Frequently Asked Questions and Help

Weather Warehouse (WxW) Overview

The goal behind the WxW was to assemble the most comprehensive and robust database of historical and real-time weather observations and to make these data available to users in a convenient and affordable way.

In total the WxW databases contain weather information from ten of thousands of weather stations with histories in some cases extending back over 100 years. The datasets which comprise the WxW database originate from numerous government and non-government sources. The collection of original datasets are rigorously quality controlled by Weather Source and merged together to create a “super” database of weather information that is more accurate and more complete than the any of the original datasets. From the WxW database Weather Source then produces a suite of weather data products to fit a broad range of user needs. The hourly and daily WxW products are listed in the following table. A glossary of terms for these products may be found here.

Data Densities

Weather stations don't supply perfect weather data. Many stations have been around for decades, but some are defunct and no longer report, while others are new and only recently began reporting. Regardless, it is common for stations to occasionally miss reports. This can be because a station was temporarily off-line, there was an error in transmission or because government agencies do not always supply all station data.

In order to help you find the best data for your search, we analyze each station's data to quantify the completeness and present this to you with your search results. We call the completeness the Data Density. The higher the data density, the better. We provide the data density for each product. Ideally you would like to have data that is 100% complete for the time period you are requesting, but this is not always possible. Using the data densities, you can find the station with the most complete set of data for your search results. We provide density information for all stations that were active at any point during the time period you are interested in. If you cannot find a station with a high enough data density, try broadening your search to a wider radius using the Smart Search feature. If you are interested in a particular day, narrow your search to that one day. Note that, for example, if you are looking for only temperature information and the data density for temperature is 100%, it won't matter to you if the data density for snow is not 100%.

Some stations may not be in operation for the entire range of your request. The listing "Available period" will tell you whether the station was working during the period you have requested. The fact that the station was working does not mean that data for all dates will be there (see above), but this will let you know if the station was not even returning data during part of the period of your request. If the station was not in operation at the beginning or end of your requested period, we will only provide (and charge you for) the date range when the station was operational.

Just because a station may not have 100% complete data does not mean it will not be suitable for your needs. If, for example, you are looking for temperatures and snow data is missing, the data would still be good for your uses.

Example 

- Data Availability: expand to determine suitability

Data Completeness:   Hi Temp = 99.73%   Lo Temp = 99.73%   Precipitation = 99.73%   Snow = Not Available

In the example we see that the station will report virtually all temperature and precipitation, but there is no data for snow. Use this information as a guide in choosing a station, remembering that you will be able to see exactly what data is missing ont he Final Review and Purchase page.

On the station selection page, each station provides additional information.  Open this information by clicking on the "+" symbol. There is informatoin on the station itself and also the data densities. Where it says: Data Density: expand to determine suitability you can get a general idea of what data is being reported by the station.  It is also possible to obtain exact information on the station via the Data Availability Map.

The Data  Availability Map is a visual representation of all data points within the station's operational time frame. You can quickly look at any date available and see whether there is data present before paying for it. The data map is displayed on the Final Review and Payment page. If no station is suitable we suggest that you go back and enlarge the search area so that you may find another station, perhaps farther away, but which suits your needs.

Credit Cards

Credit Card Verification Value Code Location The Card Verification Value Code (CVV) is a number used to help prevent fraud in credit card transactions where the card is not present (such as phone or internet orders).
American Express The 4 digit number is on the front right or left of the card	Visa, Mastercard The number is on the back of the card, the last 3 digits on the signature line.

Reporting Period of Data

There are many types of weather stations which report data compiled by the US Government.  Some may be automated stations that report back "in real time", others may be manned by real human beings who typically record their observations for the previous 24 hours each morning, usually 7am. Thus, for example if it rained on Oct. 26, the observerwould not report the amount in his rain guage until 7am on the 27th, and this amount would go down into the books for the 27th.  The same thing might apply to snow. For this reason if you are interested in a specific day, it might be a good idea to "bracket" that day with a day or so on either side.

Glossary of Weather Parameters

• CDD (Cooling Degree Days)
• Cloud Cover
• Dew Point
• Feels Like Temperature
• GDD (Growing Degree Days)
• HDD (Heating Degree Days)
• Heat Index Temperature
• Max
• Mean
• Min
• Precipitation
• Relative Humidity
• Sea Level Pressure
• Significant Weather
• Station Pressure
• Snowfall
• Temperature
• Visibility
• Wet Bulb Temperature
• Wind and Maximum Wind Speed
• Wind Chill Temperature

CDD (Cooling Degree Days)

Cooling Degree Days is an index used by the power industry to reflect the need for power for cooling or air conditioning. CDD are determined by the equations and logic as follows:

CDD positive example:

CDD zero example:

CDD are computed for each day where maximum and minimum temperatures are recorded. CDD are often summed over a month or season. For example if a weather station averages 10 CDD per day, it is not uncommon to hear statement such as: “The weather station in New York City reported a total of 300 CDD for the month of June”

Cloud Cover

Cloud Cover is the amount of sky that is filled with clouds. Weather Source provides Cloud Cover in terms of percent, for example a Cloud Cover observation of 75% means that 75% of the sky is filled with clouds.

Dew Point 

Dew Point Temperature is the temperature to which air must be cooled in order for saturation to occur. At this temperature, water vapor in the air can condense to form dew on surfaces in contact with the air. The Dew Point temperature is measure and/or derived at constant air pressure and constant water vapor content.

Feels Like Temperature 

Feels Like Temperature is a derived temperature index that merges wind chill, regular temperature and heat index into a single “Feels Like Temperature” index. Feels Like Temperature is a convenient way to represent what the temperature feels like to people in a single parameter.

GDD (Growing Degree Days) 

Growing Degree Days are calculated in a similar fashion to Cooling Degree Days, except the threshold is set to 50F as opposed to 65F in the Cooling Degree Day calculation. The purpose of GDDs is to estimate how much energy crops have received. The maturity of many crops can be estimated by the number of GDD that accumulate from the point in time that they where planted. For example many varieties of corn generally ready for harvest after a period of time in which 1100 to 1200 GDD have occurred. The use of 50F as the threshold for Growing Degrees Days is the most commonly used threshold, however other thresholds are often used for specific crops. For example a threshold of 41F is often used for Alpha crops. The GDD provide by Weather Source is based on a threshold of 50.

HDD (Heating Degree Days) 

Heating Degree Days is an index used by the power industry to reflect the need for energy in producing heat for buildings. HDD are essentially the opposite of CDD and are determined by the equations and logic as follows:

HDD positive example:

HDD zero example: 

HDD are computed for each day where maximum and minimum temperatures are recorded. HDD are often summed over a month or season. For example if a weather station averages 10 HDD per day, it is not uncommon to hear statement such as: “The weather station in New York City reported a total of 300 HDD for the month of March”

Heat Index Temperature

The purpose of the Heat Index is to estimate the apparent temperature a person experiences when taking into account warm temperatures plus humidity. Normally the human body cools itself by evaporating sweat from the surface of the skin. The process of evaporation removes heat from the skin. When the humidity increases, the ability to evaporate sweat from the skin surface is decreased, which in turn decreases the ability to shed heat from the body. For example when the air temperature is 85F with a low relative humidity of 40%, the apparent temperature to the body is the same as the regular air temperature (85F), however, keeping the air temperature at 85F and increasing the relative humidity to 70% results in making the human body feel like it is actually 93F.

Max

‘Max’ is short for ‘Maximum’ and is most often used to describe the highest value that a weather parameter reaches during a 24 hour day. Usage of ‘Max’ with other time periods is typically preceded by a reference to the time period, for example Monthly Max Temperature would mean the highest temperature that occurred during a month.  See also, the entry on wind for a comparison between wind speed and max wind speed.

Mean

In most cases, ‘Mean’ refers to the arithmetic mean of a weather parameter. Specifically the sum of all parameter members divided by the number of members. For example the Daily Mean Wind Speed is the sum of the 24 hourly wind speed observations divided by 24. The exception to this is often applied to temperature where the Daily Mean Temperature has an established convention of being the mean of the max and min temperature as opposed to all available temperature observations. The reason for this is that in the US there are nearly 10,000 cooperative weather stations that only record the daily max and min temperature (and total daily precipitation and snowfall). In order for daily mean temperature to be consistent across all stations, all stations must produce the mean using the same method. Thus even though a station may record 24 hourly temperatures throughout the day, the Daily Mean Temperature is still computed as the mean of the max and min temperatures so as to be consistent with stations that ONLY record max and min temperatures. Note that any ‘special’ observations are not typically used to produce the mean.

Min

‘Min’ is short for ‘Minimum’ and is most often used to describe the lowest value that a weather parameter reach during a 24 hour day. Usage of ‘Min’ with other time periods is typically preceded by a reference to the time period, for example Monthly Min Temperature would mean the lowest temperature that occurred during a month.

Precipitation

Precipitation is any water based particle falling from the sky that reaches the ground including drizzle, freezing drizzle, rain, freezing rain, snow, snow grains, snow pellets (graupel), sleet, hail, ice pellets and ice crystals. Weather Source provides Precipitation as “inches of liquid water”. For non-frozen precipitation (rain and drizzle), this is simply the direct measurement of the precipitation. For frozen precipitation the precipitation is melted and the depth is measured and recorded. For the most part, with the exception of hail, one can safely assume that precipitation that occurs when the temperature (at 2 meters above the ground) is 34F or above will be rain. It is possible for snow to occur and accumulate when the 2 meter temperatures are slightly above freezing (32F) when the air aloft is cold enough to support snow. Also, one can safely assume that precipitation that occurs when the 2 meter temperature is below 30F will be of a frozen variety. For 2 meter temperatures in between 30 and 34 degrees Fahrenheit, one often has to reference other weather information, to determine what type of precipitation occurred. 

Instead of an amount of precipitation, you may see a report of "T" or "TRACE". Trace means that precipitation was observed falling, either by a human observer or an automated instrument, but the amount that fell was below the threshold of what the is measurable. For example a couple drops of rain, or a few stray flakes of snow. The amount is generally not evenenough to cover the surface. Generally less than 0.01 inch."

Relative Humidity

Relative Humidity is a measure of how much water vapor is mixed into the air. The specific amount of water vapor that can be mixed into the air is related to the temperature of the air. Warm air can ‘hold’ more water vapor than cold air. No matter what the air temperature is, there is a maximum amount of water vapor air at that that air can hold. For that reason Relative Humidity is provided as a percentage between 0% to 100%, where 0% means the air contains no water vapor (a very rare occurrence) and 100% means the air contains the maximum amount of water vapor for air at a specific temperature. It is possible to have a Relative Humidity of 100% at nearly any temperature that can be found on Earth, however if you were to “wring-out” the water from warm air you would collect much more water than from cold air

Sea Level Pressure

Sea Level Pressure is the Atmospheric Pressure measure at a weather station that is mathematically adjusted to present the pressure the station would measure if the station where at an elevation equal to Sea Level (elevation of 0). Weather Source provides Sea Level Pressure in units of millibars (mb).

Significant Weather

Significant weather observations are reports of any notable weather including any type of precipitation, fog, high winds or other weather phenomenon that warrants notice to aviation and/or the public.

Snowfall

Snowfall is the occurrence of accumulating snow. Weather Source provides observation of Snowfall in units of inches.

Station Pressure

Station Pressure is the Atmospheric Pressure measure at a weather station. Weather Source provides Station Pressure in units of millibars (mb).

Temperature

Unless otherwise noted, Weather Source uses the term ‘Temperature’ to refer to the temperature at a height of 2 meters above the ground. This is the standard height at which temperature at weather stations is observed. Weather Source provides Temperature in units of degrees Fahrenheit.

Visibility

Visibility is provided in units of miles. Prior to using automated equipment to measure visibility, visibility was often expressed without limit. For example some stations on a very clear day may have recorded a visibility of 30 miles. With the adoption of automated equipment, visibility is generally recorded to a maximum of 10 miles. Weather phenomenon such as fog, precipitation and haze can significantly reduce visibility.

Wet Bulb Temperature

Wet bulb temperature is the temperature measure by a wet bulb thermometer OR is derived mathematically as a function of temperature, dew point temperature and station pressure. Wet bulb temperature is the temperature air would have if the energy of the air (air temperature) were used to evaporate water into the air until the air became saturated (a relative humidity of 100%) assuming the air pressure remained constant. Wet Bulb Temperature is similar to Dew Point Temperature, except the process of achieving Wet Bulb Temperature assumes that there is no limit on the supply of moisture.

Wind and Maximum Wind Speed

Wind is the movement of air and is expressed in terms of direction and speed. The wind direction is the direction from which the wind is blowing. For example, a “North Wind” blows from the North to the South. Weather Source provides Wind Direction in units of compass degrees with 90 representing East, 180 representing South, 270 representing West and 360 representing North. Zero (0) degrees is generally use to represent calm conditions (no air movement). Wind Speed is provided in miles per hour.

Maximum Wind Speed

Maximum wind speed (ie, max wind) is the highest wind speed reported each hour. Max wind speed for any hour is calculated by taking the the greatest of three values reported for each hour: (1) wind speed, (2) wind gust, and (3) peak wind speed. Wind gusts and peak wind speed are not always reported, in which case max wind would simply be the value of the reported wind speed, and, hourly wind speed and the maximum wind speed would be the same. It is not unusual for the max wind and the hourly wind speed to be the same, as big wind events are infrequent. Wind gusts and peak wind speed are only reported when certain criteria are met. Gusts are only reported when the wind is characterized by rapid fluctuations in wind speed with a variation of 10 knots (11.5 mph) or more between peaks and lulls. In which case the maximum instantaneous speed is reported as a gust. The peak wind is only reported whenever the maximum instantaneous speed is greater than 25 knots (28.75 mph).

Wind Chill Temperature

Wind Chill Temperature is a temperature index similar to Heat Index, but designed to represent how cold a person feels when exposed to cold temperatures and windy conditions. Weather Source provides Wind Chill in units of degrees Fahrenheit.

Here are some additional questions and answers.

Also, if there is a meteorological definition you have not found here, we suggest that you take a look at the American Meteorological Society Glossary of Meteorology and the  United States National Weather Service Glossary.