Air and Water Currents
Wind direction and speed can have a marked effect on fuel efficiency. This is particularly true of aircraft. The detrimental result of travelling against the wind can be mitigated by the vehicle having a streamlined shape. Of course, travelling in the same aggregate direction as the wind can have a positive effect on fuel efficiency.
The same effects can be experienced when water currents act on shipping.
Road vehicles will use more fuel when travelling on wet roads. This situation is considerably worsened when travelling through sleet or snow. This effect can be mitigated through the use of appropriate tyres.
All vehicles, including ships and even aeroplanes and helicopters will also use more fuel when travelling through rain, sleet and snow and will need to operate windshield wipers. Even the presence of mist and fog will make the air heavier and increase fuel consumption.
Starting any vehicle that is cold requires extra energy, especially when it uses a viscous fuel. The use of heaters, dehumidifiers and air conditioners for the driver will also reduce fuel efficiency. Oil tends to be more viscous in cold weather. Pushing viscous oil around the engine will also require more power. In ambient temperatures, diesel needs to be heated in order for it to ignite. In cold temperatures it would require extra heating time.