Wednesday, September 11, 2019

The 8-inch Standard Rain Gauge

The 8-inch standard rain gauge hereafter referred to as SRG) is the workhorse of the National Weather Service Coop Program. In fact, this gauge is the world-wide standard for measuring precipitation. The gauge is your basic straight-sided cylinder and is the big brother of the 4-inch rain gauge we use in CoCoRaHS. The components are the same. Each has and outer cylinder which catches overflow from the inner measuring tube. Both rain gauges have funnels which direct the precipitation into the inner measuring tube. The SRG includes another component, a measuring stick graduated to hundredths of an inch.

For a long time the SRG outer cylinder (aka "the can") and funnel were made of copper, and the inner tube was made of brass. Better and less expensive materials have for the most part replaced copper. The outer cylinder of the SRG pictured here is stainless steel, the inner tube is made of poly carbonate plastic, and the funnel is made of fiberglass. The outer cylinders are also made of aluminum. The measuring stick is a laminated fiberglass that "wets" so that you can read the precipitation measurement. Even with these newer materials there are issues. The outer cylinders can develop leaks along the seams on the bottom, and those can be hard to detect. Both the brass and plastic inner measuring tubes can be damaged and develop leaks in freezing weather.

Funnel and inner measuring tube of 8-inch SRG. Credit: NWS
One of the advantages of the SRG is its capacity. The inner measuring tube holds two inches of rain, and the outer cylinder holds 20 inches of rain. However, that advantage is also a big disadvantage. The 8-inch outer cylinder holds about 4.3 gallons of water, and a gallon weighs 8.3 pounds. An observer would have a difficult time lifting 35 pounds of water and even a more difficult time trying to pour anything into the inner measuring tube. Four inches of water in the outer cylinder would be about a gallon, and that plus the weight of the cylinder itself can be cumbersome to handle. It is definitely not a one-person job. My personal experience with the 8-inch "can" and pouring led me to build a stand to hold the inner tube while pouring from the can. The funnel is placed on the tube before pouring.

Once I became a CoCoRaHS observer I designed and built a smaller version for the CoCoRaHS gauge. You can find the plans to build one at this link.

The 8-inch SRG is the standard at NWS primary Coop sites, but states "The four-inch plastic rain gauge is a suitable substitute for the eight-inch standard rain gauge because it meets the accuracy requirements". That accuracy is specified as ±0.02 inches. (National Weather Service Instruction 10-1302: Requirements and Standards for NWS Climate Observations, April 2018) 

Tuesday, September 10, 2019

The Rain Gauge - How Can Something So Simple Be So Complex?

The rain gauge. At its most basic it is just a straight-sided cylinder, with a bottom, of course. Like the mousetrap, someone is always trying to build a better one. A simple web search of "rain gauge" will display a gallery of images of different types of rain gauges.

While the basic concept for a rain gauge is simple, the complications start to come in with calibration, measurement, and siting/exposure. Add to that the increasing complexity when mechanical and electronic components become part of the measurement process and the potential for measurement errors greatly increases. There are weighing bucket rain gauges, which measure the amount of precipitation by weighing the water and recording it on a revolving chart (and now digital storage). There are also weighing rain gauges that utilize more complex technology and software to measure precipitation.

An older weighing bucket rain gauge. The recording drum is located behind the door in the bottom.

Tipping bucket rain gauges utilize a small "bucket" that tips and triggers a signal every time one one hundredth of an inch of rain is collected. Optical rain gauges utilize photo diodes, laser, or infrared to detect drops and measure rainfall rate and intensity. Acoustic rain gauges have sensors that detect the acoustic signature for each drop size on the sensor surface. From this data the drop size distribution can be determined, and from the the rainfall rate and accumulation.

Tipping bucket rain gauge

As we all know, weather also has an effect on precipitation measurement. Strong winds affect collection efficiency, typically resulting in an under-catch of rain, and even more so for snow. Speaking of winter, rain gauges that rely on collecting liquid water must be heated if mechanical or electronic, or the frozen precipitation must be melted before it can be measured, for example, as with the CoCoRaHS gauge and the NWS 8-inch standard rain gauge. Siting and exposure all can affect the measurement with any of the rain gauges mentioned, not just how close the rain gauge is to nearby objects that might affect the rain gauge catch, such as trees and buildings, but also the height of the opening above the ground.  A rain gauge installed closer to the ground is slightly less susceptible to affects from strong winds compared to one at a greater height above ground.

So, what should be a simple, straightforward measurement (how much water is in the straight-sided cylinder) is affected by many factors. Even manual measurement is subject to observer error. The more complicated rain gauges, largely developed to make measurements where or when manual measurements are not practical, introduce the potential for a variety of other errors even though minimizing the human factor.

Wednesday, September 4, 2019

Dorian Part 1 - Cluster of Thunderstorms to Tropical Beast

Tonight Hurricane Dorian is affecting the southeastern U.S. coast. The center of the eye is roughly 100 miles off the coast, and it's precipitation shield extends inland 30 miles or less from the northern Florida coast up through Georgia and into South Carolina. We have already been following Dorian for 11 days.

The first advisory for then Tropical Depression #5 was issued during the morning on August 24th. Later that day Tropical Depression #5 became Tropical Storm Dorian.

On Wednesday, August 28 Dorian strengthened into a hurricane near St. Thomas in the U.S. Virgin Islands. Dorian steadily strengthened the next 48 hours, becoming a major category 3 hurricane on Friday morning, August 30. Dorian rapidly strengthened to a Category 4 hurricane by Friday evening. Dorian became a Category 5 hurricane sometime near dawn on Sunday, September 1, just 40 miles or so east of Great Abaco island in the northwest Bahamas.

Track of Hurricane Dorian from August 24 to 5:00 p.m. EDT September 4.

Satellite infrared image of Hurricane Dorian at 6:00 p.m. EDT on August 31. At this time Dorian was a Category 3 hurricane and quickly ramped up to a Category 4 within three hours.
Radar image from the Bahamian Weather Service showing Dorian at 4:00 pm EDT on September 1. At this time the eye was moving over Great Abaco with sustained winds up to 185 mph and gusts to 220 mph

For the next 40 hours the combination of 155 to 185 mph+ winds in Dorian's eyewall and storm surge exceeding 20 feet of water produced catastrophic damage across Great Abaco and Grand Bahama islands as the storm virtually came to a standstill. Buildings not damaged or destroyed by the winds were likely "protected" by the deep water that submerged them.

These two radar images, 12.5 hours apart show how little Dorian moved during the time period while the eye was over Grand Bahama Island. During this time sustained winds were sustained around 155 mph with gusts to 195 mph.

As of this writing the death toll in the Bahamas is 20, and is likely to go higher.

Tonight (September 4) Dorian has re-intensified to a Category 3 hurricane and is likely headed to the Outer Banks of North Carolina. 2.3 million people in the Southeast are under evacuation orders. It will be another several days before Dorian is no longer something to worry about. For the latest information on Dorian visit the National Hurricane Center website.