Adiabatic Cloud Parcel
A **toy lifted parcel** keeps its water-vapor pressure **e** fixed (no entrainment/detrainment). Saturation vapor pressure **e_s(T)** follows a **Magnus-type** formula. **Dew point** **T_d** solves **e_s(T_d) = e**. While rising, the parcel cools at a constant **dry adiabatic lapse** **Γ_d ≈ 9.8 K km⁻¹** until **T = T_d**; that height is the **lifting condensation level (LCL)**, sketched as **cloud base**. Above the LCL a constant **moist lapse** **Γ_m ≈ 6.5 K km⁻¹** stands in for a pseudoadiabat — adequate for **qualitative** meteorology, not for skew-T analysis or precipitation forecasting.
Who it's for: High-school or intro atmospheric-science students linking humidity, lapse rates, and cloud base; pairs with climate toy models.
Key terms
- lifting condensation level
- dew point
- dry adiabatic lapse rate
- moist adiabatic
- relative humidity
- parcel model
How it works
Moist air lifted adiabatically cools at roughly the dry lapse rate until it becomes saturated at the **lifting condensation level (LCL)** — then condensation is drawn as cloud. The profile above LCL uses a simplified moist lapse for illustration.
Frequently asked questions
- Why does RH increase even before the cloud?
- The parcel keeps the same vapor pressure e while temperature drops; saturation pressure e_s(T) falls with T, so RH = e/e_s(T) climbs until it reaches 100% at the dew point.
- Is Γ_m = 6.5 K km⁻¹ exact?
- No. Real moist-adiabatic lapse rates depend on pressure, temperature, and latent heating; they are curved on a thermodynamic diagram. The constant Γ_m here is a simple continuation for visualization.
- Does the cloud draw water conservation?
- The cartoon cloud marks where the model switches to a moist lapse. It does not track condensed water, precipitation, or entrainment of dry air.
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Joule Expansion
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