August 2009

Radar and Rainfall

Radar is an acronym meaning Radio Detection and Ranging.  During its initial development in WW II, weather was treated as "clutter", a problem that kept radar operators from seeing enemy targets.  Shortly after the end of the war, scientists realized the great benefit of using radar to study storms.  The radar displayed previously unseen patterns of storm growth and structure.  Doppler capabilities allowed for the detection of tornados and downbursts.  Radar also showed its utility in estimating precipitation.

The traditional way of turning radar measurements into rainfall data is to relate the power returned from the cloud to the radar (meteorologists refer to this as the "radar reflectivity") to some estimate of rain intensity measured on the ground.  Once this so called Z-R relationship is determined, the radar data can be converted directly to rainfall over the entire coverage region of the radar.  Usually, the rainfall estimates in the Z-R relationship come from rain gauge networks, like CoCoRaHS. 

The great benefit of using radar is that it can estimate rainfall over a huge area, including most places where there are no rain gauges.  However, it turns out that using radar to estimate rainfall is much more difficult than one might think for a number of reasons:

1. The radar reflectivity and rainfall relationship is not unique, it changes constantly within a storm, among different types of storms, and from place to place.
2. The radar looks within the cloud while the gauge is at the ground.  The difference in height between where the radar is looking and the gauge can be thousands of feet. A lot can happen to those drops as they fall out of the cloud: some might evaporate and others might be blown far downwind from where the radar is looking.
3. The radar beam might be blocked by hills, trees or building and might not be able to see the cloud producing the rain.

More information and links to information on radar and rainfall can be found on the CoCoRaHS web site: http://www.cocorahs.org/content.aspx?page=radar

January 2009

It looks like I have 0.02" of dense fog in my gauge!

An observer from Washington State writes: "For the past week - we have had very dense fog and a few days ago I checked my rain gauge and there was 0.02" in it.  This morning I checked it again and there was an additional 0.04" in it.  Should I report this as precipitation even though it isn't rain or snow?"
Great question!  Here's our answer: 

Moisture collected from fog deposition should be reported just like rain.  It is useful to mention in your comments that it came from fog.

Moisture from dew or frost does NOT count as precipitation (these form from condensation from usually clear skies) and should not be reported as such, although you are welcome to let us know that you did observe dew or frost by making a comment in the observation notes field.

October 2008


We all know that the amount of precipitation falling from the sky varies a lot, depending on where we live: some places get loads of water and other places get hardly anything.  Ever wonder how much water falls over the entire earth over the course of a year? According to a recent article in Scientific American (August 2008), the amount is 110,000 cubic kilometers - that's nearly 10 times the volume of lake Superior!

That huge amount of water would be enough to easily fulfill the requirements for everyone around the planet, if the water arrived where and when we needed it.  Unfortunately, this is not the case.  Not only is the rainfall supply highly variable in time and space,  most of the water that does fall cannot be captured.

Here's a rough breakdown of where all that precipitation goes:
* 61.1% absorbed by soil and plants and then released back into the air, unavailable for withdrawal
*  36% ends up in the oceans, also unavailable for withdrawal
* 1.3% evaporates from open water, also unavailable for withdrawal
* only 1.5% is directly used by people.  Of this amount, 1.4% is used for farm irrigation and 0.1% is used by municipalities and industry

August 2008


The minimum water that each person requires, on average, for drinking, hygiene and growing food is about 1,000 cubic meters per year.  That's about 2/5 of an olympic size swimming pool.  Because the distribution of global water resources varies widely, some people get much more than this and some people get less.

Researchers have recently developed the concept of a "water footprint".  As indicated on the water footprint web page (http://www.waterfootprint.org), "The water footprint of an individual, business or nation is defined as the total volume of freshwater that is used to produce the goods and services consumed by the individual, business or nation."

You can figure out your own water footprint using the calculator (quick and more extensive versions) on this site as well learn the total water required to produce various products.  Some interesting facts about our water use from the water footprint web site:

*The production of one kilogram of beef requires 16 thousand litres of water.
*To produce one cup of coffee we need 140 litres of water.
*The water footprint of China is about 700 cubic meter per year per capita. Only about 7% of the Chinese water footprint falls outside China.
*Japan with a footprint of 1150 cubic meter per year per capita, has about 65% of its total water footprint outside the borders of the country.
*The USA water footprint is 2500 cubic meter per year per capita.

In addition to the water footprint web site, check out the August 2008 issue of Scientific American to learn more about the global variability of fresh water.

August 2008


The word "monsoon" comes from the Arabic word mausim, meaning season.   Basically, it describes a seasonal wind shift over a region that is  usually accompanied by a dramatic increase in precipitation.  Many of  us are familiar with the Indian-Asian monsoon that brings heavy rains  during the summer months over widespread areas of India and SE Asia.   Although these rains often produce major flooding, they are vital to  agriculture and the economy.  Because so much of the world's  population live in this region, a delayed or reduced rainfall season  can have a devastating effect on the livelihood of a significant  fraction of the world's population.

Many other parts of the world experience monsoons, including North  America.  Our North American monsoon (also known as the Mexican  monsoon) typically occurs between July-September and is relatively  small compared to the Asian monsoon.  However, in parts of NW Mexico,  over 50% of the annual rainfall comes in this 3-month period.  The  rains provide a critical source of replenishment for water resources  of Mexico and the SW United States.  This year, our CoCoRaHS volunteers in southern New Mexico reaped the benefits (and flood dangers) from very heavy monsoonal rains.

To learn more about the North American monsoon, check out: http:// www.wrh.noaa.gov/twc/monsoon/monsoon_tracker.php

April 2008


Based on pictures we’ve seen in magazines, on TV, and elsewhere, most of us would guess that raindrops are shaped like a teardrop.  It turns out that this is not the real shape of a raindrop.  Small raindrops (< 1mm in diameter) are spherical, like a round ball.  This is because a sphere is the shape that requires the least amount of energy for the drop to hold itself together.

As drops grow bigger than a millimeter or so, they start to become flat along their bottom edge as they fall, due to the resistance of air flowing around the drop.  By the time a drop reaches 2-3 mm in diameter, it looks more like a hamburger bun than a sphere.  Drops bigger than about 6 mm in diameter are relatively rare because the air resistance tends to cause the drops to breakup as they fall.  

February 2008


Many of us are familiar with the verse “Water, water everywhere, nor any drop to drink” from the poem: “The Rime of the Ancient Mariner”.  Most of us, however, probably don’t realize how true those words are in terms of the amount of freshwater available to us. 

Have you ever wondered where our water is stored?  As you probably guessed, most of it is in the oceans.  In fact, over 97% of all water on the planet is too salty for us to drink.  Of the remaining 3% that is freshwater, about 2% is tied up in ice caps and glaciers and about 0.9% is stored as water in the ground.  That leaves less than 0.1% of the world’s total water supply to be split up between rivers, lakes, and the atmosphere (stored as water vapor)!

Next time you empty your gauge, remember how precious that freshwater is.

More information about the water cycle can be found at: http://ga.water.usgs.gov/edu/watercycle.html

February 2008


Water is an amazing substance.  Besides nourishing life, it is unique in it’s physical aspects.  Unlike most substances, water’s density (the amount of mass that can be stuffed into a fixed volume) doesn’t continually increase as temperature decreases.  For fresh water, the maximum density (~1000 kg/m3*) occurs at about 4 C.  That is, water is most dense above freezing.  That means that ice (frozen water) is less dense than liquid water.  Thus, ice forms on top of the water’s surface (as opposed to freezing from the bottom up).

The fact that water bodies like lakes and ponds freeze on the surface (but not usually through their entire depth) is good news for all the fish and other creatures that depend on a continual supply of liquid water and oxygen to get them through the cold winter months.  It’s also good news for people who like to ice skate!

For more information on this topic visit: http://www.piercecollege.edu/offices/weather/water.html

* this essentially means that a volume of water measuring a cubic meter weighs about a ton!

September 6, 2007


On mornings with very heavy dew, moisture may condense on the funnel of your gauge and then drip into the measurement cylinder. Should that be reported as precipitation?

The answer is no. Dew is condensation on the surface, not precipitation from the sky. If you find moisture in your gauge from dew, do not include that in your precipitation report.

Dew may be a valuable source of water, however, for plants, animals and insects. If you observe heavy dew, report that in your "Observation Notes"

Thanks for participating in CoCoRaHS!