By Lukie Pieterse, Potato News Today

Dew point decision-making and warm-spell ventilation tactics to prevent condensation, hotspots, and late-season rot risk.

Potato storages don’t usually get into trouble during steady cold weather. They get into trouble when winter stops behaving like winter – a warm spell rolls in, outside air turns damp, and a cold pile meets air that wants to drop water. That’s when “routine ventilation” can quietly trigger condensation events that set the stage for rot flare-ups, odour issues, CO₂ creep in weak zones, and a stressful shipping season.

This article forms part of PNT’s Storage Season Essentials series, built for the people who carry the responsibility of the pile – one practical baseline at a time. It also builds directly on the two earlier installments:

• The storage baseline that saves crops: 10 measurements every potato facility should track mid-season
• Mid-season storage health check: The 60-minute weekly audit that prevents spring surprises

What follows is a late-winter dew point decision-making playbook – designed to help storage managers ventilate through warm spells without “sweating” the pile or the building.

Why warm spells are uniquely risky in late winter

By late January into March, the pile mass is often cold and stable. That’s good – until outside air becomes warmer and holds more moisture. When that warmer, moist air contacts a colder surface, water can condense. In storage, those “colder surfaces” include:

• the top layer of tubers near walls and corners
• cold ducting and plenums
• walls, roof members, insulation breaks, doors, louvers
• stagnant or weak-airflow zones where temperature and moisture stratify

The result can be localized wetness – sometimes brief, sometimes repeated – that creates the exact conditions breakdown organisms like.

Warm spells also create human risk: they push crews toward “doing something” – and the most common “something” is long fan runs. When the air is moisture-laden, that habit can be the mistake.

Dew point – the simplest explanation that actually helps

Relative humidity confuses people because it changes with temperature. Dew point is easier: it’s the temperature at which air becomes saturated and starts depositing water on cooler surfaces.

A practical way to think about it in storage:

• If the air you’re bringing in has a dew point higher than the temperature of tubers or building surfaces it contacts, condensation risk rises.
• If the air dew point is lower than those surfaces, the risk drops.

You do not need a PhD to use dew point thinking. You need a habit: check the moisture risk before you ventilate, especially during warm spells.

The late-winter trap: “free cooling” that isn’t free

Many storages are designed to use outside air effectively. The trap is assuming outside air is always “safe” if it’s cooler than you want the pile to be.

During warm spells, outside air can be:

• warm enough to raise pile surface temperatures
• moist enough to condense when it hits cold surfaces
• unstable enough to swing within hours

In those conditions, the goal often shifts from “cooling” to stability. Stability is not doing nothing – it’s doing the right small things and refusing to do the wrong big things.

Condensation doesn’t always look like rain

A lot of mid-season condensation events are missed because people expect obvious water. In reality, condensation can show up as:

• a subtle damp smell near doors or louvers
• fogging at openings during fan changes
• new wall frost patterns or wet corners
• roof drip starting where it never dripped before
• top-of-pile dampness near walls while the centre looks fine

If you see any of these during a warm spell, treat them as decision signals, not annoyances.

The warm-spell decision workflow

Use this every time outside conditions change quickly.

Step 1: Identify the risk window
Warm spell conditions that deserve extra caution:

• outside air warming rapidly over hours
• fog, rain, thawing snow, or generally “damp” outside air
• a cold pile mass that has been stable for weeks

Step 2: Compare air moisture risk to pile/surface reality

• What is the approximate pile temperature (surface and internal)?
• What is the supply air temperature you would actually deliver (not just outside air)?
• Do you have dew point readings? If yes, use them. If not, use condensation logic.

Step 3: Decide your operating intent
During warm spells, your intent should be explicit:

• stability (avoid condensation and keep the system calm)
• CO₂ management (exchange air without importing moisture unnecessarily)
• targeted response (address a known weak zone, not the entire building)

Step 4: Choose a ventilation strategy that matches the risk

• If moisture risk is high: use shorter, controlled cycles or pause ventilation until conditions improve, depending on your facility and crop needs.
• If moisture risk is moderate: ventilate with caution and confirm outcomes quickly via inspection.
• If moisture risk is low: operate closer to normal routines.

Step 5: Verify with a quick inspection
Do not “set and forget” during a warm spell. Verify outcomes:

• do you see fogging?
• did frost patterns change?
• did any damp odour emerge?
• did pile surface conditions change near walls?

Step 6: Document the decision
A warm spell is a classic point where later questions arise. Log:

• outside conditions
• what you did and why
• what you observed afterwards

Ventilation strategies that work better in warm spells

The best strategy depends on facility design, crop type, and whether you’re in piles or boxes. But the principles are consistent.

Short, controlled cycles over long runs
Long fan runs can import moisture steadily, creating repeated micro-condensation events. Short cycles can sometimes accomplish air exchange without “soaking” cold surfaces.

Targeted ventilation over whole-building ventilation
If one zone needs attention (CO₂, temperature drift), address the zone. Whole-building ventilation during a warm spell can spread risk.

Avoid sudden transitions
Rapid temperature changes can stress tubers and create localized condensation. A calm slope is often safer than a steep drop.

Use mixing/tempering where available
If your system can blend or temper air, use it thoughtfully. The goal is to deliver air that achieves your intent without crossing condensation thresholds.

A practical checklist for spotting condensation hotspots

During warm spells, walk these points – quickly and consistently:

• doors and louvers (fogging, damp seals, unusual air movement)
• corners and wall bases (wetness, ice melt, damp patches)
• roof members and insulation breaks (new drip points, unusual condensation)
• top-of-pile near walls (damp tubers, surface sheen, localized odour)
• known weak zones from past seasons (repeat offenders often repeat)

A warm spell is not the time to assume “it’s probably fine.” It’s the time to verify.

Common warm-spell scenarios and what to do

Warm spell arrives – you feel pressure to ventilate “because we always do”
What to do instead:

• pause and re-frame the goal as stability
• do a short controlled cycle only if conditions support it
• immediately inspect for fogging or dampness
• document the decision so your team stays consistent

Fogging at openings during fan changes
What it suggests:

• air moisture is high relative to cold surfaces
• condensation risk is active right now
  What to do:
• shorten cycles or pause until conditions improve
• check corners, roof members, and top-of-pile zones
• avoid long runs that amplify the event

New wall frost patterns or a drip where none existed
What it suggests:

• moisture deposition is occurring on building surfaces
• air pathways or insulation breaks may be creating cold spots
  What to do:
• treat it as a red flag, not a curiosity
• reduce moisture-risk ventilation
• increase inspection frequency and note the location for follow-up

A localized damp odour appears after a warm spell
What it suggests:

• moisture + biology may be interacting in a weak zone
  What to do:
• cross-check temperature patterns in that zone
• if you measure CO₂, compare zones
• avoid destabilizing the entire pile with aggressive ventilation
• increase monitoring frequency and document carefully

What not to do during a warm spell

These are common mistakes that look like “action” but create risk:

• running fans for long periods during damp outside conditions without verifying outcomes
• switching rapidly between ventilation modes without a clear intent
• chasing one sensor reading without checking the physical storage environment
• ignoring localized signals (odour, fogging, wall frost changes) because “the average looks fine”
• making big changes and failing to document why

Warm spells reward calm discipline – not heroics.

Low-tech and high-tech approaches – both can work

If you’re low-tech:

• lean on consistent pile temperature checks and disciplined walk-throughs
• pay close attention to fogging, damp smells, frost pattern shifts, roof drip
• ventilate conservatively during warm spells unless you’re confident conditions support it

If you’re high-tech:

• use dew point and trend tools, but validate with physical inspection
• confirm sensor placement – moisture risk can be very localized
• avoid over-automation that runs fans purely on temperature without moisture context

In both cases, the winning move is the same: turn warm spells into a special operating mode, not “business as usual.”

One-page Warm-Spell Ventilation and Condensation Checklist

Use this as a practical tool your team can run every time a warm spell hits.

Warm-spell trigger

• outside air warming rapidly and/or damp conditions present (fog/rain/thaw)
• pile mass cold and stable
• risk of condensation elevated

Before you ventilate

• confirm pile temperature profile (surface + internal where possible)
• confirm supply air temperature (not only outside air)
• check dew point if available – if not, use condensation logic
• define your intent: stability, CO₂ control, or targeted response

Ventilation choice

• prefer short controlled cycles over long runs during moisture-risk windows
• target zones instead of whole-building ventilation if possible
• avoid sudden temperature transitions

Verify immediately after

• check for fogging at openings
• inspect walls/corners for dampness or changing frost patterns
• inspect roof members for new drip points
• check top-of-pile near walls for dampness or odour

Document

• outside conditions
• what you did and why
• what you observed
• what you will do next (re-check time)

Section	Checklist items (tick as you go)	Notes / readings
Warm-spell trigger	☐ Outside air warming rapidly (within hours) ☐ Damp outside conditions (fog / rain / thaw / “heavy air”) ☐ Pile mass cold and stable (higher condensation sensitivity) ☐ Weather forecast shows continued swing risk (next 24 – 72 hours)
Before you ventilate	☐ Confirm pile temperature profile (surface + internal where possible) ☐ Confirm supply air temperature (not only outside air) ☐ Check dew point if available (or apply condensation-risk logic if not) ☐ Walk quick “hotspot / odour” scan before changing ventilation ☐ Define operating intent: Stability / CO₂ control / Targeted response
Ventilation choice	☐ Prefer short, controlled cycles over long runs during moisture-risk windows ☐ Target the zone (if possible) – avoid whole-building ventilation by habit ☐ Avoid sudden transitions (big temperature jumps in delivered air) ☐ If blending/tempering is available, use it to reduce condensation risk
Verify immediately after	☐ Check for fogging at openings during/after the cycle ☐ Inspect doors / louvers / seals for dampness ☐ Inspect corners and wall bases for wet patches or changing frost patterns ☐ Inspect roof members / insulation breaks for new drip points ☐ Check top-of-pile near walls for dampness, sheen, or localized odour
Document (5 minutes)	☐ Outside conditions recorded (temp + “dry/damp” notes) ☐ What we did – and why (intent stated clearly) ☐ What we observed (fogging / frost shift / odour / damp zones) ☐ What we will do next (re-check time and trigger conditions)
Do NOT do during warm spells	☐ Do not run long fan cycles in damp air without verifying outcomes ☐ Do not chase one sensor reading without a physical walk-through ☐ Do not make big mode changes without intent and documentation ☐ Do not ignore localized odours, fogging, or new frost/drip patterns

Takeaway – warm spells don’t have to become rot season

Late-winter warm spells are not unusual anymore. In many regions, they are becoming a defining feature of storage season – and they can punish routines that worked fine during steady cold weather. The good news is that warm spells are manageable when you treat them as a predictable risk event, not a surprise.

Dew point decision-making doesn’t need to be complicated. It needs to be consistent. When your team pauses, checks moisture risk, chooses controlled ventilation, and verifies outcomes with a walk-through, you dramatically reduce the chance of “sweating” the pile or the building. That reduces the conditions that accelerate breakdown, keeps CO₂ and temperature behaviour more predictable, and protects the calm that storage managers need to make good decisions through to shipping.

Author: Lukie Pieterse, Potato News Today
Image: Credit Potato News Today