Consider the reaction of formation of ammonia from nitrogen and hydrogen by Haber's process:

${\mathrm{N}}_2\left(\mathrm{g}\right)+3{\mathrm{H}}_2\left(\mathrm{g}\right)\stackrel{ }{\rightleftharpoons }2{\mathrm{NH}}_3\left(\mathrm{g}\right); ∆\mathrm{H}=-92.4 \mathrm{kJ}$

The forward reaction is exothermic while, the reverse reaction is endothermic. If the temperature of the system in equilibrium in increased, according to Le Chatelier's principle, the system tries to decrease the temperature by absorbing heat. So, an increase in temperature favours the endothermic reaction. Therefore, at higher temperature the yield of ammonia is low.