A hot surface does not heat the whole fluid at once — it only warms the thin film touching it. Follow that boundary layer as it thickens upward, watch the warm fluid rise and cool fluid flow in, and read the velocity and temperature profiles that describe the layer.
Convection is heat carried by a moving fluid. Unlike conduction, where energy is passed between atoms in a solid, convection simply takes warm fluid away from the surface and replaces it with cooler fluid. That makes convection one of the fastest ways to move heat — it is how a radiator warms a room and how a cooling fan keeps electronics alive.
The whole behaviour of convection is decided in a very thin film of fluid hugging the surface, called the thermal boundary layer. Heat must cross this film by conduction before the bulk fluid can carry it away, so the layer's thickness sets how easily heat escapes. Seeing it makes the whole process — and why moving air cools so much faster — obvious.
A heat source at the base keeps the plate hotter than the surrounding fluid. That temperature difference is the driving force: heat always wants to flow from the hot plate into the cooler fluid beside it.
Right at the surface the fluid cannot move — friction holds it still — so the first heat can only creep across by conduction. That creates a thin, warm film, the start of the thermal boundary layer.
As the film warms it expands and becomes less dense than the fluid around it. The denser, cooler fluid pushes the lighter warm fluid upward, so it climbs along the plate carrying its energy with it. This is natural convection.
As the warm fluid leaves the top, fresh cooler fluid is drawn in from the sides to fill the gap. A continuous loop is set up: fluid heats at the wall, rises, dumps its heat, cools and returns.
The velocity profile shows the fluid sticking to the wall (zero speed), speeding up away from it and levelling off in the free stream. The temperature profile starts at the wall temperature and decays to the free-stream temperature. Where these profiles reach the free stream marks the thickness of the boundary layer.
It is the thin region of fluid next to a surface where the temperature differs from the surrounding fluid. Heat has to cross it before the bulk fluid can carry energy away, so it controls how fast convection moves heat.
Near the bottom edge the fluid has only just met the heated surface, so only a thin film is warmed. Further up, heat has had longer to spread into the flow, so the warmed layer grows steadily thicker with height.
It plots fluid speed against distance from the wall. The fluid sticks to the surface, so the speed is zero right at the wall; it rises with distance and levels off at the free-stream speed of the surrounding fluid.
Heating makes a fluid expand and become less dense. The lighter fluid next to the plate is pushed upward by the denser, cooler fluid around it — this buoyancy is what drives natural convection.
Natural convection is driven only by buoyancy — warm fluid rises on its own. Forced convection uses a fan, pump or the motion of a vehicle to push fluid across the surface, which thins the boundary layer and moves far more heat.