There have been several reports of smaller than expected nut size for the 2021 harvest. Nonpareil sizes have been reported to be in the range of 28-30/ounce rather than the typical 24/25 nuts per ounce. Many have suggested that this is due to drought induced water shortages, but this has occurred even on fully irrigated orchards. Water stress has played a role, but earlier conditions of the season should also be considered. Nut weight is influenced by a few different factors. These include: Reduced number of cells; Reduced cell size; Reduced cell weight. Cell number is mostly impacted by temperatures that occur during cell division, which occurs early in the season, typically within 6-8 weeks following fertilization. Cell enlargement is influenced by plant stress. It begins in April and continues through May. Lastly, weight accumulation begins in May and continues until early August. It is influenced by plant stress, which reduces the production of carbohydrates. To maximize nut size, carbohydrates (sugars) must be available during these periods for cell building processes. Carbohydrate availability is influenced by the rates of photosynthesis and respiration. These two processes are heavily influenced by plant temperature, which are influenced by ambient air temperature and water availability. Temperature influences photosynthesis and respiration. Both processes increase rapidly as temperatures warm, and begin to drop as temperatures increase above a certain point. The peaks, however, are different, with 86F and 103F for photosynthesis and respiration, respectively (figure 1). Since these peaks are different, there is a net gain of carbohydrates when temperatures are below 86F, but a net loss as temperatures increase above this point. Furthermore, as temperatures warm towards 86F, there is more energy being directed towards respiration, which reduces availability to be used for other processes, like nut development and plant growth. Internal plant temperature is