Meteorological day

This page applies to both the original Cumulus software and MX.

Traditionally

The modern Cumulus user tends to forget that computerisation is a fairly recent change (in the last few decades) for weather recording, which has been taking place for at least a century in most nations.

Older people will remember how weather was traditionally recorded. Weather instruments were not directly connected to a computer. Observers would manually read the instruments at least once a day, traditionally at 9 a.m. (one hour later in daylight saving time). Some observers made measurements every hour, or more frequently at places like airports.

Certain instruments, like barographs and sun recorders, did record readings on a continuous process, but they still needed to be manually read and interpreted. For temperature, James Six invented an instrument that could store the highest and lowest temperatures between manual readings.

Measurements were manually logged onto a sheet, or into a notebook, as they were made. Subsequently, longer period derivatives could be calculated from the manual measurements and these might be either phoned to the main weather service, transmitted electronically (e.g. by telex), or entered into an electronic record.

Modern era

Towards the end of the 20th century, not only did computers become small enough to be located on a widespread basis, but electronic sensors that could record various weather parameters started to become cheap, reliable, and widely available.

Thus the concept of recording weather in real-time finally became available.

Another change soon followed, in most nations, the concept of relying on a lot of weather stations recording local weather has been abandoned. Just a few stations remain, generally only to serve the needs of pilots, at airports. Instead, most weather observations nowadays rely on satellites to sense the weather, and mathematical models to interpolate to a local variation. A few terriestrial recordings are still made, mostly to provide consistency with past measurments and to inform climatological models over a decade or more.

Daily assignment

Steve Loft created Cumulus software, back in 2003, because he owned a weather station and he could not find any software that allowed him to process measurements in the way he wanted. The first public release, allowing other people to use his software, was on 27th January 2004.

Most weather organisations in the world still to some extent remain consistent with the traditional way of working. This means that they report by meteorological day. Therefore, a key requirement for Steve Loft was to report daily derivatives based on a day that started at 9 a.m. Most weather software can only report daily measurments on a calendar day basis (since midnight).

Steve Loft did make a simplifying assumption that does not align with the practice of most Meteorological Offices, Cumulus implements minimum temperature and rainfall differently. The World Meteorological Office (WMO) allows individual nations some flexibility, and does not prescribe total international alignment.

The following table tries to illustrate the divergence if you choose a rollover time other than midnight:

Derivative	Typical national practice	Cumulus approach
Minimum Temperature	The daily minimum temperature in the 12 hours (9 p.m. to 9 a.m.) prior to the traditional observation time is assigned to the date of that observation	The daily minimum temperature in the 24 hours starting at the selected rollover time is assigned to the date of that rollover
Maximum temperature	The daily maximum temperature in the 12 hours (9 a.m. to 9 p.m.) after the traditional observation time is assigned to the date that applies to both times	Reported for day starting at 9 a.m.
Average temperature	The WMO says that daily temperature average is calculated in different ways. Its preference is the average of the minimum in previous 12 hours and maximum in subsequent 12 hours, this ensures consistency with traditional climate statistics. Prior to automatic recording, some stations would have manual recordings taken a few times a day, and if these were at 0600, 12:00, and 18:00, the mean might be calculated by adding the 3 readings, then adding the last again, and dividing by 4! Nations are permitted to use integrated averages for their internal purposes.	Cumulus maintains a total and a count for each day. The total is incremented by the actual temperature every time Cumulus processes a measurement (may be every minute for Cumulus 1, and every second for MX). The count is incremented by one each time a measurement is processed. The integrated average reported at the end of the day is just the total divided by the count.
Rainfall	Some nations report the daily rainfall for the 24 hours prior to the 9 a.m. observation time on a particular date. Others, throw their rainfall total back to previous date, i.e. the total for the 24 hours after 9 a.m on the particular date.	Cumulus assigns it to the date when the day started.
Sunshine Hours	Always reported by calendar day (midnight to 23:59)	Regardless of which rollover time is selected, Cumulus always reports sunshine hours from one minute past midnight until midnight for any day
Snowfall	A snow day is reported by calendar day. The snow depth may be only reported at standard observation time of 9 a.m. (or as maximum that day)	For Cumulus 1, you select a particular time (SnowDepthHour) and that is when the date changes, i.e. prior to that snow depth is assigned to yesterday, a snow index is calculated for any longer period (e.g. month or season). Cumulus 1 allows other information to be recorded in the Weather Diary. MX implements its diary.db slightly differently, and reports snow falling and lying as well as depth.
Highest and lowest pressure	Reported for day starting at 9 a.m.	Reported for day starting at 9 a.m.
Highest wind speed	Reported for day starting at 9 a.m.	Reported for day starting at 9 a.m.

Read more

thunder, rainfall, mean sea level pressure, snow days, fog day, minimum and maximum temperature

minimum temperature, and rainfall, a day out

WMO guidance for climate normals see table 1 on page 3 where it says " Different methods are in operational use for the calculation of daily mean temperature."