EnergyPlus Weather File (EPW) Format

Background

Hourly weather files downloaded from Climate Analytics are in the EnergyPlus / ESP EPW format and this page provides details on the EPW file format. Information has been sourced from the EnergyPlus Auxiliary Programs help guide.

The EPW weather data format was originally developed for use with two major simulation programs EnergyPlus and ESP-r (Crawley et al. 1999) and has since been adopted as a standard format by most other building simulation tools.

The format is simple, text-based with comma-separated data. It is based on the data available within the older TMY2 weather format but has been rearranged to facilitate visual inspection of the data. The TMY2 data are a strict, position-specific format-filling missing data with nines and zero values with zeroes. The EPW weather data format contains commas to facilitate data reading and analysis with spreadsheet programs. By eliminating redundant ‘fill’ values, the size of each file is only slightly larger than the original TMY2 format.

All EPW data is in SI units.

The traditional distribution of data source and uncertainty flags within the raw data fields carries with it not only the need for many field separators, it obfuscates the relationships between non-numerical data. In a set of minute data, which could easily require hundreds of thousands of records, the space penalty is considerable. In the EPW file format, all data source and uncertainty fields have been clumped together as a single field immediately following the day and time stamp. For applications where uncertainty is not an issue such data can be easily ignored. When it is important, a single text field is conceptually and computationally easy to parse.

Another difference between the EPW format and TMY2 is the addition of two new data fields: minute and infrared sky. The minute field in theory facilitates use of data observed at intervals of less than one hour such as measured data from a research study of energy efficiency for a particular building, although we understand that it is currently not used by EnergyPlus. Climate Analytics generates hourly data only. Note also that the infrared sky field allows simulation programs to calculate the effective sky temperature for re-radiation during nighttime.

The last difference is that, in principle, a full year of data (8760 hours) is not required and subsets of years are acceptable. The period covered by the data is described in the files, however note that in practice most simulation tools, DesignBuilder included, require a full year of hourly values and don't support part-year weather files or intervals other than hourly.

EPW Data Dictionary

The first eight lines or header at the top of each EPW weather file define basic location information such as longitude, latitude, time zone, elevation, annual design conditions, monthly average ground temperatures, typical and extreme periods, holidays/daylight saving periods, and data periods included. There is also space for users to document any special features or information about the file such as sources of data.

The header is typically followed by 8760 lines of data, one line for each hour in the year.

The EnergyPlus data dictionary format is used to describe the EPW weather data set. Each line in the format is preceded by a keyword such as LOCATION, DESIGN CONDITIONS, followed by a list of variables beginning either with A or N and a number. A stands for alphanumeric; N for numeric. The number following A/N is the sequence of that number in the keyword list. Commas separate data. (Refer to the IDD Conventions document in “Input Output Reference” for further explanation of the format). The header information consists of eight lines (keywords): LOCATION, DESIGN CONDITIONS, TYPICAL/EXTREME PERIODS, GROUND TEMPERATURES, HOLIDAYS/DAYLIGHT SAVINGS, COMMENTS 1, COMMENTS 2, and DATA PERIODS. This is followed by the time step data.

The “data dictionary” for EnergyPlus Weather Data is shown below. Note that semi-colons do NOT terminate lines in the EnergyPlus Weather Data. It helps if you have familiarity with the IDD conventions please view them in the EnergyPlus Input Output Reference document. Briefly, the EPW Data Dictionary format uses similar “\” conventions that are important for reading the following tables:

\minimum, \minimum> - values for this field must be either > = or > than the following number

\maximum, \maximum< - values for this field must be either < = or < than the following number

\missing - if values in this field are > = the following number, it is considered “missing” and missing data rules will apply

\default - blank fields will receive the following as “default” values

\units - expected units for the field. Standard EnergyPlus units are shown in the Input Output Reference Document.

Note that in the header records where “date” is used, the interpretation is shown in the following table.

Weather File Date Field Interpretation
Field Contents	Interpretation	Header Applicability
< number >	Julian Day of Year	All date fields
< number > / < number >	Month / Day	All date fields
< number > / < number > / < number >	Month / Day / Year	DataPeriod only - special multiple year file - ref: RunPeriod:CustomRange object in IDF / Input Output Reference document
< number > Month	Day and Month	All date fields
Month < number >	Day and Month	All date fields
< number > Weekday in Month	Numbered weekday of month	Holiday, DaylightSavingPeriod
Last Weekday In Month	Last weekday of month	Holiday, DaylightSavingPeriod

In the table, Month can be one of (January, February, March, April, May, June, July, August, September, October, November, December). Abbreviations of the first three characters are also valid.

In the table, Weekday can be one of (Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday). Abbreviations of the first three characters are also valid.

The Location header record duplicates the information required for the Location Object. When only a Run Period object is used (i.e. a weather file), then the Location Object Is not needed. When a Run Period and Design Day objects are entered, then the Location on the weather file (as described previously) is used and overrides any Location Object entry.

The Design Conditions header record encapsulates matching (using WMO# – World Meteorological Organization Station Number) design conditions for a weather file location. Currently only those design conditions contained in the ASHRAE Handbook of Fundamentals 2009 are contained in the weather files. These conditions can be used as desired. In addition, Design Day definition files have been created of all World, Canada, and United States Design Conditions. These files are available in the DataSet folder of the EnergyPlus installation.

Using a heuristic method, the weather converter can determine typical and extreme weather periods for full year weather files. These will then be shown on the Typical/Extreme Periods header record. These are also reported in the statistical report output from the Weather Converter.

The weather converter program can use a full year weather data file to calculate “undisturbed” ground temperatures based on temperatures. Since an important part of soil heat transfer includes soil properties such as conductivity, density and specific heat AND these cannot be calculated from simple weather observations, this header record is provided primarilyfor user information. However, with the FC construction option, these are automatically selected (.5 depth) for use if the user does not include values in the Site:GroundTemperature:FcfactorMethod object.

As noted in the statistics report, the “undisturbed” ground temperatures calculated by the weather converter should not be used in building losses but are appropriate to be used in the GroundTemperatures:Surface and GroundTemperatures:Deep objects. The reasoning (for building losses) is that these values are too extreme for the soil under a conditioned building. For best results, use the Slab or Basement program described in this document to calculate custom monthly average ground temperatures (see the Ground Heat Transfer section). This is especially important for residential applications and very small buildings. If one of these ground temperature preprocessors is not used, for typical commercial buildings in the USA, a reasonable default value is 2C less than the average indoor space temperature.

The Holidays / Daylight Saving header record details the start and end dates of Daylight Saving Time and other special days such as might be recorded for the weather file. These can be used by keying “Yes” for appropriate fields in the Run Period Object.

Note: EnergyPlus processed weather files available on the EnergyPlus web site have neither special days specified nor daylight saving period.

For example, using a RunPeriod:

Will use any holidays specified in the Holidays / Daylight Saving header record of the weather file but will not use the Daylight Saving Period that is specified there (if any). In addition, the user can specify Special Day Periods via the Special Day Period object and/or Daylight Saving Period via the Daylight Saving Period object to additionally specify these items.

The Comment header records may provide additional information about the weather data source or other information which may not fit in other header record formats.

A weather file may contain several “data periods” though this is not required (and, in fact, may be detrimental). In addition, a weather file may contain multiple records per hour BUT these must match the Number of Time Steps In Hour for the simulation. Multiple interval data files can be valued when you want to be sure of the weather values for each time step (rather than relying on “interpolated” weather data). A weather file may also contain several consecutive years of weather data. EnergyPlus will automatically process the extra years when the Number of Years field is used in the RunPeriod object. Sorry - there is no way to jump into a year in the middle of the EPW file.

Note that a Run Period object may not cross Data Period boundary lines.

For those interested in creating their own weather data in the CSV or EPW formats or reading the .csv and .epw files that are produced by the Weather Converter program, the fields are shown in the following “IDD” description. Items shown in bold are used directly in the EnergyPlus program.

Data Field Descriptions

The Descriptions of the EPW fields below are taken from the IWEC manual.

EPW files can be loaded to a spreadsheet and the column letters are provided below for each field to help you match up descriptions in the help with the data.

Year

This is the Year of the data. It is not really used in EnergyPlus but is used in the EnergyPlus Weather Converter program for display in audit file.

Field Index	1
IDD	N1
Spreadsheet	Column A

Month

This is the month (1-12) for the data. Cannot be missing.

Field Index	2
IDD	N2
Spreadsheet	Column C

Day

This is the day (dependent on month) for the data. Cannot be missing.

Field Index	3
IDD	N3
Spreadsheet	Column C

Hour

This is the hour of the data. (1 - 24). Hour 1 is 00:01 to 01:00. Cannot be missing.

Field Index	4
IDD	N4
Spreadsheet	Column D

Minute

The minute field (values 0..60) is currently not used by EnergyPlus.

Field Index	5
IDD	N5
Spreadsheet	Column E

Data Source and Uncertainty Flags

The data source and uncertainty flags from various formats (usually shown with each field) are consolidated in the EPW format. More is shown about Data Source and Uncertainty in Data Sources/Uncertainty section later in this document.

Field Index	6
IDD	A1
Spreadsheet	Column F

Dry Bulb Temperature

This is the dry bulb temperature in C at the time indicated. Note that this is a full numeric field (i.e. 23.6) and not an integer representation with tenths. Valid values range from -70°C to 70°C. Missing value for this field is 99.9.

Field Index	7
IDD	N6
Spreadsheet	Column G

Dew Point Temperature

This is the dew point temperature in C at the time indicated. Note that this is a full numeric field (i.e. 23.6) and not an integer representation with tenths. Valid values range from -70°C to 70°C. Missing value for this field is 99.9.

Field Index	8
IDD	N7
Spreadsheet	Column H

Relative Humidity

This is the Relative Humidity in percent at the time indicated. Valid values range from 0% to 110%. Missing value for this field is 999.

Field Index	9
IDD	N8
Spreadsheet	Column I

Atmospheric Station Pressure

This is the station pressure in Pa at the time indicated. Valid values range from 31,000 to 120,000. These values were chosen from the standard barometric pressure for all elevations of the World. Missing value for this field is 999999.

Field Index	10
IDD	N9
Spreadsheet	Column J

Extraterrestrial Horizontal Radiation

This is the Extraterrestrial Horizontal Radiation in Wh/m2. It is not currently used in EnergyPlus calculations. It should have a minimum value of 0; missing value for this field is 9999.

Field Index	11
IDD	N10
Spreadsheet	Column K

Extraterrestrial Direct Normal Radiation

This is the Extraterrestrial Direct Normal Radiation in Wh/m2. It is the amount of solar radiation in Wh/m2 received on a surface normal to the rays of the sun at the top of the atmosphere during the hour preceding the time indicated. It is not currently used in EnergyPlus calculations. It should have a minimum value of 0; missing value for this field is 9999.

Field Index	12
IDD	N11
Spreadsheet	Column L

Horizontal Infrared Radiation Intensity

This is the Horizontal Infrared Radiation Intensity in Wh/m2. If it is missing, it is calculated from the Opaque Sky Cover field as shown in the following explanation. It should have a minimum value of 0; missing value for this field is 9999.

HorizontalIR=ϵσT4drybulb

where

• HorizontalIR is the horizontal IR intensity {W/m2}

• ϵ is the sky emissivity

• σ is the Stefan-Boltzmann constant = 5.6697e-8 W/m2-K4

• Tdrybulb is the drybulb temperature {K}

The sky emissivity is given by:

ϵ=(0.787+0.764ln(Tdewpoint / 273))(1+0.0224N−0.0035N2+0.00028N3)

where:

• Tdewpoint is the dewpoint temperature {K}

• N is the opaque sky cover {tenths}

Example: Clear sky (N=0 ), Tdrybulb=273+20=293K , Tdewpoint=273+10=283K :

ϵ=0.787+0.764∗0.036=0.815

HorizontalIR=0.815∗5.6697e−8∗(2934)=340.6W/m2

 

References (Walton, 1983) (Clark, Allen, 1978) for these calculations are contained in the references section at the end of this list of fields.

Field Index	13
IDD	N12
Spreadsheet	Column M

Global Horizontal Radiation

This is the Global Horizontal Radiation in Wh/m2, sometimes referred to as GHI in the literature. It is the total amount of direct and diffuse solar radiation in Wh/m2 received on a horizontal surface during the hour preceding the time indicated. It is not currently used in EnergyPlus calculations. It should have a minimum value of 0; missing value for this field is 9999.

See AMS Glossary

Field Index	14
IDD	N13
Spreadsheet	Column N

Direct Normal Radiation

This is the Direct Normal Radiation in Wh/m2, sometimes referred to as DNI in the literature. It is the amount of solar radiation in Wh/m2 received directly from the solar disk on a surface perpendicular to the sun’s rays, during the hour preceding the time indicated. If the field is missing (=9999 ) or invalid (<0 ), it is set to 0. Counts of such missing values are totalled and presented at the end of the runperiod.

See AMS Glossary

Field Index	15
IDD	N14
Spreadsheet	Column O

Diffuse Horizontal Radiation

This is the Diffuse Horizontal Radiation in Wh/m2, sometimes referred to as DHI in the literature. It is the amount of solar radiation in Wh/m2 received from the sky (excluding the solar disk) on a horizontal surface during the hour preceding the time indicated. If the field is missing (=9999 ) or invalid (<0 ), it is set to 0. Counts of such missing values are totalled and presented at the end of the runperiod.

See AMS Glossary

Field Index	16
IDD	N15
Spreadsheet	Column P

Global Horizontal Illuminance

This is the Global Horizontal Illuminance in lux. It is the average total amount of direct and diffuse illuminance in hundreds of lux received on a horizontal surface during the hour preceding the time indicated. It is not currently used in EnergyPlus calculations. It should have a minimum value of 0; missing value for this field is 999999 and will be considered missing if greater than or equal to 999900.

Climate Analytics calculates Global Horizontal Illuminance from Global Horizontal Radiation based on spectral average method using a fixed average luminous efficacy of 110 lm/W:

Global Horizontal Illuminance = 110 x Global Horizontal Radiation.

Field Index	17
IDD	N16
Spreadsheet	Column Q

Direct Normal Illuminance

This is the Direct Normal Illuminance in lux. It is the average amount of illuminance in hundreds of lux received directly from the solar disk on a surface perpendicular to the sun’s rays, during the hour preceding the time indicated. It is not currently used in EnergyPlus calculations. It should have a minimum value of 0; missing value for this field is 999999 and will be considered missing if greater than or equal to 999900.

Climate Analytics calculates Direct Normal Illuminance from Direct Normal Radiation based on spectral average method using a fixed average luminous efficacy of 105 lm/W:

Direct Normal Illuminance= 105 x Direct Normal Radiation.

Field Index	18
IDD	N17
Spreadsheet	Column R

Diffuse Horizontal Illuminance

This is the Diffuse Horizontal Illuminance in lux. It is the average amount of illuminance in hundreds of lux received from the sky (excluding the solar disk) on a horizontal surface during the hour preceding the time indicated. It is not currently used in EnergyPlus calculations. It should have a minimum value of 0; missing value for this field is 999999 and will be considered missing if greater than or equal to 999900.

Climate Analytics calculates Diffuse Horizontal Illuminance from Diffuse Horizontal Radiation based on spectral average method using a fixed average luminous efficacy of 119 lm/W:

Diffuse Horizontal Illuminance = 119 x Diffuse Horizontal Radiation.

Field Index	19
IDD	N18
Spreadsheet	Column S

Zenith Luminance

This is the Zenith Illuminance in Cd/m2. It is the average amount of luminance at the sky’s zenith in tens of Cd/m2 during the hour preceding the time indicated. It is not currently used in EnergyPlus calculations. It should have a minimum value of 0; missing value for this field is 9999.

Field Index	20
IDD	N19
Spreadsheet	Column T

Wind Direction

This is the Wind Direction in degrees where the convention is that North = 0.0, East = 90.0, South = 180.0, West = 270.0. It is the wind direction in degrees at the time indicated. If calm, direction equals zero. Values can range from 0 to 360. Missing value is 999.

Field Index	21
IDD	N20
Spreadsheet	Column U

Wind Speed

This is the wind speed in m/s at the time indicated. Values can range from 0 to 40. Missing value is 999.

Field Index	22
IDD	N21
Spreadsheet	Column V

Total Sky Cover

This is the value for total sky cover (tenths of coverage). A value of 1 is 1/10 covered and 10 is total coverage. It is the amount of the sky dome in tenths covered by clouds or obscuring phenomena at the hour indicated at the time indicated. Minimum value is 0; maximum value is 10; missing value is 99.

In everyday language, total sky cover includes opaque sky cover such as "thick" clouds that blocks everything above (the sky, the higher cloud layers) plus "thin" clouds that are partially transparent or translucent.

See also AMS Glossary

Field Index	23
IDD	N22
Spreadsheet	Column W

Opaque Sky Cover

This is the value for opaque sky cover (tenths of coverage). A value of 1 is 1/10 covered and 10 is total coverage. It is the amount of the sky dome in tenths covered by clouds or obscuring phenomena that prevent observing the sky or higher cloud layers at the time indicated. This value is not used unless the field for Horizontal Infrared Radiation Intensity is missing and then it is used to calculate Horizontal Infrared Radiation Intensity. Minimum value is 0; maximum value is 10; missing value is 99.

In everyday language, opaque sky cover includes opaque sky cover such as "thick" clouds that blocks everything above (the sky, the higher cloud layers) but not "thin" clouds that are partially transparent or translucent.

See also AMS Glossary

Field Index	24
IDD	N23
Spreadsheet	Column X

Visibility

This is the value for visibility in km. (Horizontal visibility at the time indicated.) It is not currently used in EnergyPlus calculations. Missing value is 9999.

Field Index	25
IDD	N24
Spreadsheet	Column Y

Ceiling Height

This is the value for ceiling height in m. (77777 is unlimited ceiling height. 88888 is cirroform ceiling.) It is not currently used in EnergyPlus calculations. Missing value is 99999.

Field Index	26
IDD	N25
Spreadsheet	Column Z

Present Weather Observation

If the value of the field is 0, then the observed weather codes are taken from the following field. If the value of the field is 9, then “missing” weather is assumed. Since the primary use of these fields (Present Weather Observation and Present Weather Codes) is for rain/wet surfaces, a missing observation field or a missing weather code implies no rain.

Present Weather Observation Values
Element	Values	Definition
Observation Indicator	0 or 9	0 = Weather observation made; 9 = Weather observation not made, or missing

Field Index	27
IDD	N26
Spreadsheet	Column AA

Present Weather Codes

The present weather codes field is assumed to follow the TMY2 conventions for this field. Note that though this field may be represented as numeric (e.g. in the CSV format), it is really a text field of 9 single digits. This convention along with values for each “column” (left to right) is presented in Table 3. Note that some formats (e.g. TMY) does not follow this convention - as much as possible, the present weather codes are converted to this convention during WeatherConverter processing. Also note that the most important fields are those representing liquid precipitation - where the surfaces of the building would be wet. EnergyPlus uses “Snow Depth” to determine if snow is on the ground.

Weather Codes Field Interpretation
Column - Position in Field	Element Description	Possible Values	Definition
1	Occurrence of Thunderstorm, Tornado, or Squall	0 - 2, 4, 6- 9	0 = Thunderstorm-lightning and thunder. Wind gusts less than 25.7 m/s, and hail, if any, less than 1.9 cm diameter 1 = Heavy or severe thunderstorm-frequent intense lightning and thunder. Wind gusts greater than 25.7 m/s and hail, if any, 1.9 cm or greater diameter 2 = Report of tornado or waterspout 4 = Moderate squall-sudden increase of wind speed by at least 8.2 m/s, reaching 11.3 m/s or more and lasting for at least 1 minute 6 = Water spout (beginning January 1984) 7 = Funnel cloud (beginning January 1984) 8 = Tornado (beginning January 1984) 9 = None if Observation Indicator element equals 0, or else unknown or missing if Observation Indicator element equals 9
2	Occurrence of Rain, Rain Showers, or Freezing Rain	0 - 9	0 = Light rain 1 = Moderate rain 2 = Heavy rain 3 = Light rain showers 4 = Moderate rain showers 5 = Heavy rain showers 6 = Light freezing rain 7 = Moderate freezing rain 8 = Heavy freezing rain 9 = None if Observation Indicator element equals 0, or else unknown or missing if Observation Indicator element equals 9 Notes: Light = up to 0.25 cm per hour Moderate = 0.28 to 0.76 cm per hour Heavy = greater than 0.76cm per hour
3	Occurrence of Rain Squalls, Drizzle, or Freezing Drizzle	0, 1, 3-9	0 = Light rain squalls 1 = Moderate rain squalls 3 = Light drizzle 4 = Moderate drizzle 5 = Heavy drizzle 6 = Light freezing drizzle 7 = Moderate freezing drizzle 8 = Heavy freezing drizzle 9 = None if Observation Indicator element equals 0, or else unknown or missing if Observation Indicator element equals 9 Notes: When drizzle or freezing drizzle occurs with other weather phenomena: Light = up to 0.025 cm per hour Moderate = 0.025 to 0.051cm per hour Heavy = greater than 0.051 cm per hour When drizzle or freezing drizzle occurs alone: Light = visibility 1 km or greater Moderate = visibility between 0.5 and 1 km Heavy = visibility 0.5 km or less
4	Occurrence of Snow, Snow Pellets, or Ice Crystals	0 - 9	0 = Light snow 1 = Moderate snow 2 = Heavy snow 3 = Light snow pellets 4 = Moderate snow pellets 5 = Heavy snow pellets 6 = Light ice crystals 7 = Moderate ice crystals 8 = Heavy ice crystals 9 = None if Observation Indicator element equals 0, or else unknown or missing if Observation Indicator element equals 9 Notes: Beginning in April 1963, any occurrence of ice crystals is recorded as a 7.
5	Occurrence of Snow Showers, Snow Squalls, or Snow Grains	0 - 7, 9	0 = Light snow 1 = Moderate snow showers 2 = Heavy snow showers 3 = Light snow squall 4 = Moderate snow squall 5 = Heavy snow squall 6 = Light snow grains 7 = Moderate snow grains 9 = None if Observation Indicator element equals 0, or else unknown or missing if Observation Indicator element equals 9
6	Occurrence of Sleet, Sleet Showers, or Hail	0 - 2, 4, 9	0 = Light ice pellet showers 1 = Moderate ice pellet showers 2 = Heavy ice pellet showers 4 = Hail 9 = None if Observation Indicator element equals 0, or else unknown or missing if Observation Indicator element equals 9 Notes: Prior to April 1970, ice pellets were coded as sleet. Beginning in April 1970, sleet and small hail were redefined as ice pellets and are coded as 0, 1, or 2.
7	Occurrence of Fog, Blowing Dust, or Blowing Sand	0 - 9	0 = Fog 1 = Ice fog 2 = Ground fog 3 = Blowing dust 4 = Blowing sand 5 = Heavy fog 6 = Glaze (beginning 1984) 7 = Heavy ice fog (beginning 1984) 8 = Heavy ground fog (beginning 1984) 9 = None if Observation Indicator element equals 0, or else unknown or missing if Observation Indicator element equals 9 Notes: These values recorded only when visibility is less than 11 km.
8	Occurrence of Smoke, Haze, Smoke andHaze, BlowingSnow, BlowingSpray, or Dust	0 - 7, 9	0 = Smoke 1 = Haze 2 = Smoke and haze 3 = Dust 4 = Blowing snow 5 = Blowing spray 6 = Dust storm (beginning 1984) 7 = Volcanic ash 9 = None if Observation Indicator element equals 0, or else unknown or missing if Observation Indicator element equals 9 Notes: These values recorded only when visibility is less than 11 km.
9	Occurrence of Ice Pellets	0 - 2, 9	0 = Light ice pellets 1 = Moderate ice pellets 2 = Heavy ice pellets 9 = None if Observation Indicator element equals 0, or else unknown or missing if Observation Indicator element equals 9

For example, a Present Weather Observation (previous field) of 0 and a Present Weather Codes field of 929999999 notes that there is heavy rain for this data period (usually hourly but depends on the number of intervals per hour field in the “Data Periods” record).

Field Index	28
IDD	N27
Spreadsheet	Column AB

Precipitable Water

This is the value for Precipitable Water in mm. It is the depth of water in vapour form in a column of the atmosphere, if all the water in that column were precipitated as rain. It does not take into account water droplets from clouds. Precipitable Water data is commonly presented in kg/m2 form, which is effectively the same unit as mm.

This data is provided by Climate Analytics when downloading Actual Year epw files but not it is not included in Design Year data.

Note that it is not rainfall which is stored in the Liquid Precipitation Depth field (if provided).

Precipitable Water is not currently used in EnergyPlus calculations. Missing value is 999.

Field Index	29
IDD	N28
Spreadsheet	Column AC

Aerosol Optical Depth

This is the value for Aerosol Optical Depth in thousandths.

Tiny solid and liquid particles suspended in the atmosphere are called aerosols. Windblown dust, sea salts, volcanic ash, smoke from wildfires, and pollution from factories are all examples of aerosols. Depending upon their size, type, and location, aerosols can either cool the surface, or warm it. They can help clouds to form, or they can inhibit cloud formation.

Aerosol Optical Depth is not currently used in EnergyPlus calculations. Missing value is 999.

Field Index	30
IDD	N29
Spreadsheet	Column AD

Snow Depth

This is the value for Snow Depth in cm. This field is used to tell when snow is on the ground and, thus, the ground reflectance may change. Missing value is 999.

Field Index	31
IDD	N30
Spreadsheet	Column AE

Days Since Last Snowfall

This is the value for Days Since Last Snowfall. It is not currently used in EnergyPlus calculations. Missing value is 99.

Field Index	32
IDD	N31
Spreadsheet	Column AF

Albedo

The ratio (unitless) of reflected solar irradiance to global horizontal irradiance. It is not currently used in EnergyPlus and not currently written out by Climate Analytics.

Field Index	33
IDD	N32
Spreadsheet	Column AG

Liquid Precipitation Depth

The amount of liquid precipitation falling at the surface (mm) observed at the indicated time for the period indicated in the liquid precipitation quantity field. If this value is not missing, then it is used and overrides the “precipitation” flag as rainfall. Conversely, if the precipitation flag shows rain and this field is missing or zero, it is set to 1.5 (mm).

This data is provided by Climate Analytics when downloading Design Year epw files but not it is not included in Actual Year data.

Field Index	34
IDD	N33
Spreadsheet	Column AH

Liquid Precipitation Quantity

The period of accumulation (hr) for the liquid precipitation depth field. It is not currently used in EnergyPlus and is not currently written out by Climate Analytics.

Field Index	35
IDD	N34
Spreadsheet	Column AI

References

1. Walton, G. N. 1983. Thermal Analysis Research Program Reference Manual. NBSSIR 83-2655. National Bureau of Standards, p. 21.
2. Clark, G. and C. Allen, “The Estimation of Atmospheric Radiation for Clear and Cloudy Skies,” Proceedings 2nd National Passive Solar Conference (AS/ISES), 1978, pp. 675-678.