Hot, dry weather is not good news for sowing seeds outdoors!
The main reason for poor seed germination or failure in hot weather, is lack of moisture at critical times in the seed's development. Together with this, there are some varieties of seeds that will 'lock-up' the germination process if the temperatures are too hot.
Whereas an established, or even young plant, can survive a period of drought, a seed in the process of germination, needs sufficient moisture to survive. It cannot obtain its own moisture from the surrounding environment, for it will have no root system with which to collect soil moisture, nor leaves with which to extract atmospheric moisture.
It is important to ensure that your germinating seed and also the resultant seedling receive adequate moisture for proper development.
As vegetable seeds sown outdoors are normally in the top few millimeters of soil - which dries out the quickest - they can often fail to germinate properly because of lack of moisture.
During sowing time, a little care can help to prevent the problems of poor germination and drying out of the emerging seedling. After making your seed drill - row - give the drill a good soaking with a watering can aimed at the drill area, Allow it to soak well in - repeat, then sow your seeds. Then cover the sown seed with fine dry soil, which will actually act as a mulch and conserve the water in the moist soil below. This normally allows for quick germination of the seeds.
After a day or so, you can start watering as normal, but do NOT allow the seedlings to dry out at ant time.