Dry Bulb Temperature and Relative Humidity: Why Both Numbers Drive HVAC Sizing in Kerala

Ask most building owners what their air conditioning needs to do and the answer comes back in degrees Celsius. Hold the room at 24. Hold the server hall at 22. Hold the cold storage at 4.

That answer is incomplete in Kerala. Two rooms held at exactly the same dry bulb temperature can feel completely different, cost very different amounts to operate, and require very differently specified equipment. The difference is moisture, and that means any honest HVAC calculation in our state has to deal with two numbers, not one: dry bulb temperature and relative humidity.

What dry bulb temperature actually measures

Dry bulb temperature (DBT) is what an ordinary thermometer reads when its sensing element is dry and shielded from radiant heat. It captures the kinetic energy of the air molecules and nothing else. It does not tell you how much water vapour the air is carrying.

That is the whole point. DBT is a clean measurement of one thing. The problem is that in a humid climate, that one thing is only part of the cooling problem.

What relative humidity is, and why "relative" matters

Relative humidity (RH) is the amount of water vapour in the air expressed as a percentage of the maximum the air can hold at that temperature. The phrase "at that temperature" is the trap that catches almost everyone.

Warm air holds more water vapour than cold air. So the same absolute amount of moisture can be 60% RH at noon and 95% RH at 4 AM without a single drop of water entering or leaving the air. The number that does not change with temperature is the humidity ratio (also called specific humidity or moisture content), measured in grams of water per kilogram of dry air. When HVAC engineers do real calculations, the humidity ratio is what they reach for, because it is conserved across temperature changes and tells you the actual mass of water the cooling coil must remove.

RH is what occupants feel and what mould reacts to. Humidity ratio is what the equipment has to deal with. Both matter, for different reasons.

Kerala's design conditions, in numbers

For comparison, the international AHRI standard rates split AC capacity at 27°C DBT, 19°C wet bulb indoors with 35°C outside. That is a dry, mild benchmark.

Kerala does not behave anything like that benchmark:

Kochi, summer design day: roughly 34°C DBT and 28°C wet bulb outdoors, which corresponds to about 65 - 70% RH.
Thiruvananthapuram coastal sites: 33°C DBT, 27.5°C wet bulb, RH 70%+ for long stretches of the year.
Kozhikode and the northern coast: 34°C DBT with wet bulb temperatures touching 28.5°C through April and May.
Monsoon, statewide: DBT drops to 27 - 30°C but outdoor RH sits at 85 - 95% for weeks. The air is cooler and far wetter.

That last point is what catches builders out. People assume the monsoon is "easier" for an AC because the outdoor air is cooler. The cooling coil disagrees. The latent load (the moisture you have to condense out of the air) is at its absolute worst in July and August.

How DBT and RH change the load calculation

A cooling load splits into two parts:

Sensible load: the heat you remove to bring the dry bulb temperature down. Driven by ΔT between outdoor and indoor DBT, building envelope, glass area, equipment heat, lighting, and the temperature component of occupants and fresh air.
Latent load: the heat you remove to condense water vapour out of the air. Driven by the difference in humidity ratio between outdoor air and the indoor setpoint, plus moisture generated indoors (people breathing and perspiring, kitchens, wet processes, plants, swimming pools).

The ratio between them is the Sensible Heat Ratio (SHR):

SHR = sensible load / (sensible load + latent load)

In Delhi or Bengaluru summer design, a typical office sees SHR around 0.80 to 0.85. In coastal Kerala, the same office often computes to 0.65 to 0.75, because the latent share is larger. A bus depot canteen, a hotel ballroom, or a hospital OPD with high fresh air rates can drop below 0.65.

That single number changes equipment selection. Standard comfort split units are designed around an SHR of 0.75 to 0.85 at rated conditions. Run one against a Kerala load with an SHR of 0.65 and the unit will hit setpoint on temperature long before it pulls the room down to comfortable humidity. The thermostat is happy. The occupants are not.

Capacity derating: the tonnage tag lies

Every AC sold in India has a nominal tonnage stamped on it, rated at the AHRI or ISO test condition mentioned above. Take that same machine and operate it in Kochi in April and its actual delivered capacity drops, because the evaporator coil is working against warmer, wetter return air and the condenser is rejecting heat into a hotter ambient.

For Kerala design conditions, the working derating factors we use are typically:

Sensible capacity: 88 - 94% of nameplate, depending on indoor RH and outdoor DBT.
Total capacity: 90 - 96% of nameplate.
Latent capacity: highly variable, often 110 - 130% of the nameplate latent share because the coil sees more moisture to condense.

The practical implication is that a "1.5 tonne" unit selected purely off a rule-of-thumb sq.ft. chart frequently delivers around 1.3 tonnes of useful cooling in a Kochi April afternoon, with most of that capacity going into wringing water out of the air rather than dropping the temperature.

What this means at the design table

These are the calculation discipline points HRS applies on every commercial project in the state:

Use Kerala design conditions, not Indian Standard generic ones. ISHRAE handbook values for the specific district are the floor, not the ceiling, and coastal sites need a separate look. Designing to Bengaluru numbers is the single most common reason a Kerala system underperforms.
Compute the latent load explicitly. That means counting people, fresh air CFM, infiltration through doors that open onto humid corridors, and any process moisture. Skipping this and just adding a "humidity safety factor" is how oversized, short-cycling systems end up installed.
Match SHR at selection, not just total tonnage. Two units of the same nominal capacity can have very different SHRs. The lower-SHR unit will hold humidity better in Kerala, even if its rated total capacity looks identical on paper.
Plan the fresh air handling separately. Outdoor air in Kerala can carry 18 - 22 grams of water per kilogram of dry air during monsoon. Forcing the main DX coil to do all that dehumidification is wasteful. A dedicated outdoor air system (DOAS), an Energy Recovery Ventilator, or a treated fresh air unit usually pays back inside two to three years on operating cost alone.
Insulate for dew point, not for habit. Ducts, chilled water lines, and refrigerant pipes will sweat if their surface temperature drops below the room dew point. With Kerala indoor dew points often above 20°C, insulation thicknesses that work in Pune or Hyderabad will drip in Ernakulam. We size insulation against the actual local dew point, not against generic IS 661 minimums.

For the practical owner-side companion to this piece - what an oversized or wrongly specified system actually feels like, and what to do about it in an occupied building - see humidity is Kerala's real HVAC problem.

Why this matters to building owners, not just engineers

Owners do not need to read a psychrometric chart. They do need to know what to ask for, because the consequences of treating DBT as the only number are visible and expensive:

AC bills that climb every monsoon while the rooms still feel damp.
Mould on plasterboard within a year of handover.
Condensation staining around grilles, diffusers, and duct take-offs.
Server rooms holding 22°C cleanly but corroding electronics because indoor RH sits at 70%.
Cold rooms tripping on high-pressure cut-outs every May because the unit was sized off DBT load and never saw the latent design point.

These are not equipment failures. They are calculation failures, made at the design stage, that no amount of subsequent service work can fully undo.

The short version

Dry bulb temperature tells you how hot the air is. Relative humidity, properly converted to a humidity ratio, tells you how much water that air is carrying. In Kerala, the second number is often the larger problem and almost always the more expensive one to fix after the fact. Any HVAC calculation that does not handle both, with our state's actual design conditions, is producing answers that look right on a spreadsheet and underperform on site.

If you are scoping a commercial project in Kerala and want the load calculation done against real district-level design data rather than generic averages, that is exactly the conversation our commercial HVAC team is set up to have.