How do I get the daytime temperature to read accurite?

For accurate measurements, the unit must not be placed near any heat sources or direct sunlight. If placed outside, it must be under permanent shade with plenty of circulation.

It is possible that the heat from the sun is still warming the sensor after it has passed the point of engaging the aspirating fan through the solar panel. You may want to consider this optional part; http://www.acurite.com/5-in-1-sensor-dual-solar-panel-wind-cups-add-on-5n1dssa.html