Weather packages: finding the right scale and composition of climate in ecology

1. Animals are affected by local weather variables such as temperature, rainfall and snow. However, large-scale climate indices such as the North Atlantic Oscillation (NAO) often outperform local weather variables when it comes to explain climate-related variation in life history traits or animal numbers. 2. In a recent paper, Hallet et al. (2004, Nature, 430, 71-75) document convincingly why this may happen. In this perspective, we identify from the literature three mechanisms why this is so: (1) the time window; (2) the spatial window; and (3) the weather composition component of climate. 3. Such an understanding may be used to derive even better 'weather packages' than the NAO.