Why is there no wave for atrial repolarisation in the electrocardiogram?

While atrial repolarisation exists, you actually can't see it in the ECG! I can draw you a diagram of a normal cardiac cycle: this is the P wave, that you'll know represents the DEpolarisation of the atria. The QRS complex here represents the huuuuge wave of ventricular depolarisation, and then the T wave is the ventricles REpolarising.

So when does atrial repolarisation happen in the cardiac cycle? It happens very shortly after its contraction! Almost immediately after a heart muscle depolarises and contracts, it repolarises again! So the atrial repolarisation - what's called a Ta wave - is actually ABSORBED by the QRS complex, so you don't end up seeing it on an ECG!