It had already reached 30c by early afternoon, and with a UV index of 8, Islanders are being urged to take extra care in the sun.
The Health Department said a number of people have needed treatment after suffering breathing difficulties or fainting in the hot weather. Several others have been treated for sun burn.
Five people in Guernsey needed hospital treatment for heat-related issues on Tuesday.
Jersey Met are forecasting another hot day tomorrow, with the temperature likely to rise to 31c.
But fresher Atlantic air is expected to push across the Channel Islands overnight on Wednesday.
It is likely to remain dry and fairly sunny, but it will be much cooler, with highs of 22c on Thursday and 20c on Friday and Saturday.
Although it has been hot – it is nothing compared to the heatwave gripping parts of America.
In Phoenix, Arizona, the mercury was due to rise to 49c today. And the heat is so extreme that 40 flights have been cancelled – because it is too hot for planes to fly.
A Health spokesman said: ‘There have been an increased number of patients who have had breathing difficulties or fainted as a result of the high temperatures, and also some cases of sunburn, with the sun’s rays being at their strongest around the summer solstice.’