What is the specific heat capacity of dry air at ambient temperature?

At standard ambient temperature (300 K / 27°C), the specific heat capacity of dry air at constant pressure (Cp) is 1.005 kJ/(kg·K) or 0.240 Btu/(lb·°F), and at constant volume (Cv) is 0.718 kJ/(kg·K).

How does humidity affect the specific heat capacity of air?

Moisture increases the specific heat capacity of air because water vapor has a specific heat of approximately 1.884 kJ/(kg·K), which is significantly higher than dry air (1.005 kJ/(kg·K)). The humid air specific heat is calculated as Cp' = 1005 + 1884 * w in J/(kg_dry_air·K).

What is the isentropic expansion coefficient (k) for dry air?

For dry air at ambient temperature, the ratio of specific heats k = Cp / Cv is approximately 1.40.

Humid Air : Specific heat capacity

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1. Introduction
2. Humid air specific heat capacity calculation
3. Dry air specific heat capacity
4. Dry air isentropic ratio
5. Interactive Online Calculator

1. Introduction

The absolute humidity is calculated from the specific heat of dry air and the specific heat of water ponderated by the absolute humidity.

2. Humid air specific heat capacity calculation

How to calculate the specific heat capacity of humid air ?

The specific heat capacity of humid air at ambient temperature can be calculated thanks to the following formula :

\[ C_p' = 1005 + 1884 \cdot \omega \]

**Cp' = 1005 + 1884*ω**

With :
\(\omega\) = Absolute humidity of air (\(\text{kg}_{\text{water}} / \text{kg}_{\text{dry air}}\))
\(C_p'\) = specific heat of humid air (\(\text{J}/(\text{K}\cdot\text{kg}_{\text{dry air}})\))

⚡ Interactive Humid & Dry Air Specific Heat Calculator

⚠️ ENGINEERING NOTICE & EDUCATIONAL DISCLAIMER: This interactive calculator is provided exclusively for preliminary estimation and educational purposes. It is not intended for detailed design or equipment procurement without certified vendor rating. No warranty, expressed or implied, is provided, and no liability is assumed.

Unit System: SI Metric (°C, kJ/kg·K, kg/kg) US Customary / Imperial (°F, Btu/lb·°F, lb/lb)

Calculation Results:

Humid Air Specific Heat (\(C_p'\)):
- kJ/(kg·K)

Dry Air \(C_p\):
- kJ/(kg·K)

Dry Air \(C_v\):
- kJ/(kg·K)

Isentropic Ratio (\(k = C_p / C_v\)):
-

💡 Engineering Rules of Thumb & Practical Design Notes

Standard Ambient Air Cp: At standard temperature and pressure (\(20^\circ\text{C}\) / \(293.15\,\text{K}\)), dry air specific heat capacity is approximately \(1.005\,\text{kJ/(kg}\cdot\text{K)}\) (\(0.240\,\text{Btu/(lb}\cdot^\circ\text{F)})\).
Impact of Moisture: Water vapor has a much higher specific heat capacity (\(C_p \approx 1.884\,\text{kJ/(kg}\cdot\text{K)}\)) than dry air. Increasing absolute humidity noticeably increases total humid air thermal capacity (\(C_p'\)).
Isentropic Expansion Index (\(k\)): For ideal diatomic dry air, \(k = C_p/C_v \approx 1.40\). As humidity increases, the effective isentropic exponent decreases slightly toward water vapor's ratio (\(k \approx 1.33\)).
HVAC & Dryer Sizing: Always use humid air heat capacity (\(C_p'\)) based on dry air mass flow rate (\(\dot{m}_{\text{dry}}\)) when performing heat exchanger, coil, or air dryer heat balance calculations: \(Q = \dot{m}_{\text{dry}} \cdot C_p' \cdot \Delta T\).

3. Dry air specific heat capacity

What is the specific heat of air at usual temperature and pressure ?

The specific heat capacity of air at 300K is \(C_p = 1.005\,\text{kJ/kg/K}\). The specific heat capacity of air is varying with the temperature as reported in the table below :

Temperature in K	Cp in kJ/kg/K	Cv in kJ/kg/K
250	1.003	0.716
300	1.005	0.718
350	1.008	0.721
400	1.013	0.726
450	1.020	0.733
500	1.029	0.742

4. Dry air isentropic coefficient k

For air, at ambient temperature \(k = C_p/C_v = 1.4\)