It follows on from a similar warning issued yesterday evening by gardai as heavy fog hit the commute home for many people.
A Met Office spokesman said: "If there are clear skies, the heat radiates back into space leading to cooling at the earth's surface".
Weather forecaster Mark Wilson says radiation fog is "not smog" and is a natural event.
"Radiation fogs usually dissipate soon after sunrise as the ground warms".
"This reduces the ability of the air to hold moisture, allowing condensation and fog to occur". Some patches of fog will form and also some frost.
Where is radiation fog in the United Kingdom?
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The yellow weather warning is in place from 5pm today through to Friday 10am with much of north east Wales and the north west of England affected.
A Met Office severe weather warning covers a portion of western Britain through tonight until 11am tomorrow.
Conditions will become cooler again on what is forecast to be a cold and misty night on Thursday, with frost, fog and temperatures as low as -2 degrees.
He said: "Fog is expected to become increasingly widespread through Wednesday evening and overnight into Thursday". It will be mainly dry at first with a few bright spells in eastern areas, but rather cloudy overall, with patches of drizzle and mist scattered about.
Temperatures are expected to remain low climbing no higher than 4C in the northwest but slightly warmer temperatures of 7C in London and the southeast.