Outline the conditions needed for a tropical storm to form

Hurricanes need a lot of heat to form, which is why they usually occur over tropical seas (at least 26°C).
The sun is close to the equator, providing energy to heat the ocean.
The warm ocean heats the air above it causing it to rise rapidly.
Water evaporates quickly from the hot surface of the ocean, so the rising air contains great amounts of water vapour.
The rising air starts to spin (anti-clockwise in the northern hemisphere)
The centre of the storm - the eye - is calm.
As the air rises it cools, condenses and forms towering cumulonimbus clouds.
The rapidly rising air creates an area of intense low pressure. The low pressure sucks in air, causing very strong winds.
Once the storm moves over land it starts to lose energy and fades.

Well, a and b together would be negative. b alone would also be negative.