A new record July temperature has been set in Wales, the Met Office has confirmed.
The weather station at RAF Mona on Anglesey, recorded 33.9C (93F). It beat the previous record set in Usk, Monmouthshire, in July 1976, by 0.3C.
The 30-year-old July record had already been equalled at RAF Valley, Anglesey.
The heatwave has brought delays on the railway and caused some roads to leak bitumen. A north Wales primary school sent its pupils home at lunchtime.
South east Wales is still favourite to set a new July record temperature, but it is uncertain whether the all-time Welsh temperature record - 35.2C at Hawarden Bridge on 8 August 1990 - will fall.
A Met Office weather station in Usk is one of 37 across Wales where readings - including the maximum temperature achieved the previous day - are taken at 9am each day.
The River Usk has been a popular place for people to cool off
However, the weather station at Valley on Anglesey - one of six which report their figures automatically every 10 minutes - was the official hottest spot in Wales for much of the day, reaching 33.6C, and matching the Usk July record.
But that figure was passed later by the weather station at nearby Mona - setting a new record temperature for Wales in July.
"It's pretty exciting," said BBC Wales' weather presenter Derek Brockway.
"It's also means it was the hottest day ever on Anglesey."
Among the temperature records to fall was that in the west Wales coastal town, Aberporth.
The forecaster added: "My old recorded record of 31.8C has been replaced by at least 32.7C at 1400 BST.
"Valley's 33.6C at 1410 BST equals the previous July record in Usk, and probably breaks the station record."
The heat caused problems on the railways with some train companies reporting delays of up to 45 minutes, and speed restrictions on around half of the Wales' networks.
Worst affected was the First Great Western inter city service from London to south Wales.
The conditions cause rails to expand beyond the limit to which they are pre-stretched, with the risk of buckling.
Network Rail also said some overhead power lines expanded so much they started to droop, with trains having to slow down so that they did not become entangled with the cables.
Gwynedd Council said it had nine gritters spreading dust on the county's roads. A decision would be made later on whether the gritters would be back out on Thursday.
Winter gritters were also in action in the Vale of Glamorgan as some roads started to bleed bitumen and appeared to be melting in the hot temperatures.
Bodedern Primary school, near Holyhead, closed at lunch time and the children were sent home because of the heat.
Wales' chief medical officer has issued guidance on keeping children safe in the sun and avoiding heat exhaustion.
Dr Tony Jewell's 10-point guide includes keeping children inside during the hottest part of the day, dressing them in loose, cotton clothing, putting high-factor sunscreen on them and giving them plenty of non-fizzy, non-sugary drinks.
If children develop signs of heat exhaustion - including headaches, dizziness, nausea, cramps and high temperatures - parents should get medical advice immediately.
The RSPCA has reminded dog owners of the dangers of leaving their pets out in the sun.