Tomatoes drop their blossoms to concentrate on the plant surviving anytime they are significantly stressed. As noted one of the commonest causes of that stress is heat. Tomatoes grow best if daytime temperatures range between 70 F and 85 F . While tomato plants can tolerate more extreme temperatures for short periods, several days or especially nights with temps outside the ideal range can cause blossom drop. Select heat a heat-tolerant ("heat set") tomato variety for areas with long periods of hot or humid weather.
But cold can be a stress also. Night time temps below 55 F will stress the plants --protect them with a cover at night.
Tomatoes need some help to pollinate. Either insects, wind or hand shaking of the flowers is necessary to carry the pollen from the anthers to the stigma. During weather extremes, there are often no insect pollinators in the garden.
Over fertilizing can also be a stress. If your soil is healthy, with adequate organic matter you should only need to fertilize (eg add extra amendments) when planting and mid season.
If humidity is either too high or too low, it interferes with the release of pollen and with pollenÃ¢â‚¬â„¢s ability to stick to the stigma. So pollination will not occur. If humidity is too low, hose the foliage during the day. This will both cool the plant and raise the humidity. This is not recommended in areas with high humidity or when fungus diseases are present. .
Tomatoes have very deep roots. Shallow watering will stress and weaken the plants. Water them deeply once a week or so, depending on conditions.
Talk to your local reputable nursery -- there are a zillion different varieties, lots of which have been developed to cope with whatever the local conditions are--- heat, cold, humidity, etc. It helps to grow a variety that is adapted to your area.