Weather for skydiving. Part deux.

14 Aug

Precipitation

Precipitation is any product of the condensation of atmospheric water vapour that falls to the ground under the effect of gravity. This is anything from misty drizzle to insurance company bankrupting hail via rain, snow and the mysterious graupel.

As RH hits 100%, moisture condenses and falls – normally collecting more moisture on the way day, thus forming into droplets. The rise in RH can be caused by an increase in absolute humidity or the temperature falling to the dew point.

Rain, or liquid precipitation, is measured by either collecting it in a tube and expressing a number in millimetres, or by collecting rainfall over an area an expressing it in g/m² or kg/m² for cold blooded metric rationalists or bald eagle tears per freedom² for anyone from the three countries on earth that still use the imperial system.

As rain falls, it gathers more moisture, forming larger drops. Raindrops can be from 0.1mm to 9mm across. Raindrops aren’t teardrop shaped, but look more like a squished ball, with their largest cross-section facing the airflow – think of it as being forced down by gravity, but then compressed by drag.

Snow forms when tiny, supercooled water droplets freeze. They grow and twist and build in the clouds based on varying temperature and humidity – that’s why snowflakes are beautiful and completely unique, just like all the rest of them. The ice which makes up snowflakes is actually clear, but for complicated reasons the random facets and hollows and imperfections reflect, confuse and brutally tease the light, making them appear white.

Ice pellets are basically snow which has fallen through a warmer layer and melted and then through another below zero layer and frozen, this time in little pellets of sadness. If the lower layer of sub-zero air is only just below zero, or only very shallow in depth, the moisture may only partly freeze and fall as freezing rain, or as small ice pellets.

Hail! Forms in storm clouds, rising and falling, turning into bigger, meaner and angrier balls of hail (Satan!). Hail can grow up to 15cm across and weigh as much as 500grams – this happens with ‘wet growth’, when the outer surface of the ball melts just enough to allow other bits of hail to adhere to it.

Solid precipitation is generally collected in a tube and expressed as a number in milli- or centimetres. Sometimes the snow may be melted and the level of precipitation is expressed as a water equivalent in millimetres, but this, due to the varying density of the precipitation, may be a very inaccurate guide to the depth of any snow accumulating on the ground.

Gust Fronts and wind squalls and clouds

A great variety of cloud features can appear along the leading edge of a gust front or a wind squall. As with advancing cold fronts, they can act as a wedge, driving the warmer air up to cause sometimes spectacular shelf clouds or cloud banks. The complexities of shelf clouds, be it more humid conditions forming thicker cloud or a strong gust front producing rising scud are many and nuanced, but none are harbingers of good skydiving conditions.

High- and Low-Pressure Systems

Pressure systems are a relative peak or lull in atmospheric pressure. Mean air pressure at sea level is 1013hPa.

Low pressure systems are essentially just the product of atmospheric lift. This can be as localised as a desert gleaming with the sun’s radiation causing air to rise, or can be continent in scale and drive seasonal monsoons. They can also be caused by divergent winds in the upper levels of the troposphere created by big weather systems or ancient gods fighting – either of these equally likely events creates a pressure disparity high in the atmosphere, which lifts air from the surface.

Low pressure systems tend to be associated with cloud, rain and general wintery discontent.

The lowest air pressure recorded on this here earth of ours was 870hPa during Typhoon Tip in 1979.

High pressure systems are most often the result of a cool air parcel descending from the high troposphere, a process called ‘subsidence’– like a downburst, but much gentler. It is subject to adiabatic heating– when the pressure of a gas increases, it also experiences a rise in temperature, which tends to dry out the atmosphere. This is best visualised by putting a bit of tissue paper in a cylinder and rapidly compressing the air in the cylinder with a piston. If done quickly enough, the tissue will catch fire just as a result of the adiabatic heating. For best results, cover yourself and your loved ones with petrol before trying this shining little star of home science. This is how diesel engines ignite their fuel/air mix without sparkplugs.

High pressure systems tend to be associated with clear skies and gentle winds. The clear skies will normally result in a higher day/night temperature differential.

Cold fronts

Commonly found in the south of Australia. They often:

· Create clouds. Dense cold air wedges the warmer, moist air up, which cools and condenses into cloud and rain.

· Bring changes in wind direction. In the southern areas of Australia, the wind often transitions from a North-westerly to a South-westerly.

· Cause rain. The rainfall is driven by the cold front wedging below the warmer air, so it moves slightly ahead of the cold front. It tends to come from blanket strata-form cloud and the rain steady and consistent. Behind the front, the rain will tend to be driven by cumulus clouds, so more scattered with intervening periods of sun.

· Cause gusty winds – fire conditions in summer and rain, snow and general all round gustiness in winter.

rainfall, cloud, changes in wind direction and sudden drops in temperature.

East Coast Low

Australian East Coast Lows, or sometimes East Coast Cyclones, are storms that develop on the East Coast, between Gladstone in Queensland and the Vic/NSW border. They vary in size and intensity, but they are typically characterised by widespread rainfall. They can vary in size from mesoscale, 10-100km to synoptic scale, 100 – 1,000km. They occur mostly in winter and can occur as often as ten times a year and tend to occur in clusters when the conditions are suitable.

They are intense low-pressure systems which are often very slow moving, so the effects can last much longer than the normal lows which move across the southern states.

It is caused by a confluence of systems: cold air coming up from the south can sometimes end up as a bubble over NSW, then the east coast current supplies warm water, as the they interact, warm air rises from the sea to the bubble, intensifying the low off the coast. If there is then a high-pressure system to the south, the low whips the wind across the NSW south coast, rising over the dividing range, causing lots of sustained rainfall.

They are responsible for significant insurance claims and have been the cause of 7% of all major Australian disasters since 1967. They have caused deaths in the Sydney to Hobart yacht race, 390mm of rain in 24 hours in Nowra and the grounding of bulk carriers around Newcastle, amongst other issues.

Thunderstorms

Thunderstorms require three ingredients:

· warm, moist air

· a lifting mechanism:

A cold front moving in, forcing the warm air to rise.
Orographic lift – i.e. a mountain range driving air upwards
A sea breeze convergence
Thermal lift

· atmospheric instability – air parcels can rise high.

Normal thunderstorms exist in three stages, dictated by the magnitude and direction of the moving air masses:

· Developing: air rises. This is the cumulus stage

· Mature: air continues to rise, spreading out into an anvil shape when it cools or is too dense to continue to move vertically. Air and precipitation are falling. This the cumulonimbus stage and, excitingly, the peak lightning stage.

· Dissipating: downdrafts throughout the cell as precipitation cools any air still rising – killing the original updraft, which deprives the cell of the warm moist air which feeds the rain and precipitation.

An ordinary cell is 5-10km in horizontal extent and lasts only 15-30 minutes – destroyed, quite poetically, by its own vehemence.

Severe Thunderstorms

Multicell storms are a line of small, short lived cells all at different phases of their life cycles. The cold outflows from all the storms can combine into a gust front which acts as a snowplough or wedge, driving the warm moist air up into the approaching cells, feeding it straight into the heart of storms, allowing them to sustain for much longer and offer greater risk of weather dangerous to all concerned over a wide area.

Squall lines are multicell storms arranged in a long, basically continuous, line of storms. They tend to develop along a linear lifting front, such as a cold front. They can be dangerous to everyone concerned as they present a wall of weather that cannot readily be avoided. Strong vertical wind shear will drive the line forwards as the warm, moist air is scooped into it by the gusty front.

Supercell thunderstorms are the worst kind of storm – or the best, depending on how you see life. They rely on a very strong updraft which rotates as it moves up into its apogee – a general rule is the higher the top of the cloud, the worse the weather it will produce. They are reliant on vertical wind shear to separate out the rising and falling columns of air – so that the heavy and violent precipitation it causes doesn’t cool the rising air which feeds it, allowing the storm to continue for as long as it can find warm, moist air to feed into itself – one has been observed at its mature phase for seven hours.

Michael Tibbitts