Why does my iPad Air dim in broad daylight?

As described, in very bright ambient lighting, your iPad screen will be operating at its maximum brightness. Relatively high ambient temperature along with additional heat generated by your iPad's screen backlighting creating more internal heat, your iPad is internally overheating.

So as to protect the iPad from thermal damage, in particular to the iPad's battery, your iPad will automatically reduce the screen brightness and CPU/GPU performance - to help reduce the internal temperature.

If your iPad is within a fitted case, removing the case can help to improve thermal dissipation. Keeping the iPad in the shade will also help, both with reducing internal heating and in reducing the need for high screen brightness.

So, this is expected behavior in hot and also bright climates. iPads lack active cooling such as fans and control screen brightness as a way of reducing the heat produced by screen brightness. This also extends daily battery life.

Not sure where you are in the Southwest, but it was almost 100° F yesterday in Death Valley. Ambient temperature has a big impact on cooling an iPad.

The ultimate solution is reduce the temperature of the iPad. Apple is not going to give users the option to permanently damage internal components.

The screen doesn't have enough light to stay lit when the sun is present. This is because the sunlight is stronger and this is a hardware issue due to the limitations of your iPad's screen.