What a Hot, Humid July Day in 2026 Says About the Produce Season in Yuma, Arizona

Heat and humidity are one number: vapor pressure deficit or VPD

Growers watch temperature and humidity separately, but the crop responds to their combined effect, captured in a single term: vapor pressure deficit (VPD), the difference between the moisture the air can hold when saturated and the moisture actually present (Mohammed, 2026). VPD is the atmosphere's demand for water. Two days at the same temperature place very different loads on a crop if humidity differs, which is exactly what the monsoon delivers

What 20 days of AZMet data from Yuma Valley shows

Daily records from the Yuma Valley AZMET station (July 1–20, 2026) make the pattern concrete. Three days frame the whole period (Table 1). Maximum air temperature remained punishingly high all month, while dew point climbed from the mid-40s to the mid-60s as monsoon moisture arrived after about July 8 (Figure 1). Measured VPD peaked at 4.77 kPa on July 5, a bone-dry day with an RH minimum of 11%, and bottomed at 3.21 kPa on the humid, cloudy July 16 (Figure 2). Daily reference ET (ETo) ranged from 0.22 in on July 16 to 0.39 in on July 10, about a 75% swing with no matching swing in air temperature (Figure 3).

Table 1. Three representative days from July 1–20, 2026, Yuma Valley AZMET station.

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Crop water use follows solar radiation, VPD, and wind

Figure 3 shows the key insight, which is that crop water use follows solar radiation, VPD, and wind, not the thermometer. July 16 recorded the lowest demand of the period despite being classic "heat and humidity" weather, because monsoon cloud cover cut solar radiation to about a third of the clear-day value. The highest demand, July 10, was not the hottest day; it was a windy day (mean 7.4 mph, gusts to 27) under full sun.

How this applies to crops

The stress was not uniform across the month. Early July (roughly the 4th through the 9th) was the hard stretch, with VPD above 4.2 kPa under clear skies (Figure 2), the conditions that force midday stomatal closure and throttle photosynthesis when roots cannot resupply. Sudan grass, a C4 summer grass, tolerates this better than a C3 vegetable would, but even it trades growth for water conservation at those levels. After the monsoon moisture surged in, that daytime pull eased, but overnight lows of 84–88°F (July 10–20) raise night respiration, so some of the carbon fixed by day is burned off at night. Expect the strongest daytime growth limitation early in the month and a night-respiration drag late.

How this applies to irrigation

This is where the data pays off most directly. A fixed summer irrigation set would have over-applied on July 16, when true demand fell nearly by half under cloud, and under-applied on July 10, when wind and full sun pushed demand to the period high (Figure 3). Scheduling to daily ETo or a three-day running ETo to smooth storm-day noise rather than to a calendar rate is the single highest-value adjustment, and it carries straight into fall stand establishment. 

How this applies to integrated pest management 

The monsoon shift can change pest and disease risk. Hot, dry weather mainly stresses the crop, while humid nights can favor some diseases. After July 10, higher humidity and warm nights likely made scouting more important, especially in weak or uneven areas of the field. VPD does not predict pests by itself, but it helps show when the field environment is changing. In Yuma, tracking heat, humidity, and crop condition together can help growers time scouting and management decisions more effectively.