Glossary of Technical Terms

There are many terms used in the field of Climatology which may be confusing to non-technical users. Listed below are some terms which are commonly used in the bulletins of the Caribbean RCC:

Above-normal rainfall ( / temperature) – When used in climate outlooks: the rainfall sum ( / average temperature) of the entire forecasted period is higher than usual, i.e. wetter ( / warmer) than usual, specifically being among the top 33% of wettest ( / warmest) years in the historical past.

Below-normal rainfall ( / temperature) – When used in climate outlooks: the rainfall sum ( / average temperature) of the entire forecasted period is lower than usual, i.e. drier ( / cooler) than usual specifically being among the bottom 33% of driest ( / coolest) years in the historical past.

Climate – The mean state and other statistics (such as the occurrence of extremes) of conditions in the atmosphere, ocean and other parts of the environment as observed over many years or decades.

Climatology – A location’s climatology refers to the summary statistics (i.e. averages, extreme values, frequency of events, etc.) of weather variables (e.g. temperatures, precipitation, sunshine, etc.) or events (e.g. dry spells, wet spells, heatwaves, etc.) as computed from a historical record of observations taken over a given time span of typically 30 years or longer.

A reference climatology is a climatology using observations spanning the World Meteorological Organization’s reference periods (i.e. 1961-1990, 1971-2000, 1981-2010, etc.), with the current reference period being 1981-2010.

It is, however, customary to analyse the climatology of extremes using longer periods of record because the occurrence of extremes is rare and the summary statistics will only be robust once a large enough sample of occurrences is contained within the period of record.

NB: in other contexts, this term can refer to climate science.

Climate change – A change in the state of the climate that can be identified by changes in the mean state and/or the variability of its properties, and that persists for an extended period, typically decades or longer, as driven by natural factors (such as volcanic activity or solar activity) or man-made factors (such as greenhouse gas emissions, changes in land use).

Climate Change Projection – An assessment on climate conditions during the coming decades or longer that is based on output from one or several models of the atmosphere and, where relevant, the ocean and other parts of the environment. In such models, climate change is driven by scenarios of natural (such as volcanic activity or solar activity) or man-made factors (such as greenhouse gas emissions, changes in land use).

Climate extreme – Climate extremes are rare events that strongly deviate from usual conditions. The occurrence of a value of a climate variable or condition beyond a threshold near the upper or lower ends of the historical range or distribution of observed values. Some examples include extremely active hurricane seasons, drought, long dry spells, record warm or cool temperatures, extremely wet seasons or a rapid succession of extreme wet spells within a season.

Climate forecast – An assessment of climate conditions during the coming months or years that is based on output from one or several models of the atmosphere and, where relevant, the ocean and other parts of the environment. Forecasted climate variability is driven by the latest state of the climate conditions, with ocean temperatures being the most important driver of climate variability.

Climate impact – A physical hazard and/or socio-economic impact associated with the occurrence of a climate event or climate extreme.

Climate information – Climate information is a broad term that refers to knowledge and advice about the past, present and future characteristics of the Earth’s climate and at all relevant time and space scales. It includes summary statistics, historic time-series records, near-real-time monitoring, predictive information from daily weather to seasonal to inter-annual time scales, and climate change scenarios.

Climate monitoring – This is continuous observation and assessment of accurate, long-term records of the atmosphere and other climate indicators (e.g. global mean Earth surface temperature and precipitation). This monitoring is done to keep track of variability of the earth’s climate over different time scales, from seasons to decades to thousands of years and more, and to understand the impacts that the changes can have on all aspects of life: environment, economy and society. By carefully monitoring the climate, the effects of extreme events can be chronicled and mitigated. Monitoring also allows for the detection of climate change, its driving forces as well as its impacts around the world.

Climate Outlook – In order to understand what climate conditions to prepare for in coming months, knowledge on the recent conditions and what usually happens in the coming season is necessary alongside the forecast. A climate outlook is a forecast that is contextualized with information on recent climate conditions and their hazardous or beneficial impacts (i.e. what conditions do we start from?), usual climate conditions in the period of forecast (i.e. what conditions do we usually get in the upcoming season?), and potential implications of the forecast in terms of expected impacts (i.e. what will we have to worry about or which opportunity to seize in the next season?)

Climate prediction – In the broad sense, the art and science of making inferences on future climate conditions using models of the physical (and, where relevant, chemical, biological and geological) behavior of the environment. Climate prediction encompasses climate forecasts and climate change projections.

Climate risk – Climate risk can be defined qualitatively as the likelihood of unfavorable impacts occurring as a result of severe climate events interacting with vulnerable environmental, social, economic, political or cultural conditions. It can also be defined more quantitatively, as a function of the probability of a given hazardous climate event, a element’s vulnerability and its exposure to this hazard. As such, climate risk originates from a dynamic combination of climate hazards (e.g. extent and duration of extreme temperatures or rainfall) and the vulnerabilities (propensity or predisposition to be adversely affected) of exposed elements (e.g. communities, economic or societal sectors or ecosystems).

Climate risk management (CRM) – A systematic and coordinated process in which climate information is used to reduce the risks associated with climate variability and change, and to take advantage of opportunities, in order to improve the resilience of social, economic and environmental systems.

Climate variability – Variations in the mean state and other statistics (such as standard deviations, the occurrence of extremes, etc.) of the climate on all temporal and spatial scales beyond that of individual weather events that occur naturally and typically last from a month to several years.

Dry day – A 24 hour period during which the rainfall total is less than 1 mm.

Dry spell – A succession of at least 7 consecutive dry days.

El Niño conditions – El Niño is a naturally occurring climate pattern associated with large-scale weakening of the trade winds and warming of the ocean surface temperatures in the central and eastern tropical Pacific Ocean, which can significantly influence weather patterns, ocean conditions, and marine fisheries worldwide.

El Niño conditions are a major driver of regional drought in the Caribbean and can further decrease the Atlantic Hurricane Season activity. Furthermore, El Niño conditions tend to also increase temperatures in the Caribbean for several months.

El Niño event – Once El Niño conditions have manifested for at least five consecutive months, an El Niño event is declared.

ENSO – The El Niño Southern Oscillation is a periodic fluctuation (i.e., every 2–7 years) in sea surface temperature (El Niño) and the air pressure of the overlying atmosphere (Southern Oscillation) across the equatorial Pacific Ocean. ENSO conditions can be divided in three different categories: El Niño, neutral and La Niña conditions.

ENSO neutral conditions – The regular situation when ENSO conditions are neither consistent with El Niño nor La Niña, in other words when tradewinds and sea surface temperatures in the tropical Pacific are near average.

Extreme wet spell – 3 consecutive days of which the total rainfall is extremely high and can increase the potential for flash flooding.

Heat stress – A situation where too much heat is absorbed by a person, a plant or an animal and causes stress, illness or even death. Heat stress is manifested by elevated body temperature, hot, dry skin, lack of sweating and neurological symptoms such as paralysis, headache vertigo and unconsciousness. It can also cause heat cramps, heat exhaustion and heat stroke which may lead to death.

Heat wave – A multi-day period of continuous extremely high temperatures or feel like temperatures. Such heat waves tend to cause acute heat stress.

Hurricane – A tropical cyclone associated with maximum wind speeds above 73 miles per hour.

La Niña conditions – La Niña is a naturally occurring climate pattern associated with large-scale strengthening of the trade winds and cooling of the ocean surface temperatures in the central and eastern tropical Pacific Ocean, which can significantly influence weather patterns, ocean conditions, and marine fisheries worldwide.

La Niña conditions tend to increase chances of extreme rainfall in the Caribbean and can increase the Atlantic Hurricane Season activity. Furthermore, La Niña conditions tend to also slightly decrease temperatures in the Caribbean for several months.

La Niña event – Once La Niña conditions have manifested for at least five consecutive months, a La Niña event is declared.

Long-term (meteorological) drought – A rainfall deficit over a total period of 9 to 24 months, but specifically 12 months when concerning the CariCOF drought outlooks and drought alert maps.

(Near-) normal rainfall ( / temperature) – when used in climate outlooks: the rainfall sum ( / average temperature) of the entire forecasted period is quite the usual, in other words not too far from average, specifically being among the middle 33% of years in the historical past.

Seasonal climate forecast – the guidance offered by a forecaster or forecast centre on the climate conditions during the coming months. NB: This forecast information pertains to the 3 months highlighted in a given bulletin issue.

Seasonality of temperature in the Caribbean – Fairly consistent temperatures throughout the year, with an annual temperature range of from less than 2C in the south (i.e. the Coastal Guianas) to about 5C in The Bahamas and at high elevations in the northwest (i.e. Belize, Cuba and Jamaica), and with heatwaves possible from May to September (the Bahamas, Belize and the Greater Antilles) or October (Lesser Antilles) and from August to November (Guianas).

Seasonality of rainfall in the Caribbean – Throughout Belize and the Caribbean Islands, there is one dry and one wet season. In the Bahamas, Belize, the Greater and Lesser Antilles, the wet season runs from May/June to November/December, the dry season during the other half year, whereas in the ABC Islands (Aruba, Bonaire and Curaçao), which are much drier than the remainder of the region, there is a short wet season from October to January. In the Guianas, the northern and eastern parts experience two wet seasons (May to July or early August, and late November to early February) and two dry seasons (mid-February to April, and mid-August to mid-November) annually, whereas in the southwestern parts, there is only one wet season (April to August) and one dry season (September to March). Extreme rainfall events typically do not occur from January to March in Belize and the islands and they are rare between September and the middle of November in the Guianas. By contrast, dry spells of 7 to 15 days can and do frequently occur at any time of the year in areas westwards of Puerto Rico, whereas eastwards of Puerto Rico their frequency tends to increase towards the end of the dry season and decrease in the early wet season.

Short-term (meteorological) drought – A rainfall deficit over a total period of 3 to 6 months.

SPEI – The standardized precipitation-evapotranspiration index is a drought index used to represent the balance between rainfall as a source of moisture and evaporation as a loss of moisture over a given number of months.

The main advantage gained over the SPI is that the SPEI can account for the increase in evaporation encountered when temperatures are higher. Given a certain amount of rainfall, at higher temperatures the evaporation rates are higher, and therefore the amount of water remaining in the ground decreases, potentially worsening drought impacts. The main disadvantage over the SPI is that monthly temperature records are fewer than rainfall records, especially in the tropics, meaning a reduced geographic coverage and resolution would result from using the SPEI compared to the SPI.

SPI – The standardized precipitation index is a drought index used to represent rainfall deficits (or surpluses) over a given number of months.

As a result of the flexibility in the number of months an SPI is calculated to measure short term (1-3 months) or progressively longer term drought (up to 24 or 48 months), the SPI has been found to correlate with a broad variety of drought impacts in a diversity of socio-economic sectors, including water resources management, agriculture, health, etc. Another strength of the SPI index as a drought index is that, relying solely on monthly rainfall totals, it benefits from the largest number of observed records from weather stations available, allowing the best possible geographic coverage.

Tropical cyclone – A rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, (possibly) strong winds, and, in most cases, heavy rainbands. Tropical cyclones in the Atlantic are divided in three different categories depending on the maximum wind speed anywhere within the cyclone: tropical depression, tropical storm or hurricane.

Tropical depression – A tropical cyclone associated with maximum wind speeds below 39 miles per hour.

Tropical storm – A tropical cyclone associated with maximum wind speeds between 39 and 73 miles per hour.

Vulnerability – The propensity or predisposition to be adversely affected. Vulnerability encompasses a variety of concepts and elements including sensitivity or susceptibility to harm and lack of capacity to cope and adapt.

Weather – Conditions of the atmosphere such as temperature, winds, cloudiness, air pressure, etc. as they vary on time scales from seconds to about two weeks.

Weather extreme – A rare and strong deviation from usual weather conditions at a certain location for a certain time of the year, such as hurricanes, record high and low temperatures, heatwaves, extreme wet spells.

Wet Day – A 24 hour period during which the rainfall total is at least 1 mm.

Wet Spell – A multi-day period during which the rainfall total is large enough to cross a certain threshold.