Negative effects on germ cell development have been correlated with elevated scrotal temperatures in domestic livestock species, including the boar. To fully comprehend the effect of environmental heat stress on male fertility, a total of 1,181 boar ejaculates, with an average of 49.2 ± 1.7 per week, were collected during June–November 2012 from a commercial boar stud. Boars were housed in facilities equipped with cool cell technology and facility temperature was measured every 30 min in 2012. Environmental temperature data were obtained from the National Oceanic and Atmospheric Administration for 2012 and 2014, and expressed as a weekly average of the maximum daily temperature. Sperm were assessed using Fourier Harmonic Amplitude (FHA) analysis to determine sperm nuclear shape and harmonic values were generated (HA0–5). Using the first collection week as the control, a GLM procedure with Dunnett’s mean separation test was performed to measure changes occurring in HA0-HA5. Changes were found in HA0 and HA2 for wk 8–21, or the weeks of July 25-October 24 (P < 0.05). The HA0 at week one was 2.80 ± 0.01 µm and peaked during wk 14 at 2.87 ± 0.01 µm. Effects on HA0 appeared 3 weeks after external temperatures rose above 90°F (facility temperature = 82°F); this corresponds to when affected round spermatids first appear in the ejaculate as mature sperm. Fluctuations in HA0 followed the oscillations in temperature until the average maximum external and internal temperature fell below 68°F. To confirm these results, samples from the same facility were collected in 2014 3 weeks after external temperatures rose above 90°F. A period of heat stress was effectively captured as changes in HA0 and HA4 were again found 3 weeks post-heat event (P < 0.05). Increases seen in HA0, which is a reflection of the overall size of the sperm head, indicate that boar sperm nuclei enlarge during seasonal heat stress. Fourier Harmonic Amplitudes, particularly HA0, 2, and 4, can be utilized as an effective tool in predicting specific periods where nuclear shape may be affected by heat stress. Furthermore, FHA analysis could be utilized in the future to correlate changes in nuclear shape as a result of heat stress to fertility.