The oil price is partly determined by actual supply and demand, and partly by expectation. Demand for energy is closely related to economic activity. It also spikes in the winter in the northern hemisphere, and during summers in countries which use air conditioning. Supply can be affected by weather (which prevents tankers loading) and by geopolitical upsets. If producers think the price is staying high, they invest, which after a lag boosts supply. Similarly, low prices lead to an investment drought. OPEC’s decisions shape expectations: if it curbs supply sharply, it can send prices spiking. Saudi Arabia produces nearly 10m barrels a day—a third of the OPEC total.