The reference sensor for rain intensity, drop size & hail detection
RainFlow RF4 is a fourth-generation, no-moving-parts precipitation instrument that combines a high-precision rain gauge, a rain disdrometer and basic hail detection in a single ultra-robust sensor. It delivers real-time rain intensity, cumulative rainfall and drop size distribution, while also detecting hail episodes with very low power consumption.
Ideal for weather & hydro networks, roads, cities, energy sites.
- Acoustic impact sensing – no buckets or optical windows.
- Real-time rain intensity, DSD and hail detection.
- Designed for remote, solar-powered autonomous stations.
From "it's raining" to understanding the full event
RainFlow RF4 characterizes not just how much rain fell, but how it fell: intensity, temporal structure, drop size distribution and hail.
Conventional rain gauges only provide accumulated rainfall. They say nothing about the structure of the event: is it drizzle or convective downpour, stratiform or shower, mixed with hail, or transitioning between phases? For hydrologists, modelers and infrastructure operators, these details are crucial.
RainFlow RF4 is a dedicated precipitation sensor that measures instantaneous and averaged rain intensity (mm/h), cumulative rainfall (mm), number of drops and drop size distribution (disdrometry), and the presence and intensity of hail impacts. Using an acoustic impact principle and embedded signal processing, the RF4 analyzes each impact on its metallic sensing head, reconstructs the distribution of drop diameters in real time, and outputs both classical rain-gauge data and full disdrometric information.
The instrument is engineered for 24/7 autonomous operation in harsh outdoor conditions. Its sealed electronics, stainless-steel/aluminum body and very low power consumption make it a natural choice for remote hydrological catchments, dense urban networks, road-weather stations, renewable-energy sites and research observatories.
Why RainFlow RF4
RF4 measures standard rain gauge variables (mm/h and mm) together with the number of drops and their size distribution. This allows you to distinguish between drizzle, stratiform rain and convective cells, and to quantify kinetic energy input to surfaces and soils.
The instrument continuously classifies detected drops into size classes, yielding a drop size distribution (DSD) for each measurement interval. These data are suitable for microphysical studies, model parameterization, and calibration of weather radar or microwave links.
Strong impacts are detected and flagged as hail. The sensor counts hailstones and associates them with dedicated classes, allowing you to identify and characterize mixed rain-and-hail events or pure hail showers without a separate hail pad or camera.
There are no mechanical buckets, tipping mechanisms or optical windows to adjust or clean. The sealed metallic sensing head and embedded processing make the instrument virtually maintenance-free, with excellent long-term stability.
A stainless-steel and aluminum body, long-life elastomer damper and heavy-duty mounting arm ensure mechanical stability in strong winds, vibrations and outdoor environments, across a wide temperature and humidity range.
Configurable acquisition and averaging cycles allow you to optimize duty-cycle and power consumption. Typical average currents are compatible with small solar panels and battery systems, even on remote sites.
Dual analog outputs (continuous or pulse), SDI-12, serial ASCII and optional Modbus RTU and 4–20 mA interfaces make integration straightforward with almost any datalogger, PLC, SCADA, BMS or custom embedded platform.
An internal recorder can store measurements autonomously. The ISAW Toolbox software provides configuration, diagnostics, live scope visualization and data retrieval via a simple USB link, with identical concepts across the ISAW sensor family.
Where RainFlow RF4 makes the difference
RainFlow RF4 supports a wide range of operational and scientific applications wherever detailed precipitation information matters.
Operational meteorology & dense rain networks
Provide high-resolution rain intensity and disdrometric parameters for national and regional networks. Additional DSD and hail information help interpret radar echoes, validate nowcasting products and refine warning thresholds for intense precipitation.
Hydrology, flood warning & catchment studies
Hydrological response depends on intensity peaks and drop sizes, not only totals. RF4 provides the temporal structure and microphysical characteristics of events, improving estimation of infiltration, runoff, soil erosion and flash-flood risk in small basins and urban catchments.
Urban drainage, smart cities & civil protection
In dense urban environments, local convective storms can overwhelm drainage systems. RF4 can be deployed at strategic points in smart-city networks to track real-time rain intensity, identify rapidly developing downpours and support adaptive control of stormwater and drainage infrastructures.
Wind farms, PV plants & critical infrastructure
For renewable-energy sites, heavy rain and hail can degrade PV modules, affect turbine blades and damage equipment. RF4 helps operators monitor precipitation loads, characterize damaging events and correlate them with performance losses, insurance claims or maintenance strategies.
Transport, roads & airports
Rain intensity and drop size strongly influence road friction, aquaplaning risk and runway conditions. RF4 can be integrated into road-weather information systems and airport observation networks to provide real-time precipitation characterization, including hail episodes that may affect aircraft on the ground.
Agriculture, vineyards & crop protection
For agriculture and viticulture, the kinetic energy of rain and hail drives canopy damage and soil erosion. RF4 supports decisions for protective measures, irrigation planning and risk analysis, and provides objective data for insurance and loss adjustment.
Research, climate & microphysics
RF4 is ideal for campaigns on precipitation microphysics, model validation and climate monitoring. Disdrometric outputs can be combined with radar, microwave links, in situ sensors and numerical models to study DSD evolution, melting-layer processes, hail formation and long-term trends.
Acoustic impact & embedded disdrometry
RainFlow RF4 uses an acoustic impact and vibro-mechanical principle optimized for raindrops and hailstones.
- Raindrops and hailstones strike the metallic sensing head.
- Each impact generates a mechanical response in the sensor body.
- An embedded transducer converts this response into an electrical signal.
- Real-time signal processing algorithms analyze the amplitude and temporal shape of the signal.
- Impacts are classified into drop-size and hail classes and aggregated over the configured averaging interval.
What the instrument computes
- Number of drops and drop size distribution (DSD).
- Rain intensity and cumulative rainfall (mm/h and mm).
- Hailstone count and event indicators.
The same physical principle is used across the ISAW RF4, HF4, FC4 and SF4 instruments, ensuring consistent configuration, data formats and integration methods within the ISAW family.
Core specifications
Measured parameters (selectable)
- Rain intensity (mm/h).
- Cumulative rainfall (mm).
- Number of drops.
- Drop size distribution (size classes).
- Maximum and average rain rate over the interval.
- Hailstone count and hail disdrometry fields.
Performance
- Sensitivity from light drizzle to intense convective events.
- Wide dynamic range for both drop count and rain intensity.
- Configurable averaging and measurement intervals (A–C–M mode).
- Internal datalogger with selectable fields (intensity, cumulative, DSD, hail, etc.).
- Factory linearization and calibration coefficients stored in the instrument.
Outputs & protocols
- Dual analog outputs (OUT1 / OUT2): continuous DC voltages 0–2.5 V or 0–5 V (rain intensity, cumulative or other mapped variables), or pulse outputs for rain accumulation with configurable mm-per-pulse and pulse shape.
- SDI-12 (v1.3) for complete data frames including DSD and hail fields.
- Serial ASCII (TTL / RS-422 / RS-485 with external converter) for custom systems.
- Modbus RTU RS-485 via optional MOBUS adapter for SCADA / PLC / BMS integration.
- 4–20 mA current loops via optional AD420 adapter for industrial interfaces.
- Cloud Data Link interf. (C-LINK)
Power
- Wide DC supply range suitable for field deployments.
- Ultra-low standby current, with average consumption adjustable via duty-cycle settings.
- Designed for solar-powered and battery-operated autonomous sites.
Mechanical design
- Heavy stainless-steel and anodized aluminum construction.
- Sealed electronics and IP-rated housing.
- Universal anti-vibration mounting arm with long-life elastomer damper.
- Compatible with masts, tubular structures or flat surfaces via universal bracket and V-clamp.
- Compact form factor for multi-sensor masts and compact stations.
WMO conformity & field use
WMO conformity of the RainFlow RF4
RainFlow RF4 follows the World Meteorological Organization (WMO) recommendations for surface precipitation measurement and reporting as described in: WMO-No. 8, Guide to Instruments and Methods of Observation (precipitation measurement practices and siting), WMO-No. 306, Manual on Codes (BUFR formats for precipitation and DSD-related parameters), and relevant WMO guidance for automatic weather stations and hydrological observing networks.
RF4 is suitable for operational meteorological and hydrological networks, national weather services and research infrastructures that require traceable, WMO-consistent precipitation data.
Proven in operational & research environments
RainFlow RF4 and its predecessors have been deployed in national and regional meteorological and hydrological networks, road-weather and urban drainage monitoring systems, wind-farm and PV-plant stations, agricultural and vineyard networks, and research campaigns on precipitation microphysics and DSD characterization.
Its robust design, no-moving-parts sensing head and advanced processing have demonstrated long-term reliability in a wide variety of climates, from cold continental and mountain regions to coastal and subtropical environments.
Ready to upgrade your precipitation monitoring network?
Our team can provide configuration guidance, integration support and turnkey solutions for meteorological, hydrological, infrastructure and research applications using RainFlow RF4 and the wider ISAW sensor family.