climate:climate2
climate - water and rain
Water on earth:
water vapour in atmosphere = 20 million, million tonnes
water in lakes, river & underground = 200 million, million tonnes
water in icecaps = 20,000 million, million tonnes
water in oceans = 1.4 million, million, million tonnes (ie. ~100x that in the ice caps)
Evaporation:
evaporation is not the same as boiling, it occurs at the liquid surface at any temperature
Dalton equation:
mean evaporation rates from a pan rises approx. exponentially with mean temperature:
10deg C ⇒ ~1mm/day
15degC ⇒ ~2.5mm/day
20degC ⇒ ~4.5mm/day
25degC ⇒ ~7mm/day
in Australia, average annual evaporation rates range from:
Tasmanian wilderness = 500mm/yr
NZ = 650-950mm/yr
SE Australian coast incl. Melb, Sydney = 1000mm/yr
Qld coast, Adelaide, SW WA coast = 1250mm/yr
Kalgoorlie = 3300mm/yr
southern hemisphere avg evaporation rates vs latitude:
from oceans:
latitudes 0-30deg ⇒ 1100-1200mm/yr
latitudes 30-40deg ⇒ 890mm/yr
latitudes 40-50deg ⇒ 580mm/yr
latitudes 50-60deg ⇒ 230mm/yr
from continents:
latitudes 0-10deg ⇒ 1200mm/yr
latitudes 10-20deg ⇒ 900mm/yr
latitudes 20-50deg ⇒ 400-500mm/yr
latitudes 50-60deg ⇒ 200mm/yr
Water content in atmosphere:
Clouds:
cloud types:
3 main types of clouds:
convective:
orographic:
often up to 200km wide and up to 1km thick with updraft velocities 1-10m/s
stratocumulus clouds often have bases1.5km and extend to 3-4km elevation
altocumulus exist within 3.5 to 6.5 km elevations
layer (stratus):
can be 1000km wide, but often only 100m thick and updraft velocities of only 0.1-0.2m/sec
stratus clouds are usually in elevations below 2km & have 0.25g/cu.m water (0.5 for nimbostratus)
alto-stratus clouds are usually in elevations 2-8km & have only 0.1g/cu.m water
cirro-stratus clouds are usually in elevations 8-20km at tropics and 3-8km at poles
if saturated air is cooled to its dewpoint, water condenses out forming a cloud or dew
cloud droplets are 0.2-20um size & are no more than a millionth the size of rain droplets
clouds are usually formed by moist air being either
Rain
requires both:
cloud formation
raindrop nucleation:
a cloud droplet is so small that its terminal velocity in still air is only a few millimeters/sec which is much less than the updraught velocity in a cloud, thus the resultant motion is upwards
for rain to reach the ground, droplets or crystals must reach at least 0.1mm diameter which requires the aggregation of thousands or millions of droplets
aggregation of cloud droplets requires special active nuclei:
“warm clouds”:
clouds at low altitudes or latitudes that have temperatures above freezing
particles of hygroscopic materials such as sea salt with diameters > 5um
seeding can be done with ammonium nitrate ground to 3um
“cold clouds”:
if the droplet is cooled to minus 40degC, the tendency towards freezing is so great, that it overcomes the need for a nucleating foreign body, and forms ice spontaneously ⇒ “homogeneous nucleation”
between 0degC and minus 40degC, the readiness of a cloud to form rain depends on the presence of ice nuclei, consisting mainly of volcanic dust, clay, and soil particles with a crystalline structure similar to ice with sizes typically > 0.1um, however, if there are too many such nuclei (eg. in drought conditions when there is much dust in the air ⇒ drought begets drought), the drops do not get big enough to fall as rain.
seeding can be done by either dry ice or silver iodide
rainfall variability:
mean annual rainfall on coastal regions often correlate with adjacent sea mean annual temperatures
rainfall for a region tends to have persistence:
a wet year tends to be followed by another wet year
a wet day tends to be followed by a wet day (Melbourne: 67% chance in Sept, falling to 47% in Jan)
a dry day tends to be followed by a dry day (Melbourne: 81% chance in Jan, falling to 54% in July)
seasonal variability for a region is dealt with elsewhere, but in general is determined by probability of:
tropical cyclones (mainly summer in tropics)
onshore moist winds:
frontal systems - esp. for SE Australia, more common in winter
convection thunderstorms - esp. in summer
droughts:
are difficult to define, but generally is regarded as occurring if the annual rainfall for a region is in the range of the driest 10% of years for that region
major droughts in Australia:
climate/climate2.txt · Last modified: 2021/07/07 23:51 by gary1