A fluid flowing over a flat plate has the following properties: dynamic viscocity $$ = 25 \times {10^{ - 6}}\,\,kg/ms,$$ specific heat $$=2.0 $$ $$kJ/kgK,$$ thermal conductivity $$0.05W/mK.$$ The hydrodynamic boundary layer thickness is measured to be $$0.5$$ $$mm.$$ The thickness of the thermal boundary layer would be