Heat/cold degree days and Chill hours

Explanation

Heat Degree Days

You may have a thermostat that makes your heating come on when the temperature falls below a threshold. Cumulus calculates the time that the temperature is below that threshold and reports it as 'Heat degree days' using the unit of days. Cumulus reports this by month as part of the NOAA report. It also appears in the annual NOAA report.

So if for each day of November, there was just one hour below the threshold, the total for the month would be 30 hours reported as 1.5 days.

Chill Hours

A related measure, also reported by Cumulus counting the hours below a (different) configurable temperature threshold for the 12 month period starting on the 1st day of a configurable month, is Chill Hours.

The traditional way of calculating the accumulation of Chill Hours is the number of hours the temperature is below 45F or 7.5C for the period of 1st October to 30th April in the Northern Hemisphere. This is available on Cumulus as that threshold, and start date are the default, so you simply observe the reported value on 30 April.

The main applicability is to stone and seed fruit as their exposure to low temperatures during the winter months will have a significant effect on the following harvest. Too few cold hours can result in poor quality and quantity of the crop. Fruit tree varieties prefer a Chill Hour rating from below 200 hours to around 1500 hours.

However, the way it can be configured on Cumulus will allow you to track air frost hours, ground frost hours or any other below the threshold parameter.

Cold Degree Days

You may have a thermostat that makes a cooling system come on when the temperature rises above a threshold. Cumulus calculates the time that the temperature is above that threshold and reports it as 'Cold degree days' using the unit of days. So if for each day of June, there were 3 hours above the threshold, the total for the month would be 90 hours reported as 3 days.

Setting the threshold

The thresholds for Heat and Cold Degree days are set in the NOAA Setup option within the Configuration menu. The picture shows just parts of the setup screen. It allows you to specify the two thresholds in the units you have selected to use in your implementation of Cumulus. Cumulus stores your selected threshold values in Cumulus\Cumulus.ini, Heating and Cooling Degree Days are stored in dayfile.txt, and are available via heating/cooling degree days web tags.

CAUTION: Think carefully, about what threshold to use. If you later change the threshold, only subsequent calculations use the new threshold. All retrospective calculations remain based on the old threshold, so values shown on a NOAA report for a month or a year with a threshold change will be rubbish, and need to be manually recalculated. The figures are on the reports, so make any edits directly on past reports.

The Chill Hours defaults are configurable by adding entries to the [Station] section in Cumulus.ini:

ChillHourSeasonStart=10

ChillHourThreshold=45

The season start figure is the month number, and the temperature threshold is specified in the units that you use in Cumulus.

The current figure is viewable on the 'This Year' screen, and is available via web tag <#chillhours>
Sfws 07:42, 29 November 2012 (UTC)