 
The 1870 Atlantic hurricane season marked the beginning of Father Benito Viñes investigating tropical cyclones, inspired by two hurricanes that devastated Cuba that year; Viñes consequently became a pioneer in studying and forecasting such storms. The season featured 11 known tropical cyclones, 10 of which became a hurricane, while 2 of those intensified into major hurricanes. However, in the absence of modern satellite and other remote-sensing technologies, only storms that affected populated land areas or encountered ships at sea were recorded, so the actual total could be higher. An undercount bias of zero to six tropical cyclones per year between 1851 and 1885 and zero to four per year between 1886 and 1910 has been estimated.
Reanalysis by meteorologists José Fernández-Partagás and Henry F. Diaz in 1995 led to the inclusion of seven previously undocumented Atlantic tropical cyclones – the second, third, fifth, seventh, eighth, tenth, and eleventh storms. However, Atlantic hurricane reanalysis project did not add any new systems during their review of the 1870 season in 2011. Little information is available on many of these storms, including three in which their tracks consist of merely a single data point in HURDAT. Climate researcher Michael Chenoweth conducted another reanalysis, published in 2014, which adds a net of three storms, with the tracks of four storms combined into two and five new storms proposed. However, Chenoweth's reanalysis has yet to be added to HURDAT. 
The first known storm was reported near Mobile, Alabama, on July 30 and caused about $250,000 (1870 USD) in damage. Nearly a month later, the next system existed over the western Atlantic Ocean and produced severe impacts in Nova Scotia. Two other storms left significant effects on land, both in Cuba and Florida in October. The first of these, the sixth known system of the season, particularly devastated modern-day Matanzas Province, causing at least 800 deaths, approximately $12 million in damage, and the destruction of hundreds, if not thousands, of dwellings. Later, another cyclone, the season's ninth, wrought substantial impacts to western Cuba, killed 43 people and leaving about $5 million in damage. While in the vicinity of Florida, the hurricane also capsized a ship near the Jupiter Inlet, leading to 52 fatalities. Overall, the tropical cyclones of the 1870 season caused more than $17.25 million in damage and at least 895 deaths.
Season summary.
The Atlantic hurricane database (HURDAT) recognizes 11 tropical cyclones for the 1870 season, 10 of which attained hurricane status. Two of those hurricanes are believed to have intensified into a major hurricane – Category 3 or stronger on the modern-day Saffir–Simpson scale. The first storm of the season was observed along coastal Alabama on July 30, but little else is known about the system's progress. HURDAT begins the track of the next cyclone a month later, on August 30. Later in its duration, the storm moved parallel to Nova Scotia before being last noted near Newfoundland on September 4. Three other storms developed in September; the fourth system peaked with maximum sustained winds of 125 mph (205 km/h) and a minimum barometric pressure of , becoming the most intense tropical cyclone of the season. October became the most active month of the season, with six known storms forming. The last of these, the eleventh storm of 1870, was last encountered by ships in the southeastern Gulf of Mexico on November 3. 
Collectively, the cyclones of the 1870 season caused at least 895 fatalities and more than $17.25 million in damage, the vast majority due to the sixth storm. However, "The Times" lists the sixth storm's death toll as high as 2000. Further, other sources estimate that the combined damage total in Cuba for the sixth and ninth storms reached as much as $27 million. Newspapers also reported that the two hurricanes damaged about one-fourth of Cuba's sugar crop, while "the fruit of the island is half destroyed." Captain General Antonio Caballero y Fernández de Rodas ordered that the Spanish government expend $500,000 in aid for both storms combined.
Father Benito Viñes, a Spanish Jesuit priest, became the director of the meteorological observatory at the Colegio de Belén in Havana, Cuba, in 1870. Following two storms that devastated the country in October of that year, Viñes began studying those hurricanes and previous systems – including their circulations, organization, and movements – becoming a pioneer in tropical cyclone forecasting. Later, Viñes issued possibly the first-ever hurricane warning in 1875 and founded a network of weather stations across the Caribbean Sea in 1876 to improve his forecasts.
The season's activity was reflected with an accumulated cyclone energy (ACE) rating of 88, which was then the highest total since records began in 1851, but has since been tied or surpassed numerous times. ACE is a metric used to express the energy used by a tropical cyclone during its lifetime. Therefore, a storm with a longer duration will have higher values of ACE. It is only calculated at six-hour increments in which specific tropical and subtropical systems are either at or above sustained wind speeds of , which is the threshold for tropical storm intensity. Thus, tropical depressions are not included here.
Systems.
Hurricane One.
The Mobile Hurricane of 1870
The first officially recognized tropical cyclone of 1870 made landfall near Mobile, Alabama, on July 30, though little is known about its genesis or track. According to one report, barometric pressure fell to , but this is considered dubious. The storm's passage was brief, with the strongest winds confined to a period of about two hours in the early afternoon. Storm surge flooding damaged coastal installations along the shores of Mobile Bay, and several steamboats were either sunk or blown ashore. The winds uprooted trees, damaged roofs, and severed telegraph wires throughout Mobile. At the height of the storm, a floating dry dock broke free from its moorings and traveled about up the Mobile River, crushing wharves and boats along its path. Overall, damage from the storm amounted to an estimated $250,000.
Reanalysis by climate researcher Michael Chenoweth, published in 2014, extended the track of this system back to July 29 as a tropical storm over the north-central Gulf of Mexico. Chenoweth also theorizes that the hurricane dissipated near Tuscaloosa, Alabama, on July 31.
Hurricane Two.
Although the ship "Daniel Marcy" likely encountered this storm as early as August 22, the vessel's crew did not record their location. Thus, the official track for this system begins as a tropical storm about northeast of the Lesser Antilles on August 30, the same day the "R.M.S. Shannon" encountered it.
The storm moved northwest towards the East Coast of the United States and intensified into a hurricane on the following day. Early on September 2, the cyclone reached sustained winds of 105 mph (165 km/h), equivalent to a Category 2 hurricane on the modern-day Saffir–Simpson scale. However, the hurricane curved north-northeastward and then northeastward on September 3, remaining offshore. It then travelled parallel to the coast of Nova Scotia until it was last seen on September 4 near Newfoundland.
The coast of Maine experienced abnormally high tides, prompting vessels to seek refuge at a safe harbor. Nova Scotia reported severe impacts, especially along its south coast. At the Halifax neighborhood of South End, a group of 16 to 18 vessels became "entirely broken to pieces beyond the possibility of repair." with a few exceptions, according to the "North British Daily Mail". Strong winds led to the streets being littered with bricks, shingles, and slates, while the storm toppled many large trees and fences in the Spring Garden section of Halifax. A carriage factory was nearly destroyed, suffering about $4,000 in damage. The storm demolished many barns at the North West Arm and destroyed a building in Dartmouth. Significant damage was inflicted to trees and plants at the Nova Scotia Horticultural Society Garden. Additionally, the "Portland Daily Press" reported that the hurricane "destroy[ed] a large amount of marine property and life".
Chenoweth's 2014 study begins the track of this storm on August 25 as a tropical storm east of the Lesser Antilles. The study also indicates that the cyclone struck Nova Scotia on September 4 and Newfoundland later that day after crossing the Gulf of St. Lawrence.
Tropical Storm Three.
A tropical storm was detected to the west of Cabo Verde Islands on September 1. Tracking westward, it was encountered by another ship on September 4. The storm's subsequent progress is unknown. Chenoweth extended the track of this system back to August 29 and combined it with the subsequent storm, while also postulating that the cyclone reached hurricane intensity on August 30.
Hurricane Four.
This hurricane was first encountered by a ship about northeast of San Juan, Puerto Rico, on September 9. The storm tracked generally northward, and on September 10, it passed west of Bermuda. There, the storm produced tropical storm-force winds that extensively damaged banana and other fruit trees. Based on a central barometric pressure reading of from the brig "Lizzie M. Morrell", the storm is believed to have achieved major hurricane intensity. Curving northeastward, the hurricane underwent extratropical transition by September 13. Chenoweth considers this storm and the previous to have been the same cyclone.
Hurricane Five.
Numerous ships in the western Atlantic reported stormy weather from September 17 through September 20. Consequently, the track for this storm begins on the former date about southwest of Bermuda, as a hurricane with winds of 90 mph (150 km/h). By weighting these reports according to their credibility and consistency, Fernández-Partagás constructed an approximate recurving track for the hurricane. Early on September 19, the brig "Eolus" recorded a barometric pressure of , the lowest in relation to the storm. As the system accelerated northeastward, it probably transitioned into an extratropical cyclone late on September 19 about east of Newfoundland. 
Chenoweth estimates that this storm formed on September 9 as a tropical storm about halfway between the Lesser Antilles and the Cabo Verde Islands. The storm then moved northwestward and reached hurricane status by September 11. Additionally, Chenoweth theorized that the extratropical transition occurred on September 20.
Hurricane Six.
First Key West Hurricane of 1870 or Hurricane of San Marcos of 1870
Based on reports from contemporary Cuban meteorologists, a tropical storm was first observed in the Caribbean just south of Haiti's Tiburon Peninsula on October 5. The storm moved west-northwestward and intensified into a hurricane south of Cuba on the following day. Thereafter, the cyclone rapidly strengthened further and likely peaked as a Category 3 hurricane with maximum sustained winds of prior to making landfall in the Ciénaga de Zapata municipality of Cuba's Matanzas Province on October 7. After crossing Cuba, the hurricane moved slowly northeastward through the Straits of Florida, tracking close to the Florida Keys as a Category 2 hurricane for a few days. The storm then struck Grand Bahama and the Abaco Islands in the Bahamas early on October 12 while still a Category 2 hurricane, but soon weakened to Category 1 status. It moved out to sea, last being seen west of Bermuda on October 14.
In Cuba, the hurricane produced high winds and intensive rainfall. Two rivers, the San Juan and the Yumuri, burst their banks, devastating the city of Matanzas and causing the loss of some 800 lives. These impacts destroyed approximately 410 homes, with the Cuban Meteorological Society stating in 2000 that "this number probably refers only to the city [Matanzas]." Floodwaters swept away two bridges, a railroad station, and a train, along with its passengers. The nearby city of Cárdenas reported major damage to warehouses and wharves, as well as the destruction of several dwellings; many more lost their roofs or otherwise suffered damage. Throughout the country, a loss of roughly one-third of sugar crops occurred – roughly 300,000 to 400,000 boxes. At least 800 people died in Cuba, making it one of the deadliest Atlantic hurricanes on record. Damage was estimated at over $12 million (USD). In Florida, the lower parts of Key West became "one complete sheet of water", according to the "Key West Dispatch", while tides swept away a few bridges. Significant damage to vegetation and trees also occurred. Numerous vessels wrecked throughout the Florida Keys and along or just offshore the mainland as far north as the Jupiter Inlet. In the Bahamas, Bimini reportedly began experiencing the storm as early as October 7. Abnormally high tides inundated South Bimini, leaving people "reduced to a state of starvation", according to "The Nassau Guardian". The hurricane also capsized and dismasted a number of ships sailing in the vicinity of the Bahamas.
Chenoweth's 2014 study indicates that this storm remained well south of the Florida Keys and rapidly intensified over the Straits of Florida. On October 9, the cyclone briefly peaked as a Category 5 hurricane with winds of 160 mph (260 km/h), based on a peripheral atmospheric pressure of