precipitation Flashcards
generally winter is getting _______ and summer is getting _________. though annually it doesn’t make that much of a difference
wetter
drier
why is summer drier?
warm air can hold more water
what are the Met Office Standards for rain gauges
- funnel and bottle
- there must be a standard height above the ground
- narrow spout of funnel to limit evaporation.
- standard diameter 5inches
explain briefly how tipping bucket rain gauges work
-funnel to collet water then to a metal scope on a pivot.
-when one side of the scope is full, it then tips and fill water on the other scoop
-the rate at how fast It tips measure how much water there is.
each tip is equivalent 0.5mm of rainfall
-can measure the amount of rain over a period of time to find rat
-on graph, one bar=pne tip
explain briefly how weighing rain gauges work
- it has a sensor that measure the mass of water collected, hence record the rate of rainfall in a period of time
- it can measure both liquid and solid precipitation
- big container, doesn’t need to be empty often
- oil floats on top of water to reduce evaporation
why do rain gauges have to be set on short grass
vegetation will shelter the rain gauge if its not short, hence will affect accuracy as vegetation absorbs water
why does the height at which the rain gauge is set matter
closer to surface- more splash in and splash out
higher up- no splashes but more Windfield. less blown in and blown out.
how can splash in and blow out be reduced
- the rain gauge is levelled with the ground to reduce splashing
- shielded to reduce windfield
true or false
raindrops have a higher velocity than snowflakes
True. snowflakes has a lower density hence lower velocity
what are the techniques to measure snowfalls
- need shielded gauges, can capture more snow
- snow gauges can’t be built too low; can be buried
how do we find the snow mass per unit area?
snow depth x snow density
why does snow density vary
snow can be compacted. therefore density is fresh snow is lower than old compacted snow
how does a visibility sensor work
- made up of two sensor, one is to shine light, another is to receive
- clear day- light will shined to the ground from the first sensor, no reflection will be picked up at the detector
- foggy day; particles in the air will reflect the light and the detector will pick this up. less light resulted on the ground
how does precipitation detector work
circuit is work when rain touches the electrodes. the amount of water running across the surface gives the rate of rainfall.
compare how the optical disdrometer and impact disdrometer work
optical- 2 sensors; one to emit radiation dn one to pick it up. when there’s rainfall in between, radiation gets interfered and rainfall rate will be given
impact- measure the sound of the rain and generate electrical impulse
true or false
larger drops correspond with higher rainfall rate
True
how does rainfall radar work
- a pulse electromagnetic radiation is transmitted
- signal is reflected back from the raindrops
- distance of rain is given by the time of return signal
- size of drops is given by the strength of signal.
- stronger strength = larger raindrops = higher rainfall rate.
True or false Radar scans by rotating about vertical axis at four elevation angles between 0.5 and 4 – one complete scan takes 5 minutes
True
true or false
both rain and drizzle can be detected by radar
false. drizzle is too small to be detected
what are some rainfall radar errors
- echoes can be from birds or a group of insects
- echoes due to permanent obstacles (mountains)
- rain detected at high levels may evaporate before it reaches the ground. so data not useful for public
- wet snowflakes at melting stage looks like large raindrops and can give false results
why can we eliminate rainfall radar errors
- use a network of rainfall radar to create overlapping of radar, this helps us to recognise the pattern
- compare the results with rain gauges
why is rainfall radar useful
helps us to get immediate data on the amount and location of the rainfall. helps us to forecast events like flash-floods
how can satellite measure rainfall and humidity
measure radiation field at particular wavelength and relate it to rainfall
how can doppler radar be used to monitor storms and tornado
- doppler radar measure how fast a target (eg. storm) is moving towards or away from the radar
- it does it using the frequency shift of the return signal
1mm rainfall = _____ kg m^-2
1
