Bringing the Cooling Effects of Aerosols Down To Earth

Next up in what has become mini-series of "is air pollution still a big problem"...

New research shows that aerosol pollution offsets the heating effects of greenhouse gas pollution to a much more significant degree (no pun intended) than originally thought.

As aerosol particles collectively block out sunlight, they have a cooling effect. Because of this, if today we suddenly stopped all forms of air pollution, Earth's atmosphere would get even hotter.

aerosols: While "aerosol" is often used in everyday conversation to describe canned products like hairspray, it also refers to tiny particles suspended in the air.

If you are at a construction site and see dirt being stirred up, you are seeing an example of aerosols. Large amounts of aerosols from volcanic eruptions are one suspected cause of a "little ice age" in the past (Parry, 2012).

Aerosols can also pose direct threat to human health. (Marvel)

Why

Heat-trapping greenhouse gasses remain in the atmosphere longer than aerosols which have a cooling effect. If all air pollution came to a sudden stop we would quickly lose the cooling aerosols, but not the heating greenhouse gasses so quickly. This would allow the GHGs (greenhouse gasses) to have a more full heating effect.

According to the international team of scientists who did the calculations, the difference would be about 0.5 to 1.1 degrees Celsius warmer. (Samset et al., 2018)

Implications

The report highlighted in the summary that "to keep within 1.5 or 2° of global warming, we need massive reductions of greenhouse gas emissions. At the same time, aerosol emissions will be strongly reduced.... This means that it does not only matter whether or not we reach international climate targets. It also matters how we get there."

The report also mentions that the Northern hemisphere is especially sensitive to aerosol removal because of the location they are being emitted from. Also, more precipitation could be expected in places making significant reductions, as aerosols have a drying effect.

In an interview with Yale Environment 360, Bjorn H. Samset, lead author of the report made clear that he does not want the findings to be taken as encouragement to go ahead with intentionally spraying more aerosols into the atmosphere, likening its uncertain consequences to Russian Roulette. (Schiffman, 2018)

Bringing it Down to Earth

"This means that it does not only matter whether or not we reach international climate targets. It also matters how we get there." (Samset et al., 2018)

The headlines surrounding the findings may be surprising, but the average North American carbon footprint is way beyond having to reconsider reducing pollution. The evidence still points to greenhouse gases as a serious problem, and aerosols themselves pose their own threat to human health. (Marvel)

However, this information can help illustrate the different benefits between making needed cutbacks in consumption, and actively increasing carbon sinks.

carbon sink: Natural systems that absorb carbon dioxide. This includes trees and other plants, oceans, and soil. (Thompson, 2012)

carbon sequestration: The process of capturing and storing carbon dioxide, namely from the atmosphere. (USGS). This term often references natural processes, but can also refer to artificial ones.

ecological footprint: A metric used to illustrate how much biologically productive land is needed to support a person or group's lifestyle or consumption. (Global Footprint Network, 2018) Carbon footprint is the amount of that land is needed to sequester the resulting carbon emissions, though it can alternatively refer to tonnes of carbon dioxide emitted as a result.

These findings give a pat on the back to the benefits of conservation and increasing green space. Many innovative and handy products are making reusing and making cutbacks more convenient, but activities like increasing green spaces and enhancing carbon sequestration holds special value in their own right.

So what are some ways to help conserve, expand, and enhance green spaces?

Physically planting trees and other plants is one way, as well as supporting conservation initiatives locally and globally through organizations like WWF. Also, reducing or eliminating meat consumption reduces the amount of land space required to grow foods that are used to feed livestock.

In the end, when it comes to cutting our CO2 and making sure there are lots of trees, both are needed (PIK Press), and there are many things we can do to improve in to both "cutting the carbon", and actively increase its sequestration.

Sources

Global Footprint Network. (2018, December). Climate change and the Ecological Footprint and carbon footprint. Retrieved April 8, 2019, from https://www.footprintnetwork.org/2017/11/09/ecological-footprint-climate-change/

Marvel, K. (n.d.). Ask The New York Times. Retrieved April 2, 2019, from https://www.nytimes.com/ask/answers/what-are-aerosols-particulates-role-in-climate-change

Parry, W. (2012, January 30). Volcanoes May Have Sparked Little Ice Age. Retrieved April 1, 2019, from https://www.livescience.com/18205-ice-age-volcanoes-sea-ice.html

PIK Press. (2017, May 18). Climate stabilization: Planting trees cannot replace cutting CO2 emissions. Retrieved April 8, 2019, from https://www.pik-potsdam.de/news/press-releases/climate-stabilization-planting-trees-cannot-replace-cutting-co2-emissions

Samset, B. H., Sand, M., Smith, C. J., Bauer, S. E., Forster, P. M., Fuglestvedt, J. S., . . . Schleussner, C. (2018, January 24). Climate Impacts From a Removal of Anthropogenic Aerosol Emissions. Retrieved March 3, 2019, from https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017GL076079

Schiffman, R. (2018, March 8). How Air Pollution Has Put a Brake on Global Warming. Retrieved March 3, 2019, from https://e360.yale.edu/features/air-pollutions-upside-a-brake-on-global-warming

Thompson, A. (2012, December 21). What is a Carbon Sink? Retrieved April 1, 2019, from https://www.livescience.com/32354-what-is-a-carbon-sink.html

USGS. (n.d.). What is carbon sequestration? Retrieved from https://www.usgs.gov/faqs/what-carbon-sequestration?qt-news_science_products=0#qt-news_science_products

Title Edited January 2020

'Climate Change' vs 'Global Warming'

      "How is there global warming if it is snowing in Hawaii?"

      "I don't remember winters in Ontario being this cold before. Does this mean global warming is not a problem anymore?"

These are not bad questions. Often the terms 'climate change' and 'global warming' seem to be used interchangeably; they both are typically brought up in the context of human activities harming the natural environment and messing with temperature. This has led some to think that these are just two different terms that mean the same thing.

However, while both have to do with the the way human activities and carbon emissions are affecting our planet, they actually have different meanings. So what's the difference?

Global Warming

"[The] phenomenon of increasing average air temperatures near the surface of Earth over the past one to two centuries" - Britannica.com

      If you measured the temperature in different places all over the world, all that data might not necessarily tell you whether or not you'll need a warmer jacket for life in your hometown. However, with enough representative points of data, you could use it to calculate the Earth's overall average temperature.
     The increase in the Earth's overall average surface temperature is what we call Global Warming.
     Considering how since the industrial revolution there has been so much more production, more cars, and fewer trees to filter carbon dioxide out of the air, it's easy for many people to picture the greenhouse effect increasing the planet's average temperature.
     But then how do colder temperatures in some places come into play?

Climate Change

"Climate change occurs when long-term weather patterns are altered — for example, through human activity. Global warming is one measure of climate change..." - davidsuzuki.org (italics added)

While weather is short-term atmospheric conditions, climate describes an area's normal, long-term weather patterns. For example, a desert may have rainy weather for a day, but it does not have a wet climate, and only a certain amount of rain would be reasonably expected.
    From area to area, "normal" climate will mean something different. It's normal to have snow in Alaska, but much hotter weather is normal in LA. Therefore, a "change" in climate can mean different things in different places. A specific area's climate change may not necessarily be manifested as cool-to-hot, but it can cause extreme weather events to increase in general.
     Even human infrastructure is designed based on predictable ranges of day-to-day weather that the region might be expected to experience, with some margin for error. However, climate systems are very complex, so different areas are experiencing problems because their infrastructure was not designed for what is becoming current conditions.

      We might not be able to observe global warming directly in our own backyards, but we still could perceive climate change through a lot of weird weather lately.
      So if you happen to be on a coast where the waterline has risen far past your favourite spot, feel free to shake your head at global warming. Conversely, if you happen to be in Hawaii and you find it has been cold enough to be snowing at oddly low elevations, you can go ahead and blame climate change.

Updated 2019-03-17

Resources

Kennedy, C., & Lindsey, R. (2015, June 17). What's the difference between global warming and climate change? Retrieved February 25, 2019, from https://www.climate.gov/news-features/climate-qa/whats-difference-between-global-warming-and-climate-changeLam, K. (2019, February 12). 

There's 'no place on the planet' – not even Hawaii – to escape climate change, experts say. Retrieved February 25, 2019, from https://www.usatoday.com/story/news/2019/02/11/hawaii-extreme-weather-example-climate-change-experts/2843281002/Selin, H., & Mann, M. E. (2019, January 30).

Global warming. Retrieved February 25, 2019, from https://www.britannica.com/science/global-warming

What is climate change? (n.d.). Retrieved February 25, 2019, from https://davidsuzuki.org/what-you-can-do/what-is-climate-change/

What's in a name? Weather, global warming and climate change. (2019, February 06). Retrieved February 25, 2019, from https://climate.nasa.gov/resources/global-warming/

Kennedy, C., & Lindsey, R. (2015, June 17). What's the difference between global warming and climate change? Retrieved February 25, 2019, from https://www.climate.gov/news-features/climate-qa/whats-difference-between-global-warming-and-climate-change

Lam, K. (2019, February 12). There's 'no place on the planet' – not even Hawaii – to escape climate change, experts say. Retrieved February 25, 2019, from https://www.usatoday.com/story/news/2019/02/11/hawaii-extreme-weather-example-climate-change-experts/2843281002/

Selin, H., & Mann, M. E. (2019, January 30). Global warming. Retrieved February 25, 2019, from https://www.britannica.com/science/global-warming

What is climate change? (n.d.). Retrieved February 25, 2019, from https://davidsuzuki.org/what-you-can-do/what-is-climate-change/

What's in a name? Weather, global warming and climate change. (2019, February 06). Retrieved February 25, 2019, from https://climate.nasa.gov/resources/global-warming/

Kennedy, C., & Lindsey, R. (2015, June 17). What's the difference between global warming and climate change? Retrieved February 25, 2019, from https://www.climate.gov/news-features/climate-qa/whats-difference-between-global-warming-and-climate-change

Lam, K. (2019, February 12). There's 'no place on the planet' – not even Hawaii – to escape climate change, experts say. Retrieved February 25, 2019, from https://www.usatoday.com/story/news/2019/02/11/hawaii-extreme-weather-example-climate-change-experts/2843281002/

Selin, H., & Mann, M. E. (2019, January 30). Global warming. Retrieved February 25, 2019, from https://www.britannica.com/science/global-warming

What is climate change? (n.d.). Retrieved February 25, 2019, from https://davidsuzuki.org/what-you-can-do/what-is-climate-change/

What's in a name? Weather, global warming and climate change. (2019, February 06). Retrieved February 25, 2019, from https://climate.nasa.gov/resources/global-warming/

Lingering in the Air

Sometimes it's surprising to see how long it takes to clear lingering tension from the air.

It has been discovered that a parallel principle applies to the literal air surrounding a chemical plant in St. Louis, Muchugan, that had been shut down long ago.

In 1978 the Michigan Chemical (also called Velsicol Chemical) was closed because it posed a threat to the environment. However, despite years of cleanup the air and ecosystems surrounding the Superfund site remain contaminated 36 years later.

• Environmental Word of the Day - Superfund: A program run by the American government for the long-term cleanup of hazardous waste sites that are dangerous to ecosystems and/or (therefore by default) human health.

The contamination of the atmosphere was measured by testing the concentrations of pollutants in the barks of trees in the area, including chemicals that had been produced by the plant like DDT, PBB, and HBB. For comparison, they also tested chemicals that were not produced by the plant and found that within a 6km radius there were still "relatively high levels of HBB, PBBs, and DDTs in the air they [the ones living there] breathe" (Proscia, 2014).

Don't Hold Your Breath

Angela Peverly, the lead author of the study, stated it well: "People can't control what they breathe" (Proscia, 2014). Although the exact potential threat for long-term effects on the health of people in the area has not yet been investigated, we know that it is there.

An article by Amanda Proscia regarding the study published September 20, 2014 in Great Lakes Echo also included a summary of the history behind the plant concluding that after cleanup of a leakage into the Pine River contamination of fish in the river "dramatically dropped, but a 1982 no consumption warning for all species of fish remains in effect" (Proscia, 2014).

Taking into consideration all these things, these lingering chemicals are not safe for humans or other animals, and a lack of a numerical value to measure exactly how harmful it is certainly is not a reason to ignore it. The article mentions the idea of discarding contaminated trees instead of exporting them, but states that "The EPA did not respond to numerous requests for comment" (Proscia, 2014), though on their website they state their involvement (Environmental Protection Agency, 2014). Still, there are other sources that we can pull information from to piece together what all this could mean, and what the takeaway is.

What do the chemicals do to living things?

It is important to remember that as the first law of ecology states, everything is connected to everything else. Even if only one of these elements were effected, it inevitably would affect others in the ecosystem. 

DDT

• Low levels (even 0.1 micrograms/litre) can slow photosynthesis and growth in green algae (Cruising Chemistry, n.d.).
• DDT has proven to greatly harm birds, especially ones that eat fish, and cause eggshells to thin (Cruising Chemistry, n.d.).
• High levels can cause reproductive problems in fish (Gilbert, DeCarvalho, 2013).
• Although  absorbed through skin, it is also toxic to humans (Cruising Chemistry, n.d.) and if ingested it can be stored in fatty organs and contaminate breast milk
• Carcinogenic, an endocrine disruptor, and can have reproductive and developmental effects (Gilbert, DeCarvalho, 2013).

• Endocrine disruptor: A chemical that messes up the hormone system.
• Carcinogenic: Cancer-causing

PBB

*much was learned about the harmful effects of PBB when mislabeling/mismanagement of chemicals at a chemical plant in Michigan during the 70s (we can only guess which one) caused contamination of this flame-retardant in feed for farm animals.

• PBB proved to cause skin disorders, effects on the nervous system, liver, kidney, thyroid glands, and immune systems in animals tested with PBB, and death of given high amounts. 
• The thyroid is a major organ in the endocrine system.
• Carcinogenic; Animals fed PBB in developed cancer
• Accumulates in the body, and effects can worsen in the long-term (ATSDR, 2011)

HBB

(a type of PBB)

• An endocrine disruptor
• Possible carcinogen
• "lipophilic and able to bioconcentrate" (POPs Toolkitm n.d.)... meaning it can be stored and accumulate in fat, and the concentration can be greater in the organism than the contaminated environment.

Now What?

As of July 2014, the EPA and Michigan Department of Environmental Management are continuing the cleanup in the nearby residential areas, cleanup design work at the actual former site, and replacement of municipal drinking water.

Still, a question worth asking is, 'Who pays for pollution?' 

...the cleanup? Taxpayers/people.

...the research? Universities/students/people.

...the pollution? Everyone.

When we make choices that are better or worse for the environment, it comes back to us. The the dirt and the atmosphere and the river are not the ones that feel unwell when they are polluted.

An even better question to ask 'is what can we do about it?'

Note, the mislabeling of chemicals was an accident. The contamination is not evidence that mismanagement is motivated by an intent to harm, but how easily it can happen, and about how important it is to be aware.

Despite having a limited reach, here is something we can do:

• Keep aware. Look into where your products are coming from, and avoid ones containing toxic chemicals or emit toxins in their production.

This will help protect people from exposure to these dangerous chemicals through the products they buy and ecosystems overall if the consumer demand for such products goes down. It is not fair for anyone to be exposed to toxic chemicals in air or water; it is not something that an individual can simply choose not to do, but our power lies in the choices we do have every day.

Sources
Agency for Toxic Substances and Disease Registry. (n.d.). Public Health Statement for Polybrominated Biphenyls (PBBs). Agency for Toxic Substances and Disease Registry. Retrieved October 1, 2014, from http://www.atsdr.cdc.gov/phs/phs.asp?id=527&tid=94

Cruising Chemistry. (n.d.). Effects of DDT. Cruising Chemistry. Retrieved September 30, 2014, from http://people.chem.duke.edu/~jds/cruise_chem/pest/effects.html

Environmental Protection Agency. (n.d.). Superfund. EPA. Retrieved September 28, 2014, from http://www.epa.gov/superfund/

Environmental Protection Agency. (2014, July 18). Velsicol Chemical Corp. (Michigan) - Region 5 Cleanup. EPA. Retrieved October 1, 2014, from http://www.epa.gov/region5/cleanup/velsicolmichigan/

Gilbert, S. G., & DeCarvalho, J. P. (n.d.). DDT. Toxipedia. Retrieved September 30, 2014, from http://www.toxipedia.org/display/toxipedia/DDT

POPs Toolkit. (n.d.). Hexabromobiphenyl (HBB). Persistent Organic Pollutants (POPs) Toolkit. Retrieved October 1, 2014, from http://www.popstoolkit.com/about/chemical/hbb.aspx 

Peverly, A. A., Salamova, A., & Hites, R. A. (2014, September 11). Air is Still Contaminated 40 Years after the Michigan Chemical Plant Disaster in St. Louis, Michigan. ACS Publications. Retrieved October 1, 2014, from http://pubs.acs.org/doi/abs/10.1021/es502809f

Proscia, A. (2014, September 30). Air near chemical plant remains polluted long after it closed. Great Lakes Echo. Retrieved October 1, 2014, from http://greatlakesecho.org/2014/09/30/air-near-chemical-plant-remains-polluted-long-after-it-closed/?utm_source=feedburner&utm_medium=email&utm_campaign=Feed%3A+greatlakesecho%2Fall+%28Great+Lakes+Echo+%28All%29%29

A Report on Flooding and Climate Change

Climate Change Effects - Flooding

Melody Tadeo

    While there are many impacts climate change has on the environment and therefore also human society, one of the most concerning ones is the increased risk of flooding. This is an issue involving both water and atmosphere as flooding may occur because of increased amounts of precipitation in some regions, spring runoff, and may also along the coast as sea levels rise. During "storm surges", changes in the wind and atmospheric pressure due to storms already cases damage which will only be increased with a rise in sea level. Floods are a threat to human health and security, economy, and natural ecosystems.

     One of the implications of atmospheric change is higher potential for rainfall in some regions. Around the world many communities have been built around bodies of water, putting people living near rivers and on floodplains at risk during times of heavy rainfall. The UK Environmental Agency has already provided an online map available to citizens to locate areas that are at a higher risk of flooding, specifically near rivers and the coast. After the Somerset Levels Flood, it took three months for the land to dry and it may be two years before the soil returns to levels where viable crops can be grown. Land animals are also vulnerable to floods, as reported during flooding in Britain in February 2014. Many animals such as ground-dwelling hedgehogs and badgers also drown in floodwaters, and surviving animals may suffer from a lack of food resources. Floodwaters may also carry pollutants such as lead, waste, toxic chemicals from industrial areas, and pesticides, which can affect both wild animals in flooded areas and livestock that may ingest contaminants while grazing. Populations of bumblebees that were hibernating as well as earthworms would also greatly decrease for many drown during floods or their hibernating cycle is disrupted.bumblebees play the vital role of pollination, and the affects on creatures such as earthworms will likely make its way up the food chain.

     In other places such as the Sacramento San-Joaquim River Delta, flooding may threaten water source quality and supply for people due to runoff. The University Corporation for Atmospheric Research (UCAR) states "[on] average, U.S. flooding kills more than 100 people a year --more than any other single weather hazard, including tornadoes and hurricanes" and that the toll due to flooding has actually increased in recent decades. During times of unexpected flash floods the strength of the water is sometimes underestimated, and about half of related deaths are due to driving into flooded areas to cross streams or drive on the highway. 

     In other communities, however, driving is not the greatest threat to human safety but rather complete displacement of communities and illness; from 1970 to 2008 over 95% of disaster-related deaths occurred in developing nations. In August 2010 over 20 million people in Pakistan lost their homes and over 1,500 people lost their lives in the most devastating flood in the country's history. Even after the initial damage was done water-related illness continued to inflict people and World Health Organization said "waterborne, airborne and vector-borne diseases, including acute watery diarrhea, measles, malaria and acute respiratory infections, are threats due to overcrowding, lack of hygiene and breakage in waterlines". In crowds of people that found themselves homeless there was fear of political unrest, "flood riots", and people fought over limited amounts necessities such as food. According to the federal flood commission, 557,000 hectares of agricultural land was flooded in Pakistan, over 10,000 cows died, and overall the damage was estimated to cost about $7.1 billion.

     During the following year a number of countries in the Global Climate Risk (GCR) Index --including Brazil, El Savadore, Laos, and Thailand-- had record-breaking natural disasters. The GCR reported that floods and landslides caused over 1,000 deaths in Brazil, and 45 deaths and $43 billion of flood damage when Thailand was hit by tropical storm Nock-ten also in 2011. These are only a few examples of floods that cause damage to land, including communities and ecosystems, threaten human health and safety, and have negative economic effects. For coastal countries like Cuba that largely depend on tourism, a rise in sea level could also put a major industry in jeopardy, as well as the natural ecosystems that attract so many visitors.

     Aquatic ecosystems are already vulnerable to a number of environmental threats, and spring runoff could also affect the water quality of coastal areas, such as Chesapeake Bay and the Gulf of Mexico where "dead zones" are already being created due to agricultural fertilizer runoff, threatening the lives of animals in aquatic ecosystems as well as biodiversity. In saltwater and freshwater ecosystems flooding of rivers and creeks also causes erosion of soil that can prevent vegetation from being able to grow --also making it even more vulnerable to erosion by later rainfall-- and damaging habitats of animals such as turtles. Although some species can still survive, many are forced to move to higher ground and floodwaters can also spread species of fish and other aquatic wildlife into neighboring ecosystems as foreign and potentially invasive species if they are able to survive. Chemicals such as fertilizers can be transferred and pollute ecosystems during floods, as well as various other contaminants including sediments, nutrients, and even fresh water that can change the water quality that local species require to survive.

     Especially under the influence of other environmental and social concerns, including rising sea levels, increased tropical storm activity, and changes in rainfall which can also affect inland areas, flooding poses severe threats to human lives, societies, and natural ecosystems. While flooding is a natural phenomenon, it is important that needs to be prepared for as the effects of climate change become ever more apparent and complex.

References

Batty, D. (2010, August 14). Impact of Pakistan floods as bad as 1947 partition, says prime minister. theguardian.com. Retrieved March 27, 2014, fromhttp://www.theguardian.com/world/2010/aug/14/pakistan-flooding-disaster-partition-gilani

Bawden, T. (2014, February 17). UK weather: Floods could have devastating environmental impact as animals drown or die from lack of food . The Independent. Retrieved March 27, 2014, from http://www.independent.co.uk/news/uk/home-news/uk-weather-floods-could-have-devastating-environmental-impact-9132299.html

CNN Wire Staff. (2010, August 21). Death toll from Pakistan flooding tops 1,500 as risk of diseases rises. CNN. Retrieved March 27, 2014, fromhttp://www.cnn.com/2010/WORLD/asiapcf/08/21/pakistan.floods.death.toll/

Environmental Protection Agency. (n.d.). Southwest Impacts & Adaptation. EPA. Retrieved March 26, 2014, from http://www.epa.gov/climatechange/impacts-adaptation/southwest.html

Irin news. (n.d.). CLIMATE CHANGE: Natural disasters made history in 2011. IRIN news - Humanitarian news and analysis. Retrieved March 27, 2014, fromhttp://www.irinnews.org/report/96895/climate-change-natural-disasters-made-history-in-2011

MCEER. (n.d.). Pakistan Floods 2010: Latest Facts, News, Photos & Maps. Pakistan Floods 2010: Facts, News, Photos & Maps. Retrieved March 27, 2014, fromhttps://mceer.buffalo.edu/infoservice/disasters/Pakistan-Floods-2010.asp

University Corporation for Atmospheric Research. (n.d.). Flooding Fact Sheet. UCAR. Retrieved March 26, 2014, from http://www.ucar.edu/communications/factsheets/Flooding.html

Wharton, J. (2014, March 27). Finally dry: The Somerset Levels are free from floods - THREE months after the rain. Daily Express UK RSS. Retrieved March 27, 2014, from http://www.express.co.uk/news/uk/467251/Finally-dry-The-Somerset-Levels-are-free-from-floods-THREE-months-after-the-rain

Wildlife Preservation Society of Qld. (n.d.). Environmental Impact of Floods - February 2011. Wildlife Queensland - Environmental Impact of Floods. Retrieved March 27, 2014, from http://www.wildlife.org.au/news/2011/flooding5.html

     

The Shape of a Raindrop

     Despite weather channel depictions and after years of drawing raindrops with a pointed top and rounded bottom, I was surprised to learn that raindrops are not shaped like teardrops.

     Air resistance is not the cause of the typically-drawn shape that appears as raindrops streak across windows or on peoples' face when we dry; that is caused by surface tension of the liquid water on the surface.

     As they fall through the air, raindrops are actually spherical in shape. Smaller raindrops have less air resistance so they are more spherical than larger ones. The larger the raindrop, the more air resistance there is, and greater air resistance actually just flattens the bottom; a pointed tail/trail is actually not formed by air resistance. When a raindrop gets large enough (usually more than 4mm), it will actually split in half and drastically decrease in surface tension, possibly splitting so small that the water just rejoins as moisture the atmosphere.

Sources

Baird, Christopher S.. What Makes Raindrops Tear-Shaped? December 17, 2012 Science Questions with Surprising Answers. http://sciencequestionswithchris.wordpress.com/2012/12/17/what-makes-rain-drops-tear-shaped/

Oblack, Rachelle. The Shape of Raindrops. http://weather.about.com/od/cloudsandprecipitation/a/rainburgers.htm