L4. Precipitation I

Question 1

Considerations when using a pluviometer (raingauge)

Answer 1

• height (wind profile changes near the ground)
• colour (dark surfaces heat up and cause evaporation
• wind (wind deforms the pressure field around the gauge causing less precipitation to fall in)
• methods (wind protection can create more accurate results)
• location matters (avoid wind effects, don’t be close to tall things)
• timing (frequent measurements keep evaporation losses low)
• Person (human error)
• technology (instantaneous automated measurements reduce evaporation losses, tech could fail during storms)
• size (larger size means less inspections needed, evaporation may become significant
• temperature (cold can lead to huge potential errors, hard to measure in extreme conditions)

Question 2

What is a lysimeter?

Answer 2

A device typically used to measure the amount of evapotranspiration. Operates by recoding the amount of precipitation that an area receives and the amount of water that drains from soil. Can also tell the amount of water that falls on area

Question 3

What is the saturation vapour pressure curve

Answer 3

A graphical representation of the relationship between temperature and the maximum amount of water vapor that air can hold

Question 4

What is the dew point temperature

Answer 4

The temperature at which the air becomes saturated with moisture, and water vapor in the air begins to condense into tiny water droplets
- Cloud formation

Question 5

What is relative humidity (RH)

Answer 5

• a measure of how much moisture is in the air compared to the maximum amount of moisture the air could hold at a given temperature.
• It is expressed as a percentage.
• tells you how close the air is to being saturated with moisture.
• Warm air can hold far more moisture than cold air meaning that the relative humidity of cold air would be far higher than warm air if their absolute humidity levels were equal.

Question 6

What is absolute humidity

Answer 6

measure of the actual amount of water vapour present in the air
- mass of water per water per unit of air

Question 7

Convective uplift cloud formation

Answer 7

• classic white clouds
• The convective process is driven by the buoyant force of the warm air which eventually cools. When the air reaches the dew point then clouds form.

Question 8

Frontal (or advective) uplift cloud formation

Answer 8

• occurs at the boundaries, or fronts, where two air masses with different temperature and humidity characteristics meet
• typically a cold front and a warm front meeting
• As the two air masses meet, the warmer, less dense air is forced to rise over the colder, denser air, creating a sloping or inclined surface along the front
• the warm, moist air is lifted along the frontal boundary and cools, of the air reaches its dew point temperature, condensation occurs, leading to cloud formation

Question 9

Why does it typically get colder when you walk up a mountain

Answer 9

As you ascend a mountain, you are moving to higher altitudes. The air pressure decreases with altitude. This decreased air density leads to a drop in temperature because there are fewer molecules to absorb and retain heat.

Question 10

What is the adiabatic lapse rate?

Answer 10

the rate at which the temperature of a parcel of air changes as it rises or descends through the atmosphere without exchanging heat with its surroundings

Question 11

What is the dry and wet adiabatic lapse rates?

Answer 11

Dry rate: 1C/100m
Moist rate: 0.6C/100
- Less because condensation releases latent heat

Question 12

What causes orographic precipitation?

Answer 12

• When moist air meets mountains it is forced to rise with the slope. It cools and may reach the dew point. It will condense and may become heavy enough to precipitate
• On the windward side of the mountain (the side where the moist air is rising), orographic precipitation results in significant rainfall or snowfall. However, on the leeward side (the downwind side), a rain shadow effect occurs

Question 13

Where does it rain the most and why?

Answer 13

The tropics
- due to temperature, global atmosphere circulation patterns, topography, and distance to ocean

Question 14

How do raindrops form?

Answer 14

Formation of raindrops is aided by differential condensation.
- when water vapour gas gathers onto tiny airborne particles called cloud condensation nuclei (CCN)
- More and more water condenses, they collide with others and grow, eventually heavy enough to fall as rain or snow

Question 15

What is rain?

Answer 15

• drops when moisture in the air condenses and becomes heavy enough to fall to the ground
• 0.5-5mm

Question 16

What is drizzle?

Answer 16

• smaller than 0.5mm
• lighter than rain

Question 17

What are showers

Answer 17

• vary in intensity
• general fall from cumulonimbus or large cumulus clouds
• usually scattered and intermittent, and they can be sudden and intense before tapering off

Question 18

What is snow

Answer 18

• Forms when water vapour turns to ice without first condensing
  into a liquid
• Snow flakes are composed of microscopic water crystals that
  cluster together in groups of 50 or more

Question 19

What is hail?

Answer 19

• Formed when water droplets freeze in high, very cold clouds
• Hailstones grow bigger as they are coated with successive layers
  of ice. This can occur when they are pushed up and down through
  the atmosphere by strong updraughts and downdraughts.
  Hailstones can also get bigger when they bang into each other
  and stick, forming conglomerate hailstones.

Question 20

Rules for rain gauge networks and interpolation

Answer 20

• Gauges should be equally distributed in space
• More gauges are needed in mountainous regions (orographic
  effects) or in tropics (small scale convective precipitation)
• In most cases, we do not have enough gauging stations to
  conduct proper assessments
• Typically, we average measurements from a network of
  gauges; this introduces an error (in addition to the
  measurement error at the gauge)

Question 21

Methods of spatial interpolation for gauges

Answer 21

• arithmetic averages
• theissan method
• Isohyets
• Kriging

Question 22

What are remote precipitation measurements good for?

Answer 22

• great for measuring the spatial distribution of events
• difficult to calibrate so we still need ground measurements

Question 23

Important rainfall characteristics

Answer 23

• Magnitude: amount of precipitation (Depth mm)
• Duration: how long it rains (time)
• Intensity: magnitude/duration (Mm/time)
• Frequency: how often it rains

Question 24

What is a rainfall hyetograph?

Answer 24

a graphical representation of the distribution of rainfall over time