Humidity

Longmont at 5,000 feet: 14% outdoor humidity on spring afternoons. March has more VPD stress hours than August. The misters ran 203 pulses on the hottest day.

Water cost, gallons consumed, and climate compliance. When compliance drops, it’s almost always VPD — not temperature — that’s out of band. The misters and fog are the front line, and they’re fighting geography.

The VPD Map

This is THE chart. Each cell is an hour of VPD data — green is comfortable, yellow is stress, red is emergency territory (>2.0 kPa). The counterintuitive pattern: March shows 12.5 hours above 2.0 kPa vs August’s 8.3 hours. August has monsoon moisture (55% outdoor RH). March has bone-dry continental air (14–18% RH) combined with strong solar heating — the worst possible combination for VPD.

VPD Band Tracking

Actual VPD vs the target band, with mister activation events overlaid. Watch the sawtooth pattern — each mister pulse drops VPD by 0.04–0.15 kPa depending on zone, then it climbs back as dry outdoor air infiltrates. The controller cycles through zones every 45–60 seconds, trying to keep VPD within band.

Zone-Level VPD

VPD varies dramatically by zone. South zone runs the highest VPD — it’s the exhaust side, closest to the fans pulling in dry outdoor air. East zone benefits from the hydroponic system’s evaporation. The spread between zones can hit 0.5+ kPa at peak, which means the same greenhouse has plants in comfortable and stressed conditions simultaneously.

Mister Activity

Timeline of every mister pulse. On hot dry days, this becomes a wall of activity — 200+ pulses. On cool cloudy days, the misters may not fire at all. The pattern mirrors solar radiation with a few hours’ offset as humidity lags temperature.

Zone Runtime Distribution

Which zones are doing the work. South zone typically leads — it has the highest VPD so the firmware routes pulses there first. Water pressure limits operation to ONE zone at a time. The firmware enforces this hard constraint and rotates based on which zone has the highest current VPD.

Mister Effectiveness

VPD drop per mister pulse by zone. South dominates at 0.15 kPa average — wall-mounted, two rows of nozzles (mid + upper height), 30 active nozzles. West is close at 0.13 kPa. Center severely underperforms at 0.04 kPa despite 25 nozzles — likely a geometry or nozzle adjustment issue that needs investigation.

The Dry Air Problem

Indoor vs outdoor humidity tells the story. When outdoor RH drops to 14%, the greenhouse fights to maintain 40%+ indoors. Every time the exhaust fans run for cooling, they flush out humid air and pull in bone-dry replacement air. This is the fundamental tradeoff: open vents improve temperature but destroy humidity.

Atmospheric Demand

Left: VPD & dew point spread — the gap between air temperature and dew point drives evapotranspiration demand. Right: ET₀ (reference evapotranspiration) — how aggressively the atmosphere is pulling moisture from every surface. High ET₀ days are when the misters earn their keep.

Equipment

AquaFog XE 2000 HumidiFan

Centrifugal disk atomizer producing sub-10µm aerosol droplets. 1,644W, oversized for 234 sq ft (rated for 2,000 sq ft). Fills the room in minutes. Positioned center-overhead, blowing N→S aligned with primary airflow. Best for: rapid humidity boost when vents are closed. Limitation: fine droplets exit through exhaust fans — ineffective with ventilation active.

Micro Drip 360° Mister System

19 heads across three zones, producing 50–100µm ballistic droplets. Heavy enough to work even with fans running — they fall and evaporate rather than being swept out. Water temp 86°F from Rinnai tankless heater for faster evaporation.

Zone	Active Heads	Nozzles	VPD Drop
South	6	30	0.15 kPa avg (best)
West	3	15	0.13 kPa avg
Center	5	25	0.04 kPa avg (investigate)

→ See Cooling for how ventilation creates the humidity vs temperature tradeoff. → See Irrigation for water usage — on hot days, 90% goes to humidity, not plant roots. → See Soil for how atmospheric VPD correlates with root zone moisture drawdown.