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If a buoyant thermal receives volume inflow $J$ with reduced gravity $g_p$ and receives no other fluid entrainment, its mass and volume balances imply $\dot{V}=J$ and $d(g_{t}V)/dt=g_{p}J$. If $V\propto t^{9/4}$ and $g_t\propto t^{-5/4}$, then $g_t=9g_p/4$.

In stable density stratification, ambient density decreases with height. A rising buoyant thermal encounters progressively lighter surroundings and entrains denser fluid from below, so its net buoyancy decreases. It reaches neutral buoyancy, overshoots through its remaining momentum, and eventually stops rising, spreads as a buoyant intrusion or excites internal gravity waves. The finite rise is a consequence of this restoring effect, not just drag.

Total buoyancy has dimensions $[B_0]=L^4{T}^{-2}$ and buoyancy frequency has $[N]=T^{-1}$. For an ideal point release with no other dimensional source length or time, dimensional analysis therefore givesThe dimensionless coefficient depends on entrainment coefficient, shape and the momentum model. A finite source radius or impulse supplies additional dimensionless parameters; dimensional analysis cannot determine the coefficient by itself.

Within precisely the spherical model, $b=\alpha z$ and the evolving thermal buoyancy satisfies $dB/dz=-N^{2}V=-K{N}^2{z}^3$, so $B(z)=B_0-K{N}^2{z}^4/4$. Neutrality occurs at $z_n^4=4B_0/(K{N}^2)$. For $P=Vu$, the momentum equation and $\dot{z}=P/V$ giveThe first nonzero-height stopping point is $z_{\max }^4=8B_0/(K{N}^2)$, so $z_{\max }=2^{1/4}{z}_n$. This illustrates both the dimensional scale and the distinction between neutral height and maximum rise; the precise coefficients rely on the specified ideal fluid entrainment and impulse model.

The Boussinesq approximation retains small density differences only in the gravitational buoyancy force and uses a constant reference density in inertia and volume conservation. The entrainment coefficient $\alpha$ specifies the inward velocity of ambient fluid relative to the thermal boundary, $u_e=\alpha W$. Turbulent interface motions engulf ambient water and mix it into the coherent rising volume.

For a spherical buoyant thermal, $V=4\pi a^3/3$. To leading order in the dilute gas fraction, expansion of the gas does not dominate the total-volume balance, so fluid entrainment givesBut $dV/dt=4\pi a^{2}da/dt$ and $dz/dt=W$. Therefore, wherever the thermal is rising,This growth law for a dilute bubbly thermal is independent of stratification under the constant-$\alpha$ model. Stratification changes the speed and eventual stopping height; it does not enter this volume-per-distance relation. In real flows the entrainment coefficient itself can vary with stratification and flow structure.

In the Boussinesq approximation, a spherical buoyant thermal with conserved total buoyancy $B_0$ obeys $\dot{b}=\alpha u$ and $d(Vu)/dt=B_0$, with $V=4\pi b^3/3$. A zero-volume, zero-impulse point source gives $b=\alpha z$, $K{z}^3\dot{z}=B_{0}t$, where $K=4\pi {\alpha }^3/3$. Hence $z^4=2B_0{t}^2/K$ and $u=\sqrt{B_0/(2K)}/z$. The zero-time singularity represents an ideal source limit, not a finite initially regular sphere.

A starting plume is the unsteady flow formed when a maintained buoyancy flux first begins. An idealized model joins a steady pure plume below its growing head to a buoyant thermal above. Turner's original starting-plume study develops this plume-and-thermal description.

For a spherical buoyant thermal fed only through the moving top of a Boussinesq point-source plume, put $\lambda =R/B$ and $B=(6\alpha /5)Z$. Relative inflow $\pi B^2(w_p-\dot{Z})$ and $\dot{V}=4\pi R^2\dot{R}$ implyThe ratio of Froude numbers is $5/[\sqrt{\lambda }(5+24\alpha {\lambda }^3)]$ when both use plume reduced gravity $g_p$. Normalizing the thermal with its own $g_t=9g_p/4$ multiplies this by $2/3$. These two definitions must not be interchanged when stating bounds.

Suppose identical buoyant thermals deliver buoyancy $B_0$ at rate $f$ to a mixed lower layer of area $A$ before penetrating its interface. Neglect source volume and heat loss. Relative to the upper-layer density, let $D=gh({\rho }_L-{\rho }_2)/{\rho }_{\mathrm{ref}}$, $P=f{B}_0/A$, and $C=\sqrt{B_0/(2K)}$, where $K=4\pi {\alpha }^3/3$. The lower-layer buoyancy budget gives $D=D_0-Pt$. A cycle-averaged fluid entrainment closure $\dot{h}/u^{\ast }={\mathrm{Ri}}^{-1}$, with $u^{\ast }=C/h$, $b^{\ast }=\alpha h$, yields the displayed law. Neutrality of the thermal at the interface requires $D(t^{\ast })h(t^{\ast }{)}^2=B_0/K$. The closure must incorporate the encounter duty cycle; an isolated-event entrainment law needs an additional averaging prescription.