climate:climate2
Table of Contents
climate - water and rain
* see also:
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)
- man's use of groundwater for irrigation between 1993–2010 has moved 2150 GTon of water from underground to the oceans causing a 6mm rise in sea levels and increasing the earth's tilt by drift of Earth's rotational pole of 78.48 cm toward 64.16°E 1)
Evaporation:
- evaporation is not the same as boiling, it occurs at the liquid surface at any temperature
- Dalton equation:
- evaporation rate = Ku(liquid's vapour pressure - ambient atmospheric water vapour pressure)
- liquid's vapour pressure is dependent on its temperature
- ambient atmospheric water vapour pressure is dependent on atmospheric temperature and humidity
- K = factor dependent on stirring of surface by wind & the surface roughness
- u = wind speed, but as K is almost inversely proportional to u, Ku is almost constant at 0.5, but will almost double for the extra roughness of ocean surface when wind increases above 6.5m/s.
- 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:
- the amount can be specified in several ways:
- vapour pressure (mb) = 1.3 x absolute humidity (g/m3)
- saturated vapour pressure = 12mb at 10degC & doubles with every 11degC increase in temperature
- dewpoint temperature:
- the temperature to which air must be cooled for the moisture present to provide saturation
- air cooled below its dewpoint results in dew forming
- dewpoint = dry bulb temp - (100-RH)/5) and thus RH = 100 - 5(dry bulb temp - dewpoint)
- if wind is lifted 1km by a hill, the pressure of each of the air's components is reduced by 13%, a fall of this magnitude in the saturated vapour pressure corresponds to a reduction of dewpoint temperature by ~2degC
- see also: combating dew
- relative humidity:
- this is the ratio of the air's vapour pressure to the saturated vapour pressure at the air's temperature
- precipitable amount:
- amount of water in a column of air from the ground including vapour & cloud droplets (usually only 2.7% of total amount)
- there is a close correlation between surface dewpoint and total atmospheric moisture
- mean annual precipitable water varies with latitude, being maximal at equator and decreasing steadily away from it although there are some regional variations
Clouds:
cloud types:
- 3 main types of clouds:
- convective:
- up to 10km thick with updraft velocities of 3-20m/sec and horizontal size up to 10km
- cumulus clouds have 1g/cu.m water (1.5 for cumulonimbus) with bases often at 1km elevation
- 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
cloud formation:
- 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
- uplifted:
- orographic:
- moist wind is uplifted due to presence of hills or mountains in its path
- frontal:
- warm moist air masses are uplifted in front of an incoming cold front or local front such as that produced by a sea breeze
- convergence:
- associated with low pressure systems and tropical cyclones
- land breezes at night converging around most sides of a large lake or bay
- confluence of air from northern & southern hemispheres near the equator
- convective:
- convection upwards of moist air over a warm surface:
- hot land - eg. fair-weather cumulonimbus thunderstorms
- warm oceans - eg. off NSW coast where cold, moist air from south passes over warm East Australian Current
- tropical nocturnal thunderstorms:
- within 10deg. of equator, just before dawn, if sea temp. < 28degC, the upper air cools overnight, allowing the warm moist air over the ocean to rise
- or just cooled or mixed with colder air:
- radiation inversion:
- ground inversion:
- as the ground cools overnight, the air nearest the ground cools which may result in fog
- inhibited by clouds, urban heating, or by stirring caused by winds
- cloud tops:
- as the tops of layer clouds have high albedo, they absorb little sunshine to warm them, and so cool by losing long-wave radiation to outer space resulting in the smooth top of a layer cloud, an example of an inversion layer. Inversion layers represent extreme stability and cause:
- limitation of any ruffling of the surface layer by vertical air movement to an extent dependent on the depth of the inversion & its intensity (the difference between the top and bottom temperatures), although can be penetrated by a plume of hot gases from a bushfire or by a vigorous convection cloud.
- less aircraft lift above it as the air is warmer, thus planes experience a jolt when flying through it
- more aircraft lift flying below it, thus may cause pilots to have difficulty landing
- sound is reflected beneath an inversion since waves travel faster through the upper warmer air & so are bent back towards ground, thus to a ground observer, aircraft noise is much greater when they descend below the inversion layer
- air pollutants and clouds tend to be trapped below the inversion layer
- there is decoupling of the winds with those below it being still whilst those above it are now no longer slowed from ground friction and become faster
- advection inversions:
- cold air flowing from higher ground to pass under lower warmer air:
- eg. valleys
- warm air flowing over a cold ocean which cools the lower layers of the wind
- eg. Los Angeles onshore west winds; hot northerly winds passing over southern ocean causing a sea fog;
- cool sea-breeze passing under warm air, typically forms an inversion layer at several hundred metres elevation
- subsidence inversions:
- compression of any layer of descending air due to higher pressures at lower elevations & the spreading of air sideways, typically causing an inversion layer 300-600m deep at an elevation of 1-2km
- boundary of a cold front forms an inversion layer
- trade wind inversions:
- at low latitudes where trade winds blow over the oceans resulting in subsidence inversion is several hundred metres thick & about 500m high, increasing to 2km elevation downwind
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
- clouds 1km from coast may have 10 nuclei/L, whilst at 100km inland, only 1 nucleus/L
- 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:
- trade winds (eg. NE coast Australia in spring & summer) - less if El Nino year
- anticyclonic latitude in relation to region's latitude ⇒ Qld dry in winter
- 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:
- 1864-8; 1880-6; 1888; 1895-1903; 1911-6; 1918-20; 1940-1; 1944-5; 1946-7; 1957-8; 1965-6; 1967-8;
hail
- hail generally forms when there are these four factors present:
- deep moist convection
- adequate updraft to keep the hailstone aloft for an appropriate amount of time
- high convective available potential energy (CAPE), especially > 1000J/kg as these lead to high upward velocities within a thunderstorm
- lower precipitable water values have the potential to produce large hailstones when significant CAPE is present as there is less gravity water weight loading to oppose upward convection
- high wind shear such as in a supercell allows CAPE to be maximised by separating the updraft and downdraft
- sufficient supercooled water near the hailstone to enable growth as it travels through an updraft
- high altitude regions will be closer to cold layers of upper atmosphere and storms there are more likely to produce hail
- low freezing level (the height of atmosphere above which it is freezing)
- at low elevation, if the freezing level is closer to the surface than 650 millibars, strong thunderstorms have a good probability of producing hail that will reach the surface
- freezing level can be found by examining the morning or afternoon Skew-T Log-P plot or forecast sounding
- dry mid-level air entrained into the storm is evaporatively cooled and can lower the freezing level as well as create strong surface wind gusts
- nucleation: a piece of ice, snow or dust for it to grow upon
- hail falls when the thunderstorm's updraft can no longer support the weight of the ice
- nearly all severe thunderstorms probably produce hail aloft, though it may melt before reaching the ground
- multi-cell thunderstorms produce many hailstones, but they are not usually very large as the mature phase is short and not enough time for large hail to form
- supercell thunderstorms have sustained updrafts that support large hail formation by repeatedly lifting the hailstones into the very cold air at the top of the thunderstorm cloud
- hail 2 inches (5 cm) or larger in diameter is generally associated with supercell thunderstorms
climate/climate2.txt · Last modified: 2023/07/03 08:43 by gary1