How much rain?

The Met Office has a handy guide to making your own rain gauge. This is a great idea, and welcome inspiration for something we can do as the small boy likes to tell me about the weather. I don't think their exact design will work well for us - it uses a nifty trick of putting jelly in the bottom of the bottle to give a tube of constant diameter, but we have  several urban foxes who visit our garden and I'm pretty sure that a bottle filled with jelly won't be intact tomorrow morning.  Given the littlest one's hat was torn into shreds when it fell down off the washing line, the idea of tempting them with food doesn't appeal!

We therefore chose an empty bottle from our recycling bin which had a square cross-section - we went for this one because it was quite sturdy, and the cross-section area is constant almost to the bottom i.e. the area of rainfall that you are measuring at the top is the same as the area of the bit you are collecting the rain in at the bottom, so you can measure the depth of rain that has fallen without having to do any maths.  The Met Office example uses a round bottle, but I don't think it matters what shape you use.

I cut the bottle close to the top (without toddler assistance) to divide it into a collecting bottle and a funnel (i.e. the neck of the bottle turned upside down), and taped around the 'funnel' and the collection bit with sellotape, so as to avoid little fingers finding sharp edges.

Because I wasn't doing the jelly trick and the bottom of the bottle has an indentation, I measured the volume of water required to increase the depth in the main bit of the bottle by 1cm (55mls in our case) and then emptied the bottle and put 55ml in (measuring using our digital kitchen scales).  This gives me a mark for 1cm of rainfall, and is quite a lot more than 1cm from the bottom of the bottle due to the indentation in the bottom of the bottle that gives it strength and stability.  I also marked where half of this amount comes to, such that we can measure 0.5cm of rainfall.  Whilst not perfect, I'm using this rain gauge with a not-quite-3-year-old, so the accuracy of measuring small amounts of rainfall isn't critical.

I then got him to help me make the other marks, 1cm apart, with a permanent pen and the tape measure from his 'toolkit' (the tape measure in question is one of the paper ones that you can get to help you measure furniture in IKEA, they make a good toddler tool!).  This was a nice opportunity to talk about measuring depth and how we will be looking at how much rain falls by measuring how deep the water is.  He's got a reasonable understanding of counting and quantity (although his counting sometimes gets a little wobbly in the teens, and 30 is often called "twenty-ten") but his understanding of measuring distance doesn't seem to be quite there yet.

Our improvised rain gauge
We put the rain gauge in a large plant pot filled with soil (because a plant we'd put in it a few weeks back was dug up by the above foxes, and although the toddler carefully scooped the soil back into the pot, the plant was beyond rescuing), and moved it away from the house so it would be able to catch the rain.  It then proceeded to be sunny all afternoon!

When we went out in the rain this morning (to look at some local roadworks, as you do...), he was quite excited that it was raining so there would be something in his rain gauge.  We looked late morning (so almost 24 hours after we put it out) and there was rain in the collection bottle!  To say it was 5mm would be overstating it, but there was a little rain, and we said agreed it was more than zero, but less than one centimetre, and he seemed happy.  We watered a surviving plant in another pot with the contents, and put it back to await some more rain.