Lec 3: Precipitation I Flashcards
What factors affect rain gauge (pluviometer) readings and why? (9)
Height: Wind profile changes near ground
Color: Dark surfaces heat up and cause evaporation
Wind: deforms pressure field around gauge, causes under-measurements
Methods: wind protection & instantaneous, automated measurements increase measured quantity and reduce evaporation losses
Location: to avoid wind effects, no object should be closer than 2 times its height to the rain gauge (buildings, trees)
People: observation errors
Timing: Frequent measurements keep
evaporation losses low
* In dry regions, rain alerts may help
identify when measurements are
needed
Technology: better technology = better measurements, but technology can fail or not detect certain errors
Size: larger gauge = less inspections required, but evaporation can become significant after longer periods
Describe gadgets that account for or prevent evaporation
Tipping bucket rain gauge
Gauge that draws water level live
How accurate are rain/snow gauge in cold climates? Why? What are some solutions
- very innacurate
- wind and snow drift are very prevalent. frequent, high-quality measurement difficult due to hard to reach locations and difficult conditions
- heated gauges that melt the snow that falls in the gauge (but this may increase evaporation)
What does a lysimeter do and how?
they measure the amount of actual evapotranspiration which is released by plants
how:
- By recording the amount of
precipitation that an area receives and the amount of water that drains the
soil, the amount of water lost to evapotranspiration can be calculated.
- Weighing lysimeters measure the precise
amount (weight) of water that falls on them
What is relative humidity?
What is it an indicator of?
RH is the ratio of actual (partial) vapor pressure to saturation vapor pressure (at a given temperature), in %
RH is a good indicator for likelihood of rain,
but doesn’t say anything about amount of water vapor in air
What is saturation temperature? What is another name for it?
the temperature at which water condenses
aka dew point
Describe the saturation vapour pressure curve
Relationship between temperature and pressure. Curve represents relative pressure needed at a given temperature to reach saturation.
Above curve = supersaturated
Below curve = undersaturated
What is absolute humidity?
Absolute humidity is mass of water per unit of air
e.g. mass water per mass air OR mass water per volume air
What is the relationship between absolute and relative humidity?
Absolute humidity is higher at higher levels of relative humidity
Describe the two types of adjective (or frontal) uplift
cold front
- cold air advances into warm air
- steep boundary between air masses, warm air rises quickly
- showers and thunderstorms, clears quickly
-warm front
- warm air moves over cold air
- more gradual boundary, warm air rises slowly
- shield of steady rain or snow
How does convective uplift cause precipitation?
Convective uplift occurs when air near the ground is warmed by the sun and begins to rise. Air rises and cools, forming clouds and precipitation.
What is the adiabatic lapse rate?
How does it differ for dry and wet air?
the rate at which the temperature of an air parcel changes in response to the compression or expansion associated with elevation change
The adiabatic lapse rate for a saturated parcel is lower than that for an unsaturated parcel.
What is orographic precipitation?
Precipitation which is caused by hills or mountain ranges deflecting the moisture-laden air masses upward, causing them to cool, reach the dew point and precipitate their moisture on the windward side of the mountain.
The air that goes down the leeward side is warm and dry. As the air descends, its pressure increases, which makes its temperature go up.
What are some local names for orographic precipitation/cloud formation?
Foehn winds, Chinook
Where does it rain most?
- mountain ranges
- coastal regions
- equator
- India
