Reptiles like chameleons are cold blooded animals (ectothermic), which means they do not maintain their own body temperature by producing and preserving their own metabolic heat. They use heat from their surroundings to increase their body temperature and will not maintain a stable body temperature. This is very different from the system of mammals and birds, which keep in the heat their metabolism produces with their fur or feathers and will cool themselves down by sweating or panting. They keep their body on a very stable temperature, a slight difference in body temperature can already mean big trouble for their body.
Reptiles can only influence their body heat by their behavior. It will go to a place with heat or solar radiation to heat up, and will go to cooler places to cool down. They can loose some heat by evaporating moisture from their opened mouth when they get too hot. In nature you will notice many reptiles basking in the sun. They will lay in direct sunlight to warm their bodies up. Yemen chameleons will do the same. In their natural habitat there is plenty of sunlight, as a pet you need to provide this.
Providing heat to bask in
You cannot replicate the Saudi Arabia sunrays, but you can get pretty close. When you keep a Yemen chameleon as a pet you have to make sure there is a place they can absorb heat or light. You will have to provide your pet chameleon with a proper environmental temperature and with a heat lamp. The environmental temperature cannot be too low, as the chameleon needs this to survive. The heat lamp can be at any temperature, as the chameleon can choose to bask in it or not.
You can buy special heat lamps for reptiles in many pet shops or online. You could also use an old fashioned light bulb. Make sure the light is not too bright and provides enough heat. Do not assume but just measure this with a thermometer. The chameleon should not be able to touch the heat lamp as it is very hot and it will burn the chameleon. Chameleons are not very bright so make absolutely sure it cannot touch the lamp, you can hang it out of reach or cover it with a mesh at some distance.
Environmental temperature
A good environmental temperature for a Yemen chameleon is 82°F to 90 °F (28 to 32 °C) by day and 64 °F to 77 °F (18 – 25 °C) by night. Under the heat lamp it should reach around 100 °F (38 degrees Celsius). There should be a heat gradient inside the terrarium: this means the temperature should differ a bit within the terrarium. In this way the chameleon can choose where it will stay: under the heat lamp, in the cool part or in the warmer part. Usually this occurs naturally if you are using a heat lamp.