Measuring Precipitation

Drizzle

fine mist, 0.1 – 0.5 mm diameter

Rain

liquid precip heavier than drizzle, 0.5 – 5 mm

Snow

–precip that reaches ground as ice crystals

Sleet

raindrops frozen on descent, <0.5mm

Hail

ice balls, 5 mm – 10+ cm diameter

Problems with simple rain gauge

• what if rain is coming down at an angle: requires rain to be vertical

• What if it rains overnight or twice a day

• snow can plug it

• only measures one pt.

• you. have to dump it yourself

• left over rain

• evaporation

• rain splating

Pros of simple rain gauge

simple and very specific

cheap, can cover lots of ground if by multiple

Standard rain gauge problems

• what if rain is sideways

• What if it rains overnight or twice a day

• snow can plug it

• only measures one pt.

• you. have to dump it yourself

• left over rain

Tipping bucket rain gauge Problems

• freezing and snowing

• one pt.

• still requires maintenance

• some water could be left behind after it tips

how tipping bucket rain gauge works

• Rain falls into a funnel at the top of the gauge. The funnel directs the water into a small see-saw–like container (the “tipping bucket”) inside.

• The tipping bucket is divided into two small compartments—like two tiny cups balanced on a pivot. Each compartment holds a precisely measured amount of water

• When one side fills up to its set volume, the weight causes the bucket to tip over, emptying that side and positioning the other side under the funnel to start filling.

• Every time the bucket tips, it sends an electrical signal (a “tip count”) to a data logger or computer. Each tip represents a known amount of rainfall. By counting how many tips occur, the system calculates total precipitation.

Optical rain gauge

• the best

• uses beam of infrared light to measure the number of raindrops in a given area

issues with optical rain gauge

• expensive

• calibration(is standard) it not for all rainfall events 

  • only works for the perfect rain

Where do you want to put a rain gauge

•Open area, no potential blockages

–No overhanging branches

–No possible eddy currents(wind)

•No artificial contributors of water

•Leveled

•Protected from winds

wind as a source of error

–Increased wind deflects precip from original path

•Largest errors in snow and light rain

Solution to combatting wind

•Place gage close to ground

•Reduce wind influence

source of error: wetting loss

Moisture remains on sides of gage or funnel

•Evaporates after event

•Remains in gage due to adhesion after gage is emptied

wetting loss solution

–Use material with lower tendency for adhesion

•Recognize potential for error in measurements

Evaporation as a source of error

• Underestimation due to evaporative loss

• Most important for light rainfall in warm areas

• Evaporation can account for up to 4% of the catch

evaporation solution

•Tightly-fit funnel reduces ventilation

•Funnel should be smooth

•White gage reduces heat available

•Introduction of light oil (olive oil or glycerin)

•Condensation as a source of error

Problem: In cases of heavy dew, recordable amounts of moisture collect

condensation solution

Use a gage with an inner container

•Outer container insulates collection/measuring container and prevents heavy condensation

Rainsplash Source of error

–Problem: Raindrops splash in/out of container

rainsplash solution

• To reduce splash-out: locate funnel deep into container, funnel should have steep angle (≥45°)

• To reduce splash-in: underlying surface should not be smooth or rigid

  • Presence of air pockets reduce kinetic energy of impact

  • Place plastic beads, straw, slanted louvers, etc. around base

Freezing/snow capping source of error

• Problem: Wet snow may stick inside gage opening

  • Snow may form a cap, limiting incoming precip

  • Snow cap could melt and enter opening

Freezing/snow capping solution

• Regular inspections

•Presence of antifreeze solution

Precision depends on

• Density of gages

and

• Type of storm event

Low-density distribution

• underestimate rain amounts and intensities

• Topography has large impact

  • Areas of high relief need different distribution than areas of uniform topography

Topography has large impact

on density of gages

• • Areas of high relief need different distribution than areas of uniform topography

Type of storm event

•Localized storms more difficult than uniform events

•Arithmetic average is best for

–areas with low relief and uniform precip.

Arithmetic pros and cons

•Quick and Easy

•Provides decent estimate

but •Does not account for area differences

•Weights all values equally

why is it imp. to know rainfall intensity

• Important to know for planning purposes

• Helps to estimate runoff à flooding, storm drains, detention basins, culverts, etc.