Why Is A Candle-Flame Always Vertical

Explains Biplab Das

When a candle burns, its constituent hydrocarbon reacts chemically with atmospheric oxygen, giving off heat and light. This chemical reaction also produces water vapour and carbon dioxide. 

Since these end products are hot and lighter they change the density of air present around the candle at the beginning. In consequence, hot and less dense air goes up. 

This produces a vacuum around the flame. In other words, the pressure in the neighbourhood of the flame reduces considerably. As burning continues, oxygen rich cold and heavier air from all directions moves towards the flame to fill up the vacuum and this reduces the pressure difference. 

Downward trajectory of cold and heavier air and upward movement of hot and lighter air (water vapour and carbon dioxide) give rise to convection of air current, which makes flame vertical. 

If there is a sudden blow of air, this convection of air current is distributed and the flame loses its vertical position. This is also observed in a hurricane lamp in which small holes at the bottom and top facilitate convection of air currents.