  We use reanalysis data to investigate the daily co-variability of wind and
solar irradiance in Britain, and its implications for renewable energy supply
balancing. The joint distribution of daily-mean wind speeds and irradiances
shows that irradiance has a much stronger seasonal cycle than wind, due to the
rotational tilt of the Earth. Irradiance is weakly anticorrelated with wind
speed throughout the year ($-0.4 \lesssim \rho \lesssim -0.2$): there is a weak
tendency for windy days to be cloudier. This is particularly true in
Atlantic-facing regions (western Scotland, south-west England). The east coast
of Britain has the weakest anticorrelation, particularly in winter, primarily
associated with a relative increase in the frequency of clear-but-windy days.
We also consider the variability in total power output from onshore wind
turbines and solar photovoltaic panels. In all months, daily variability in
total power is always reduced by incorporating solar capacity. The scenario
with the least seasonal variability is approximately 70%-solar to 30%-wind.
This work emphasises the importance of considering the full distribution of
daily behaviour rather than relying on long-term average relationships or
correlations. In particular, the anticorrelation between wind and solar power
in Britain cannot solely be relied upon to produce a well-balanced energy
supply.
