The 9 a.m.-to-9 a.m. Rule, why is it? Understanding Rainfall Data Collection in Australia

Rainfall is critical to Australia’s climate, affecting everything from agriculture and water supply to weather forecasts and natural disaster planning. But have you ever wondered how this rainfall data is collected and why it’s measured from 9 a.m. one day to 9 a.m. the next? This article aims to shed light on these questions.

The Importance of Rainfall Data

Rainfall data is essential for various sectors in Australia, including agriculture, hydrology, and meteorology. Accurate data can help farmers plan their planting and harvesting, allow water resource managers to maintain reservoir levels, and enable meteorologists to make more accurate weather forecasts.

The 9am-to-9 a.m. Rule

In Australia, the standard period for measuring rainfall is from 9 a.m. on one day to 9 a.m. on the following day. But why this specific timeframe? 

Consistency with Historical Data

Historically, weather observations were often made in the morning to coincide with the daily routines of those responsible for this task, such as farmers and early meteorologists. Keeping the 9 a.m. to 9 a.m. rule allows consistency when comparing current data with historical records. It minimises the impact of evaporation, which is generally lower in the morning. 

Let’s remember we all like a sleep-in, and in the old days, when all measurements were done manually, observers such as farmers found it more convenient to collect the data as part of their morning routine. (Unfortunately the BOM is scrubbing a lot of farm-based observations)

Other meteorological observations, such as temperature and humidity, are often recorded in the morning. A consistent timeframe for all these measurements makes it easier to correlate the data.

What is Rain Rate?

Rain rate refers to the amount of precipitation that falls over a specific period, usually expressed in millimetres per hour (mm/h). This metric is crucial for various applications, such as flood forecasting, irrigation planning, and stormwater management. Rain rate can vary significantly depending on the type of weather system involved. For example, a gentle drizzle might have a rain rate of less than 2.5 mm/h, while a heavy downpour could exceed 50 mm/h.

How is Rain Rate Measured?

Rain rate is commonly measured using rain gauges or more advanced meteorological instruments like weather radars and disdrometers. The data collected can then be used to calculate the rain rate by dividing the amount of rain collected by the time over which it was collected.

How Much Rain is Needed to Fill a 5000-Litre Water Tank?

To calculate how much rain is needed to fill a 5000-litre water tank, several factors need to be considered:

  1. Area of Collection: The first step is to determine the area over which the rainwater will be collected, usually the roof area in square metres (m²).
  2. Efficiency Factor: Not all the rain that falls on the collection area will end up in the tank. Some will be lost due to factors like evaporation and runoff. An efficiency factor (usually between 0.75 and 0.90) accounts for these losses.

Formula to Calculate Rainwater Harvest

The formula to calculate the amount of rainwater that can be harvested is:

\[ \text{Rainwater Harvested (L)} = \text{Rainfall (mm)} \times \text{Collection Area (m²)} \times \text{Efficiency Factor} \]

Example Calculation

Let’s assume you have a roof area of 100 m² and an efficiency factor of 0.85.

To fill a 5000-litre tank, you would need:

\[ 5000 = \text{Rainfall (mm)} \times 100 \times 0.85 \]

Solving for Rainfall, we get:

\[ \text{Rainfall Needed} = \frac{5000}{100 \times 0.85} \approx 58.82 \text{ mm} \]

So, you would need approximately 59 mm of rain to fill a 5000-litre water tank, given the assumed roof area and efficiency factor.

How Percentage of Precipitation is Calculated

The Percentage of Precipitation is a statistical probability that represents the likelihood of a measurable amount of precipitation occurring within a specified time period and area. A measurable amount is defined as 0.01″ (one hundredth of an inch) or more, which usually produces enough runoff for puddles to form.

Formula for Calculating Percentage of Precipitation

The probability is calculated by multiplying two factors:

  1. Forecaster’s Certainty: The level of confidence that precipitation will form or move into the forecast area.
  2. Areal Coverage: The expected coverage of the precipitation over the forecast area.

The formula can be represented as:

\[
\text{Percentage of Precipitation} = \text{(Forecaster’s Certainty)} \times \text{(Areal Coverage)}
\]

For example, if the forecaster is 80% certain that rain will develop but expects it to cover only 50% of the forecast area, the Percentage of Precipitation would be \(0.8 \times 0.5 = 0.4\) or 40%.

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