Delaware Climate Projections Portal

Delaware Climate Terms and Definitions

Weather describes atmospheric conditions at a particular place in terms of air temperature, pressure, humidity, wind speed, and precipitation. Weather varies from place to place and across the globe and is measured in short time periods (days, weeks, years).

Climate describes long-term patterns of temperature,precipitation, and other weather variables. It is often described in terms of statistical averages or extremes over decades, centuries, or even millennia. Climate is generally described in a global or regional context rather than in specific locations.

Climate change describes any significant change in the measures of climate persisting for an extended period of time - decades or longer. Many climate models project that future climates are likely to increase beyond the range of variability experienced in the past. Historical data and trends may no longer be reliable indicators for future climate conditions.

Global warming describes an average increase in temperatures near earth's surface and in the lowest layer of the atmosphere. Increases in temperature in the atmosphere contribute to changes in global climate patterns. Global warming can be considered part of climate change, along with changes in precipitation, sea level, etc.

Greenhouse gases are gaseous compounds that absorb infrared radiation, trap heat in the atmosphere, and contribute to the greenhouse gas effect. These include: carbon dioxide(CO2), methane (CH4), nitrous oxide (N2O), and fluorinated gases (F-gases, which include hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6)).

Natural climate variability - Variation in seasonal, year-to-year, and even multiyear cycles that can result in wetter or drier, hotter or cooler periods than "average" weather measurements. Most natural climate variability occurs over time scales shorter than 20 to 30 years.

Global climate models (GCMs) - Complex, three-dimensional models that incorporate all the primary components of the earth's climate system, including atmospheric and ocean dynamics. Earlier versions that only modeled the atmosphere and ocean were known as general circulation models. (See detailed description in Appendix.)

Climate projections - A description of the future climate conditions based on global climate model simulations driven by a range of scenarios describing future emissions from human activities. A climate projection is usually a statement about the likelihood that something will happen over climate time scales (i.e., several decades to centuries in the future) if a given emissions or forcing pathway is followed. In contrast to a prediction (such as a weather prediction), a projection specifically allows for significant changes in the set of boundary conditions, such as an increase in greenhouse gases, which might influence the future climate. As a result, what emerge are conditional expectations (if X happens, then Y is what is expected).

Observations - Data collected from weather stations, usually daily, using measurement instruments. Data usually consists of temperature and precipitation, but weather stations may also collect data on humidity, wind speed, and other conditions.

Climate indicators - Represent the state of a given environmental condition over a certain area and a specified period of time, such as the mean annual temperature in Delaware for the period 1895-2011 or 2020-2039.

Higher and lower scenarios - Scenarios are used to describe a range of possible futures. Studies of future climate projections are often based on two or more possible future scenarios. In this analysis, the lower scenario represents a future in which people shift to clean energy sources in the coming decades, reducing emissions of carbon dioxide and other greenhouse gases. The higher scenario represents a future in which people continue to depend heavily on fossil fuels, and emissions of greenhouse gases continue to grow.

CMIP3 and CMIP5 - Two groups of global climate model simulations archived by the Coupled Model Intercomparison Project (CMIP). CMIP3 simulations were used in the Third and Fourth Assessment Reports of the Intergovernmental Panel on Climate Change (IPCC). The more recent CMIP5 simulations are used in the Fifth Assessment Report of the IPCC. (See detailed description in Appendix.)

Statistical downscaling - A method used to combine higher resolution observations with global climate model simulations in to obtain local- to regional-scale climate projections. Statistical downscaling models capture historical relationships between large-scale weather features and local climate. (See detailed description in Appendix.)

Multi-model or scientific uncertainty - Different models in a climate analysis may yield different results. In this report, the range of results, or outputs, from multiple models is expressed in the black "whiskers" (error bars) shown on the bar graphs, while the colored bar represents the multi-model mean. (See detailed description in Appendix.)

Temperature - Air temperatures over land surface, typically recorded at a height of 2 meters, in degrees Fahrenheit as °F.

Precipitation - Includes rain and snow, typically recorded as cumulative amount over a given time period ranging from a day to a year, in inches.

Maximum temperature - The highest temperature value in a given time period (daily, seasonal, or annual). Unless otherwise stated, all daily maximum temperatures in this report refer to values recorded within a 24-hour period, usually (but not always) occurring in the afternoon (also described as daytime temperatures).

Minimum temperature - The lowest temperature value in a given time period (daily, seasonal, or annual). Unless otherwise stated, all daily minimum temperatures in this report refer to values recorded within a 24-hour period, usually occurring at night (also described as nighttime temperatures).

Temperature and precipitation extremes - Extremes can be measured using fixed thresholds (e.g., days per year over 100°F) or using percentiles (e.g., number of days colder than the coldest 1% of days).

Temperature range - The range between highest and lowest temperature value in a given period (daily, seasonal, or annual).

Standard deviation of temperature - Assesses the day-today variability in daily maximum and minimum temperatures.

Dew point temperature - The temperature to which the air must be cooled to condense the water vapor it contains into water.

Relative humidity - The percentage of water vapor actually present in the air compared to the greatest amount of water vapor the air could possibly hold at the same temperature.

Heat index - A measurement that combines temperature and humidity, which affects evaporation and cooling. Sometimes referred to as the "apparent temperature", the Heat Index is a measure of how hot it really feels to the human body.

Heat wave events - A period of prolonged, unusual heat. There is no single standard definition of a heat wave. Different measures can be used to assess the frequency and severity of heat events, such as the length of consecutive days with maximum daytime temperatures exceeding a specific threshold temperature (e.g., 90°F, 95°F, 100°F). Another definition of an extreme heat wave is at least four consecutive days during which average temperatures (daytime plus nighttime temperatures) exceed the historical 1-in-10 year event.

Growing season - The "frost-free" period between the last frost in spring and the first frost in fall or winter, defined as the last and first time that nighttime minimum temperature falls below 32 degrees F.

Cooling degree-days and heating degree-days - An indicator of energy demand for heating and cooling. This represents demand for electricity in the summer (for air conditioning) and natural gas or oil in the winter (for space heating). Degree-days are typically calculated as the cumulative number of hours per year above (for cooling) or below (for heating) a given temperature threshold. For this analysis the threshold value is 65 degrees F.

Precipitation intensity - Total precipitation over a season or year, divided by the number of wet days (where wet days are defined as days with more than 0.01 inches of rain in 24 hours) that occurred in that same season or year. Higher values of precipitation intensity tend to suggest that, on average, precipitation may be heavier on any given wet day; lower values, that precipitation may be lighter on average.

Annual dry days - The number of days per year with no (or trace) precipitation (falling as either rain or snow).

Standardized Precipitation Index (SPI) and Palmer Drought Severity Index (PDSI) - Measurements of drought with negative values indicating dry (drought) conditions and positive values indicating wet conditions.

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