avo vane foni, ngavarege kufona kana kudaira foni kudzamara mvuara yagasa.

GARAI MUINE ZVINU IZVI:
Matochi ane mabhatiri ane moto
Mishonga
Chikafu nemvura yokunwa
Mishonga yekurapa
Garai mune mari yenyu uye mabhuku ekubhanga

ZVINODIWA KUITWA

Pamba/pamusha pasagara pane miti yakaora yakamira inogona kuwa kana kudona
Vanogara pamba vazive kudzima gasi , magetsi uye vanodzimura moto kana kwamudzviti
Vanhu vanofanira kuteerera yambiro inobva kunezvevemamiriro ekunze.
Dzidzisanai/panganai nzvimbo yamunosangana kana dutu remvura rapfuura napanvzimbo yenyu.

KUDZIVIRIRA NGOZI DZEMHENI MUDZIMBA
Kana mheni ichinge yarova imba, inoona zviri nyore, pakudzika kwayo pasi, kupinda nemumiviri wemunhu pane kudzika nemadziro emba. Saka izvi zvinoreva kuti avo vanhu vanenge vakatsamira madziro vanokuvadzwa kunyanya.

12/01/2022

Heavy Rains,Flash floods in Chitungwiza 11/01/2022

05/01/2022

Old MSD Training School building & Instrument site 1955

05/01/2022

Weather and Climate

Factors that Influence Climate

There are lots of factors that influence our climate

Elevation or Altitude effect climate
Normally, climatic conditions become colder as altitude increases. “Life zones” on a high mountain reflect the changes, plants at the base are the same as those in surrounding countryside, but no trees at all can grow above the timberline. Snow crowns the highest elevations.

Prevailing global wind patterns
There are 3 major wind patterns found in the Northern Hemisphere and also 3 in the Southern Hemisphere. These are average conditions and do not essentially reveal conditions on a particular day. As seasons change, the wind patterns shift north or south. So does the inter-tropical convergence zone, which moves back and forth across the Equator. Sailors called this zone the doldrums because its winds are normally weak.

Latitude and angles of the suns rays
As the Earth circles the sun, the tilt of its axis causes changes in the angle of which sun’s rays contact the earth and hence changes the daylight hours at different latitudes. Polar regions experience the greatest variation, with long periods of limited or no sunlight in winter and up to 24 hours of daylight in the summer.

Topography
The Topography of an area can greatly influence our climate. Mountain ranges are natural barriers to air movement. In California, winds off the Pacific ocean carry moisture-laden air toward the coast. The Coastal Range allows for some condensation and light precipitation. Inland, the taller Sierra Nevada range rings more significant precipitation in the air. On the western slopes of the Sierra Nevada, sinking air warms from compression, clouds ev***rate, and dry conditions prevail.

Effects of Geography
The position of a town, city or place and its distance from mountains and substantial areas of water help determine its prevailing wind patterns and what types of air masses affect it. Coastal areas may enjoy refreshing breezes in summer, when cooler ocean air moves ashore. Places south and east of the Great Lakes can expect “lake effect” snow in winter, when cold air travels over relatively warmer waters.
In spring and summer, people in Tornado Alley in the central United States watch for thunderstorms, these storms are caused where three types of air masses frequently converge: cold and dry from the north, warm and dry from the southwest, and warm and moist from the Gulf of Mexico - these colliding air masses often generate tornado storms.

Surface of the Earth
Just look at any globe or a world map showing land cover, and you will see another important factor which has a influence on climate: the surface of the Earth. The amount of sunlight that is absorbed or reflected by the surface determines how much atmospheric heating occurs. Darker areas, such as heavily vegetated regions, tend to be good absorbers; lighter areas, such as snow and ice-covered regions, tend to be good reflectors. The ocean absorbs and loses heat more slowly than land. Its waters gradually release heat into the atmosphere, which then distributes heat around the globe.

Climate change over time
Cold and warm periods punctuate Earth’s long history. Some were fairly short; others spanned hundreds of thousands of years. In some cold periods, glaciers grew and spread over large regions. In subsequent warm periods, the ice retreated. Each period profoundly affected plant and animal life. The most recent cool period, often called the “Little Ice Age,” ended in western Europe around 1850.
Since the turn of the 20th century, temperatures have been rising steadily throughout the world. But it is not yet clear how much of this global warming is due to natural causes and how much derives from human activities, such as the burning of fossil fuels and the clearing of forests.

08/12/2021

Hail Basics

What is hail?
Hail is a form of precipitation that occurs when updrafts in thunderstorms carry raindrops upward into extremely cold areas of the atmosphere where they freeze into balls of ice. Hail can damage aircraft, homes and cars, and can be deadly to livestock and people.

How does hail form?
Hailstones grow by colliding with super-cooled water drops. Super-cooled water will freeze on contact with ice crystals, frozen raindrops, dust or some other nuclei. Thunderstorms that have a strong updraft keep lifting the hailstones up to the top of the cloud where they encounter more super-cooled water and continue to grow. The hail falls when the thunderstorm's updraft can no longer support the weight of the ice or the updraft weakens. The stronger the updraft the larger the hailstone can grow.

Hailstones can have layers like an onion if they travel up and down in an updraft, or they can have few or no layers if they are “balanced” in an updraft. One can tell how many times a hailstone traveled to the top of the storm by counting the layers. Hailstones can begin to melt and then re-freeze together - forming large and very irregularly shaped hail.

How does hail fall to the ground?
Hail falls when it becomes heavy enough to overcome the strength of the updraft and is pulled by gravity towards the earth. How it falls is dependent on what is going on inside the thunderstorm. Hailstones bump into other raindrops and other hailstones inside the thunderstorm, and this bumping slows down their fall. Drag and friction also slow their fall, so it is a complicated question! If the winds are strong enough, they can even blow hail so that it falls at an angle. This would explain why the screens on one side of a house can be shredded by hail and the rest are unharmed!

How fast does hail fall?
We really only have estimates about the speed hail falls. One estimate is that a 1cm hailstone falls at 9 m/s, and an 8cm stone, weighing .7kg falls at 48 m/s (171 km/h). However, the hailstone is not likely to reach terminal velocity due to friction, collisions with other hailstones or raindrops, wind, the viscosity of the wind, and melting. Also, the formula to calculate terminal velocity is based on the assumption that you are dealing with a perfect sphere. Hail is generally not a perfect sphere!

What areas have the most hail?
Though Florida has the most thunderstorms, Nebraska, Colorado, and Wyoming usually have the most hail storms. The area where these three states meet – “hail alley,” averages seven to nine hail days per year. The reason why this area gets so much hail is that the freezing levels (the area of the atmosphere at 32 degrees or less) in the high plains are much closer to the ground than they are at sea level, where hail has plenty of time to melt before reaching the ground. Other parts of the world that have damaging hailstorms include China, Russia, India and northern Italy.

When viewed from the air, it is evident that hail falls in paths known as hail swaths. They can range in size from a few acres to an area 10 miles wide and 100 miles long. Piles of hail in hail swaths have been so deep, a snow plow was required to remove them, and occasionally, hail drifts have been reported.

How large can hail get?
Hail is usually pea-sized to marble-sized, but big thunderstorms can produce big hail. The largest hailstone recovered in the U.S. fell in Vivian, SD on June 23, 2010 with a diameter of 8 inches and a circumference of 18.62 inches. It weighed 1 lb 15 oz.

Estimating Hail Size
Hail size is estimated by comparing it to a known object. Most hail storms are made up of a mix of sizes, and only the very largest hail stones pose serious risk to people caught in the open.
• Pea = 1/4 inch diameter
• Marble/mothball = 1/2 inch diameter
• Dime/Penny = 3/4 inch diameter
• Nickel = 7/8 inch
• Quarter = 1 inch — hail quarter size or larger is considered severe
• Ping-Pong Ball = 1 1/2 inch
• Golf Ball = 1 3/4 inches
• Tennis Ball = 2 1/2 inches
• Baseball = 2 3/4 inches
• Tea cup = 3 inches
• Grapefruit = 4 inches
• Softball = 4 1/2 inches
What we do: NSSL's mPING project collects reports from the public about hail and other weather phenomena in their vicinity via a free mobile app. This data is used to refine radar algorithms that detect hail, and to enhance climatological information about hail in the U.S. Similarly, the Severe Hail Verification Experiment (SHAVE) collected data on hail by making phone calls to the public along the path of selected storms.

Hail: Types of Frozen Precipitation

Hail is only one of many types of frozen precipitation.

Hail is a form of precipitation that occurs when updrafts in thunderstorms carry raindrops upward into extremely cold areas of the atmosphere where they freeze into ice. To be considered hail, the frozen precipitation pieces must have a diameter greater than 5mm (.20”).

Graupel (a.k.a. soft hail or snow pellets) are soft small pellets of ice created when super-cooled water droplets coat a snowflake.
Sleet (a.k.a. ice pellets) are small, translucent balls of ice, and smaller than hail. They often bounce when they hit the ground.
Snow forms mainly when water v***r turns to ice without going through the liquid stage. There is no thunderstorm updraft involved in either of these processes.

Hail Detection
Hail can be detected using radar. On Doppler radar, hail generally sends a return signal that looks like extremely heavy rainfall.

28/10/2021

19/02/2020

El Nino and La Nina:
A Weather Phenomenon

What is El Nino?
• El Nino means "little boy" in Spanish and
El Nino is the powerful climate change
that occurs about every two to seven
years.
• Water near the equator warms up to a
higher temperature, which causes the air
above it to warm up too. This warmer air
and water moves east.

Changes in Weather

El Nino can cause wild weather on the west
coast.
For example:
-Winds over the Pacific Ocean can become
violent and form hurricanes
-Rainstorms and unusually high waters can
cause flooding
-Huge waves that pound into the seaside cliffs
El Nino Impact

El Nino can also cause:
• Droughts in Southeast Asia and Australia
• Wildfires in Australia and Malaysia
• Fishing shortages and crop failures
• Mudslides

La Nina
• La Nina also changes world weather.
• La Nina is what occurs when the ocean
waters are cooler than normal
• These occur about half as often as El Nino
years

• La Nina weather patterns are almost the
opposite of El Nino.
• La Nina brings wetter weather to areas
that are usually very dry during El Nino
• The effects of La Nina are usually less
extreme

Winds push from east to west and push wetter
weather to the Pacific region; drier weather in
the west coast of the US.

15/05/2019

Agrometeorology


The vagaries of weather that dominated the climate of the country over the last 20 years have brought about a phenomenal growth in the understanding of the overriding role weather plays in agricultural productivity among our populace in general and farming community in particular. The droughts and food shortages experienced in 1982/83, 1991/92 and 2001/02, to mention but a few, are ample evidence that Zimbabwe’s agricultural performance is at the mercy of the prevailing weather. Consequently, national planners are giving increasing importance to monitoring of crops, and to the assessment of the final production in terms of their dependence on weather conditions.

The provision of meteorological services to agriculture has been a major concern to the World Meteorological Organisation (WMO)-the umbrella body of all Meteorological Organisations worldwide. The WMO has, since 1974, supported the expansion and strengthening of activities in applied agrometeorology by both International and National Meteorological Organisations. Zimbabwe has also benefited from such support. An agrometeorological unit was set up in the country in the first decade of our independence in the Department of Meteorological Services. The Unit provides a wide array of services that help answer agricultural problems or situations in relation to weather and climate. However, it is regrettable that the majority of our farming community is either not aware of its existence or does not appreciate its functions and how it can assist them in decision-making.

What is Agrometeorology?

Agrometeorology (agricultural meteorology) is the application of meteorological knowledge, information and data to weather-sensitive problems in agriculture. These problems include the effect of weather, climate and their variability on land use, on crop zonation and on physical production of agriculture, as well as on management and operation of agricultural projects.

In addition, agricultural meteorology is also concerned with artificial modifications of the environment (as brought about, for example, by windbreaks and shelterbelts, irrigation, green houses, etc.); in climatic conditions of storage, weather in-doors or in the field clamps; in environmental conditions, in animal shelters and farm buildings; and during the transport of agricultural produce by land or air.

Agrometeorologists provide services that are essential in every weather-sensitive agricultural operation if the high costs of trial and error methods are to be reduced to a minimum.

What agrometeorological information can I get from Department of Meteorological services (Zimbabwe)

• data

The Department of Meteorological services provides both basic and derived climatological data for agriculture. The climate data was accumulated for over 40 years at most of the meteorological stations representatively distributed throughout the country and put in computer compatible form.

The basic data

The basic data provided by the Advisory Branch of the Department of Meteorological Services through its agrometeorological, rainfall and consultancy units include:
• Rainfall data (at daily, 5-day, weekly, 10-day, monthly and seasonal intervals)
• Maximum and minimum temperatures)
• Humidity
• Cloud cover
• Solar radiation
• Water vapour deficit
• Wind speed
• etc

The department places huge emphasis on the quality of climatological records. This is because historical weather characteristics provide the best information available for estimating the future magnitude, frequency and areal extent of thresholds of weather factors which determine the success or failure of agricultural projects. To ensure high quality of data, all the data received from our network of stations is processed in our Data Processing unit which is headed by a vastly experienced statistician.

Derived Data

Potential evapotranspiration (PET) is an important agrometeorological parameter whose use in determining the onset and cessation of the rainfall season by agronomists cannot be under estimated. However, it is not measured directly under field conditions. It is derived from values of water vapour deficit, temperature, solar radiation, wind, length of day, latitude, etc. Such data is calculated and provided by the agrometeorological unit.

Other products

Original observations of weather elements are carefully analysed, summarised and interpreted to be of maximum benefit to agricultural users for planning and decision making purposes. The products found under this group include:

• Weekly rainfall bulletins

Every Wednesday during the rainy season, the Department publishes and distributes to users a weekly rainfall bulletin which gives a summary of rainfall for the past week and the season so far in terms of the cumulative rainfall total from the beginning of the season as well as a comparison with normal such that abnormal situations can be readily identified.

The summaries help in assessing potential transpiration soil moisture deficits and occurrence of weather suitable for the spread of disease or infection.

• Weather outlooks

10-day weather forecasts:
Also on Wednesdays, from October of one year to March of the following year, The Agrometeorological unit issues out 10-day weather forecasts. This 10-day outlook is intended for on-farm operations and aids the farmer in making tactical decisions. We place emphasis on the changes of weather types, on the sequence of the rain-days, on the normal meteorological hazards in farming e.g. strong winds (exposure, physical damage), extended dry spells, extended wet spells.

The agrometeorologists also suggest agricultural implications of the forecasts.

Seasonal forecasts

Seasonal forecasts are published in September of each year. They are intended to help the farm strategise before the start of the season. These are probabilistic forecasts put in three categories, giving the probability of below normal, normal and above normal rains during the first half of the season (October-to-December) and the second half (January-to-March). If seasonal forecasts are wrongly interpreted the results are nasty for agriculture. Advice of an agrometeorologist should be sought whenever one finds difficulties in comprehending in interpreting forecasts for farmers.

09/05/2019

BRIEFING NOTESON CLIMATE SCIENCE AND CLIMATE CHANGE

What do we mean by climate and climate change?

•Climate is theaverageof weatherover time and across large regions, even the entire planet.Weather is what is happeningin one place at one time.
•Theglobalclimate has always variedformany reasons,such asinteractions between components in the climate system (oceans, atmosphere, ice sheets, etc.):-El Niño, a climatic phenomenon where the surface temperature of the eastern Pacific Ocean warms, affecting the weather worldwide, is an example of this.
•Climate change, on the other hand,occurs because the amount of energy in theentireclimate system is changedwhich affectseach and everycomponentin the system.-Changesin the Earth’s orbit, the energy received from the Sun and the amount of greenhouse gasesin the atmospherecan allcause climate change.

How can humanscauseclimate change?

•Human activitiessuch asburning fossil fuels increasethe amount of CO2and other greenhouse gasesin the atmosphere, enhancingthe natural greenhouse effect. Increasing CO2causesthe planet to heat up.-Ice cores show the concentration of CO2in the atmosphere has increased byat least40% inthelast200 years since the Industrial Revolution. The last time CO2increased this much was over a 6,000 year period as the Earth came out of an ice age,andthe average surface temperature rose by 5C.
•Burning fossil fuels and changes in land use, such asdeforestation,havealtered how much sunlight is reflected back into space(called the albedo).-Small particleslike smoke and dust in the atmosphere (aerosols)canreflect sunlight and affect clouds.-Theimpact this has on the climate is only beginning to become clear, and is expected to feature prominently in the IPCC’s 5thAssessment Report in 2014.

What effects will human activityhave on the climate?

•It will get warmer, but the exactclimatic effects oftherecentdramatic increase in CO2arenot certain.-So farwe have seen an increase of 0.8Cin theaveragesurface temperature of the Earth since 1900,with meltingin the Polar Regionsand more frequentextreme weather.
•Simple calculations and our most complex modelsallagree:if we double CO2we will see an averagewarming of between 2 and4.5C. In context,4.5Cis thedifference between today and the deepest ice-age.
•Models ofthe climate system make varying predictions of future temperatures and other effects of climate change because of two major factors:1.Climate sensitivity. Once the climatebegins to change, the effects of that change canlead tofurther changes (called ‘feedback’). Thismakes exacteffectshard to predict.Soif we double CO2, we cannot know exactly whatthe averageincrease in temperature will be.2.Future emissions. We don’t know how successful we will be in reducing greenhouse gas emissions.

Why does climate change matter?

•Small changes in average temperaturecantranslate to big changes acrossthe planet, leading torising sea levels, more extreme weatherandsomeareas becoming much harder to live in.
•Plants, animals, humans andsocietieshave developedin a climate that has variedonly a little, exceptoverextremely long timescales: we are not adapted to cope withrapid climate change.

Common misconceptionsabout climate change‘Not all scientists thinkman-madeclimate change is real’

The vast majority of climate scientists agree on the fundamentals of human-induced climate change, though there is healthy debate about the extent of changeand what to do about it. Just aswith smoking and lung cancer, the weight of evidence strongly suggests that human factorshave caused andwill cause climate change. All national academies of science agree on the existence of man-made climate change, whilemost scientists who disagree work outside climate science.‘Climategate’ and scientific misconduct:Three independent inquiries into the release of hacked emails from the University of East Anglia, which were used to suggestglobal warming was a scientific conspiracy, recommended greater transparency by UEA but found nothing wrong with the science.

‘The UK has had cold winters and poor summers’

Weather is not climate and we mustlook at averages. It is riskyto jump to conclusions about climate change by looking at small areas like theUK.At the global level,local variations average out and it is easier to make clearstatements. Nonetheless, 7of the10warmest yearsin the UKsince 1910 have occurred between2001& 2012.Extreme weather:Climate change also raises the likelihood ofextreme weather events.For instance, floods which we have come to expect once in 100 years are now likely to be seenonce in 10 yearsandsevere thunder storms are 20% more likely than 20 years ago. This is supported by observations.

‘There has been no global warming since 1998’

1998 was a particularly warm year so is an unfair placeto start measuring recent trends. But every decade since the 1950s has seen warming and 2000-2009 was around 0.15Cwarmer than 1990-1999. Temperaturefluctuates naturally, but the general trend is upwards and it moves in a series of jumps.However, the rate of rise does appear to haveslowed in the last decade and we don’t yet know exactly why.

‘Climate change has happened before’

Variations in the climate have occurred, such as the Medieval Warm Period and Little Ice-Age. There could be various causes, e.g. solar activity.But that does not changethe fact that more CO2willcause warming.‘Warming is causing more CO2, not the other way round’Ice-core recordsdo indeed showthat CO2levels rise when the Earth comesout ofanice-age. However, warming and CO2levels cause each other, soif either happens the other will follow.

‘We’re doomed, and there’s nothing we can do about it’

Some claims about the severity of climate changehave gone beyond hard evidence.For instance, climate change has beenspeculativelylinked to high numbers of deaths, and it has been claimed we are close to catastrophic‘tipping points’. While tipping points are genuine scientific possibilitiesthey are hard to predict with any certainty. Meanwhile, there is much we can do toprepare for and to slow climate change.

‘Climate change will have some good effects’

Small increases in CO2may increase crop yieldsbut larger increaseswill affectwhich crops can grow. Some colder areas will become more pleasant, but such benefits will be outweighed by rising seas, heat and drought.

What does the future hold?

How the climate changes in the future will go beyond what we have already seen and the largest effects will be in temperature-sensitive regions like the poles. Long timespans are needed to let the planet adjust. For instance, if the Greenland ice-sheet completely melts, sea levels will rise by many metres over an unknown length of time. As CO2levels increase, the risks associated with climate change become more serious.

Links to further information

Royal Society publication “Climate change: a summary of the science” published in September 2010. The guide summarises the current scientific evidence on climate change and its drivers, highlighting the areas where the science is well established, where there is still some debate, and where substantial uncertainties remain:

http://royalsociety.org/uploadedFiles/Royal_Society_Content/policy/publications/2010/4294972962.pdf

The Department for Business Innovation and Skills’ (BIS) webpages on climate science including sections: ‘Greenhouse effect’, ‘Human activities’, ‘The world is warming’, ‘Natural factors’, ‘Cause of recent warming’ and ‘Effect of rising greenhouse gas levels’

:http://www.bis.gov.uk/go-science/climatescience

Special report of the Intergovernmental Panel on Climate Change(IPCC)on extreme weather and climate change adaptation:

http://www.ipcc-wg2.gov/SREX/images/uploads/SREX-SPMbrochure_FINAL.pdf

FAQ from the IPCC’s 4thassessment report in 2007. The reports intended to assess scientific, technical and socio-economic information concerning climate change, its potential effects, and options for adaptation and mitigation:

http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-faqs.pdf

“Explaining extreme events of 2011 from a climate perspective”whichcame as a supplement to the American Meteorological Society’s State of the Climate Report 2011:

http://journals.ametsoc.org/doi/pdf/10.1175/BAMS-D-12-00021.1

Highlights of the American Meteorological Society’s State of the Climate Report 2011:

http://www.climatewatch.noaa.gov/article/2012/state-of-the-climate-in-2011-highlights

25/04/2019

Mozambique rebuilds after Cyclone Idai

It's been a week and a half since Cyclone Idai hit south eastern Africa and there are still people being rescued as its flood waters recede. (Subscribe: http...

24/04/2019

04/04/2019

We chronicle the history of cloud seeding development from Vincent Schaefer's laboratory in 1946 to its current implementation in countries like the United A...

04/04/2019

How does cloud seeding work?

During droughts water is scarce but is it possible to make it rain to provide water.

Experiments in cloud seeding suggest that it may be possible to artificially create rainfall.

Rainfall occurs when supercooled droplets of water – those that are still liquid but are at a temperature below the usual freezing point of zero centigrade – form ice crystals. Now too heavy to remain suspend in the air, these then fall, often melting on their way down to form rain.

Even in dry areas the air usually contains some water. This can be made to come together and form ice crystals by seeding the atmosphere with chemicals such as silver iodide or dry ice.

They work to promote rainfall by inducing nucleation – what little water is in the air condenses around the newly introduced particles and crystallises to form ice.

The ‘seeds’ can be delivered by plane or simply by spraying from the ground.

But does it work?

It’s hard to tell for sure. As is often the case with weather and climate, it’s impossible to carry out a controlled experiment – so, in areas of increased precipitation, we can’t know whether it would still have rained even if the clouds hadn’t been seeded.

Success has been claimed for trials in Australia, France, Spain and the US. In the United Arab Emirates, the technique is credited with the creation of 52 storms in the Abu Dhabi desert, while China boasts of having used the technology in reverse to keep the Beijing Olympic Games of 2008 dry.