Why the smoke from bushfires makes the sunrise and sunset so colourful

Monday, 11 November 2019

Smoke and dust from Australian wildfires has been turning New Zealand skies red and orange. But how exactly do smoke particles change the colour of the sky?

HOW DOES SMOKE CHANGE THE COLOUR OF THE SKY?

Niwa meteorologist Ben Noll said when a beam of sunlight hits a smoke particle in the atmosphere, something called 'scattering' occurs.

'This sends some of the light's wavelengths off in different directions and happens millions of times before that beam reaches your eye. Dust particles can help to create more scattering in the mid and upper atmosphere, which can increase the vibrancy of sunsets and sunrises.'

* Smoke and dust from Australian wildfires turns New Zealand skies red

* Smoke from Australia's wildfires sweeps across the Tasman, causing hazy NZ skies**

The lightning, thunder, rain and wind has now moved past #Whangarei leaving us with a spectacular sunset @MetService @NiwaWeather @WeatherWatchNZ pic.twitter.com/Kmi9rO9mrk

— John Macilree (@macilree) November 10, 2019

Sunrise at 5.30am in Oriental Bay, Wellington.

'However, if the dust sits low in the atmosphere, it can actually have the reverse effect, leading to muted sunsets that are less colourful,' Noll said.

MetService forecaster Andrew James said sunsets and sunrises in particular appeared brighter due to the smoke particles reflecting in the light at that time of day.

The effect of colour filtering is particularly enhanced because of the sun's lower angle in the sky.

University of Auckland physics lecturer Dr Tra Dinh said when smoke and dust are present in the upper atmosphere, the colours look deeper and darker.

'Depending on the size of the particles, light at selective wavelengths is selectively scattered, leaving the sky with the remaining colours that are not scattered and therefore changing the sky colours.'

HOW LONG IT WILL LAST?

Noll says the colourful skies will hang around until the Australian bushfires stop burning.

'Plumes of smoke and dust will be continuously dispersed into the atmosphere. This will happen until the fires are put out,' he said.

'Since the prevailing wind direction is westerly, it is blown eastbound toward New Zealand and some of it is even making it into New Caledonia and Vanuatu.'

The next plume is due to hit the North Island late Wednesday, Noll said.

'The trajectory of any smoke plume depends on the weather features and air flows in the Tasman Sea at the time. It takes approximately 36-48 hours from the time a plume of dust or smoke leaves Australia to reach New Zealand.'

'RARE EVENTS'

The Ministry of the Environment released the 'New Zealand's Environmental Reporting Series: Our air 2018' report last year, describing bushfires and dust storms from Australia as 'rare events'.

'New Zealand is an island nation, far from sources of pollution in other countries. This means that, except for rare events such as bushfires and dust storms from Australia, we generate most of our air pollution,' it says.

'Our location in the mid-latitudes means we have a windy climate that dilutes and carries pollutants away quickly, but it also increases the proportion of natural sea salt in our air.'

Noll said strong winds are forecast over the next week, so the dust and smoke is not expected to linger over the country for long.

'Each plume generally takes about 12 hours to pass across a given island,' he said.

WHAT ABOUT OUR HEALTH?

'Those that are sensitive to fine particulates will want to take extra care by closing windows and not running their air conditioner,' Noll said.

The Ministry of Health's 'Response to Major Fires: Guidelines for Public Health Units' report said people should avoid exposure to smoke.

'Smoke contains particles, gases and water vapour. Soot is basically carbon dust, although it may contain some irritant chemicals. Smoke may irritate the eyes, nose, throat and airways. Symptoms can include runny or sore eyes, dry or sore throat, sore nose, cough, tightness of the chest or difficulty breathing,' the report said.

There are currently 81 fires burning in NSW and the smoke has reached New Zealand. Feels like I’ve ended up in a Mad Max scene with stormy skies to the east! #nswfires pic.twitter.com/eaWYs0eZ3l

— Dale Wowk (@dalewowkphoto) November 9, 2019

'The small particles in smoke are more harmful than the larger particles because they can be inhaled deep into the lungs.

'In healthy people, most symptoms disappear soon after exposure to smoke ends and do not cause long-term health problems.

'Smokers, the elderly, children and those with heart disease, asthma or other lung disease are at greatest risk of harm from smoke inhalation.

'Avoid exposure if possible. Outdoors exercise such as jogging causes you to breathe more deeply and inhale more small particles.'