Tyrone and Londonderry: High levels of air pollution recorded

Health alerts have been sent to residents in parts of counties Tyrone and Londonderry due to very high levels of air pollution.

Strabane has recorded level 10 - the highest number on the scale - while Newtownstewart is at level eight.

Parts of Derry are also affected and have been recorded at level six.

Text alerts have been sent by the Department of Health and the Department of Agriculture, Environment and Rural Affairs (Daera) to some residents.

In the UK, most air pollution information services use the Air Quality Index and banding system approved by the Committee on Medical Effects of Air Pollution.

It uses a index between one and 10 to communicate information about air pollution levels in a simple way, similar to the pollen index.

Air quality is based on five pollutants including nitrogen dioxide, sulphur dioxide and other particles.

The Stormont departments are expecting the high levels to remain as weather conditions remain fairly unchanged over the next 24 hours.

It is expected that across Northern Ireland temperatures will drop to sub-zero on Sunday night.

Text message warning sent from Daera to resident

The colder than normal weather is a result of Arctic air pushing south across the UK and Ireland.

The temperature fell to -7C in Katesbridge, County Down, on Saturday night and it is expected to be as low, if not lower in some places, on Sunday.

The sub-zero overnight temperatures are to last until at least next weekend and daytime temperatures will remain below average in most parts.

A weather warning for fog and ice is in place across Northern Ireland from 16:00 GMT on Sunday until 11:00 on Monday morning.

The Met Office is warning that freezing fog could reduce visibility down to as little as 50m in places.

It says the fog could linger all day on Monday in some spots, and there is a danger of ice in untreated areas.

A partly frozen lake at Waterworks, Belfast

Meanwhile, in the Republic of Ireland, an emergency group met on Sunday to discuss what measures may be needed to assess the impact of the cold spell.

The National Emergency Coordination Group makes decisions around plans for the emergency services, transport and local authorities.

It comes after a Status Orange warning - the second highest level - was issued across the country for Sunday night.

Met Éireann has warned of severe frost, icy patches and freezing fog which could lead to disruption.

Some snowy Donegal hills

The Irish weather service has also issued a week-long yellow alert for the Republic of Ireland for low temperatures and ice.

The warning is set to last until noon on Friday.

On Saturday, freezing weather led to the cancellation of some sporting fixtures in Northern Ireland.

It also led to dozens of flight cancellations and delays at Dublin Airport and disruption at many UK airports.

Saturday's Irish Premiership contest between Glentoran and Glenavon was postponed because of a frozen pitch.

The Intermediate Football Championship Final on Saturday was also postponed at Páirc Esler due to a frozen pitch.

Frosty Hillsborough Forest in County Down

On Friday, it was announced that the drop in temperature has triggered the first of this winter's cold weather payments in Northern Ireland.

They are paid to people living in areas where the average temperature is recorded or forecast to be 0C or below for seven consecutive days.

Only those who qualify and live in certain postcode areas will receive the £25 payment.