Acronyms, terminology, and units  Last update: Sept. 2016 
Acronyms
(in alphabetic order)
Acronyms  Definition 

BHI  Beam (or Direct) Horizontal Irradiation 
DHI  Diffuse Horizontal Irradiation 
GHI  Global Horizontal Irradiation 
HRV  High Resolution Visible. Panchromatic (=broadband) channel of Seviri (Meteosat) instrument, corresponding to a silicon response, about 0.41.1 µm. 
IR  Infra Red 
MSG  Meteosat Second Generation 
SoDa  Solar Radiation Data. Originally a European project which ended in 2003, leading to the SoDa Service, populated by many different resources and databases, and now represented by 3 websites:

UV index  for human health only. Its definition is closely related to the erythemal answer of the skin of the population (most of the time caucasian type). 
UV radiation  for plants, material aging... which corresponds to a particular range of the light spectra. 
WV  Water Vapor 
Terminology: Irradiance and Irradiation
Publications: ESRA (2000) and Dogniaux et al. (1984).
Irradiation is the energy received per area. It is expressed in J m^{2} (joule per square meter). The International Solar Energy Society recommends the symbol H. Other units are J cm^{2} (joule per square centimeter) and Wh m^{2} (watthour per square meter) though the latter should not be used as it does not belong to the standard unit system (S.I., International System). See conversion factors.
Irradiance is defined as a power received per area. It is expressed in W m^{2} (watt per square meter). The S.I. (International System) recommends the symbol E.
Irradiance and irradiation values stand for area densities, i.e. no subscripts are used to indicate the surface area. All symbols refer to horizontal planes; for a tilted plane, the slope β (beta) and the plane azimuth α (alpha) are added in brackets.
Subscript 0 stands for extraterrestrial or astronomical.
Subscript h stands for hourly and subcript d for daily.
The conversion from irradiation (energy) into irradiance (power) is performed by dividing irradiation by the duration of the measurement. Reciprocally, irradiance is converted into irradiation by multiplying by a time duration.
Convert from  Into  Divide by 
Hourly irradiation in J/m^{2} (including time averaged values, e.g., monthly means, yearly means etc.)  irradiance in W/m^{2}  3600 (number of seconds in an hour) 
Hourly irradiation in Wh/m^{2} (including time averaged values, e.g., monthly means, yearly means etc.)  1  
Daily irradiation in J/m^{2} (including time averaged values, e.g., monthly means, yearly means etc.)  86400 = 24 * 3600 (number of seconds in a day)  
Daily irradiation in Wh/m^{2} (including time averaged values, e.g., monthly means, yearly means etc.)  24 (number of hours in a day) 
To convert 10days sums of irradiation, or monthly sums or yearly sums or similar quantities, into irradiance, convert them first to averaged values by dividing them by the duration of the observation (e.g., 10 days, 28 or 30 or 31 days, 365 or 366 days). Then use the table above.
Units
International system of units
The International System of Units, abbreviated SI (from the French Système International), is the internationallyagreed system of measurement.
Angles: Plane angles are expressed in radian (rad), or degrees, or sexagesimal degrees (degrees, minutes, seconds).
Energy: Energy, work, quantity of heat are expressed in Joule (J). It can be expressed by unit of surface: J/m2, J/cm2. Irradiation is the energy received per area. Other units are Wh (Watt hour), calorie, therm, Langley.
Power: Power, radiant flux are expressed in Watt (W). Irradiance is defined as a power received per area; it is expressed in W m2 (watt per square meter).
Pressure: Pressure is expressed in Pascal (Pa). In meteorology, the unit hPa (hectoPascal) is commonly used; it is 100 times a Pascal. It replaces the millibar unit and they are equivalent. Another unit is the "atmosphere".
Conversion factors
Angles:
Convert from  Into  Multiply by 
degrees (°)  radians (rad)  1.745329 E02 
radians (rad)  degrees (°)  57.29578 
minutes (')  degrees (°)  0.0167 
seconds (")  degrees (°)  1.6667 E04 
degrees, minutes, seconds  degrees with decimals  degrees + minutes/60 + seconds/3600 
Energy / Power:
Convert from  Into  Multiply by 
calorie (cal)  Joule (J)  4.1868 
Watt second (W s)  Joule (J)  1 
Watt hour (Wh)  Joule (J)  3.6 E+03 
Joule (J)  calorie (cal)  0.2388 
Joule (J)  Watt hour (Wh)  0.2778 E03 
therm (Europe)  Joule (J)  1.05506 E+08 
Watt hour per square meter (Wh / m^{2})  Joule per square centimeter (J / cm^{2})  0.36 
Joule per square centimeter (J / cm^{2})  Watt hour per square meter (Wh / m^{2})  2.778 
Joule per square meter (J / m^{2})  Watt hour per square meter (Wh / m^{2})  0.2778 E03 
Langley (Ly)  Wh / m^{2}  11.6 
Langley (Ly)  J / m^{2}  41.84 E+03 
kLy / year  W / m^{2}  1.33 
Example of conversion from Wh to J: Let's admit that the energy is 6.5 Wh. In J, E would write 6.5 Wh x 3600 J/(Wh) = 23400 J = 23.4 kJ
Pressure:
Convert from  Into  Multiply by 
atmosphere standard (atm)  Pascal (Pa)  1.01325 E+05 
atmosphere standard (atm)  hectoPascal (hPa)  1.01325 E+03 
Pascal (Pa)  atmosphere standard  0.98692 E05 