Can Tennessee enforce its weather-modification ban on activities like cloud seeding when the chemicals are released in federally controlled airspace?

Plain-English summary

Tennessee passed legislation prohibiting the intentional injection, release, or dispersion of chemicals or substances into the atmosphere with the express purpose of affecting temperature, weather, or sunlight intensity. Tenn. Code Ann. § 68-201-122. Violation is a Class C misdemeanor with civil penalties up to $25,000 per day. The legislators who requested this opinion asked: can the state actually enforce that ban when the alleged activity happens in airspace the federal government controls?

The AG split the answer along two axes:

1. Ground-based weather modification (e.g., ground-based cloud seeding): Likely enforceable. The opinion says federal airspace law seems unlikely to create a preemption problem for ground-based activity, and it does not see a Clean Air Act preemption issue in that setting.

2. Activities in navigable airspace (e.g., aircraft-based cloud seeding): The opinion says federal aviation law may preempt some applications involving navigable airspace. The Clean Air Act may separately preempt enforcement aimed at aircraft engine emissions. The result turns on the particular activity.

What this means for you

If you operate or are considering a weather-modification project in Tennessee

The opinion says a court is likely to allow enforcement against ground-based weather-modification activities, and federal preemption problems appear unlikely there. For aircraft-based activities, it identifies possible preemption under federal aviation law and the Clean Air Act, depending on the activity.

If you are an air-pollution-control staff attorney for Tennessee

The opinion says federal aviation law seems unlikely to preempt enforcement against ground-based activity and finds no apparent Clean Air Act preemption issue for that activity. It separately discusses possible preemption of certain aircraft-related applications under § 7573. The FAA's July 2023 fact sheet describes field preemption in terms of aviation safety and efficient airspace use.

If you are a state legislator considering similar legislation

The opinion concludes that Tennessee's broad statute supports preemption-resistant ground-based coverage, while regulation reaching aircraft operations or engine emissions risks preemption under 49 U.S.C. § 40103(b)(1) and 42 U.S.C. § 7573.

If you defend an alleged violator

The opinion identifies possible preemption of aircraft-related activity under § 7573 or federal aviation law, while saying enforcement against ground-based activity is likely permissible. It says each application requires its own analysis.

Common questions

Q: What is "weather modification" under Tennessee law?
A: § 68-201-122 prohibits the intentional injection, release, or dispersion of chemicals, chemical compounds, substances, or apparatus into the atmosphere with the express purpose of affecting temperature, weather, or the intensity of the sunlight. The classic examples are cloud seeding (silver iodide, sodium chloride for precipitation enhancement) and proposed solar radiation management techniques (sulfur aerosol injection).

Q: What are the penalties?
A: A knowing violation is a Class C misdemeanor with criminal fines up to $10,000 per day per violation under § 68-201-112. Civil penalties up to $25,000 per day per violation under § 68-201-116(b)(1). The Air Pollution Control Board or Commissioner of Environment and Conservation can also abate violations as public nuisances.

Q: Why does federal preemption come into play at all?
A: Two federal regimes. First, the Clean Air Act expressly preempts state regulation of aircraft engine emissions. 42 U.S.C. § 7573. Second, the Federal Aviation Act gives the FAA "exclusive sovereignty" over U.S. airspace and authority to regulate flight operations. Tennessee's law could in some applications conflict with those.

Q: Does the state have any authority over its airspace at all? A: Yes, partial. Tennessee Code Annotated § 42-1-102 declares that "[s]overeignty in the space above the lands and waters of this state is declared to rest in the state, except where granted to and assumed by the United States pursuant to a constitutional grant from the people of this state." The U.S. Supreme Court in Braniff Airways recognized that the federal declaration of sovereignty "did not expressly exclude the sovereign powers of the states" and was "an assertion of exclusive national sovereignty" only as against other nations.

Q: Can Tennessee enforce its weather-modification ban against cloud seeding? A: The statute prohibits intentional releases within Tennessee to affect weather. The opinion says enforcement against ground-based releases is likely available, while aircraft emissions may be preempted by the Clean Air Act and releases in navigable airspace depend on the facts.

Background and statutory framework

Tennessee's weather-modification ban is part of an amendment to the Tennessee Air Quality Act, codified at Tenn. Code Ann. § 68-201-122. The Act predates this provision and provides the enforcement machinery: Class C misdemeanor penalties under § 68-201-112, civil penalties under § 68-201-116, and public-nuisance abatement authority for the Air Pollution Control Board.

The federal preemption analysis turns on three doctrines:

• Express preemption. A federal statute uses explicit language to displace state law. The Clean Air Act does this for aircraft engine emission standards in 42 U.S.C. § 7573.
• Conflict preemption. State law makes compliance with federal law impossible or stands as an obstacle to congressional purposes. This doctrine has been narrowed in recent Supreme Court cases like Kansas v. Garcia.
• Field preemption. Federal law occupies an entire regulatory field, leaving no room for state supplementation. The Federal Aviation Act is widely understood to occupy the field of aviation safety and efficient airspace use, but the FAA itself reads that field narrowly.

The opinion's analytical move is to separate the purpose of the regulation (weather modification, traditionally a state-regulated field) from the medium of the regulated conduct (atmospheric release, sometimes overlapping with federal aviation regulation). When the regulation targets weather modification on the ground, no federal field is implicated. When it targets aircraft engine emissions, § 7573 expressly preempts. The middle ground (aircraft-mounted, non-emission releases of weather-modification chemicals) is the genuinely uncertain space.

The opinion notes that "[f]ield preemption of state law is disfavored" (citing the Fifth Circuit's McCraw decision) and that even Kansas v. Garcia signaled the Supreme Court's willingness to rein in implied preemption. Both points support narrower preemption of Tennessee's law than a maximalist reading might suggest.

Citations and references

Statutes:

• Tenn. Code Ann. § 68-201-122 (Weather modification ban)
• Tenn. Code Ann. § 68-201-112 (Air Quality Act enforcement)
• 49 U.S.C. § 40103 (Federal Aviation Act airspace authority)
• 42 U.S.C. § 7573 (Clean Air Act aircraft emission preemption)
• 42 U.S.C. § 7416 (Clean Air Act state authority)

Cases:

• Murphy v. NCAA, 584 U.S. 453 (2018), preemption framework
• Kansas v. Garcia, 589 U.S. 191 (2020), narrowing implied preemption doctrine
• Braniff Airways, Inc. v. Neb. State Bd. of Equalization & Assessment, 347 U.S. 590 (1954), federal airspace sovereignty does not erase state authority
• Gustafson v. City of Lake Angelus, 76 F.3d 778 (6th Cir. 1996), FAA's exclusive authority over airspace regulation
• Riggs v. Burson, 941 S.W.2d 44 (Tenn. 1997), Tennessee Supreme Court on FAA airspace authority

Source

• Landing page: https://www.tn.gov/attorneygeneral/opinions.html
• Original PDF: https://www.tn.gov/content/dam/tn/attorneygeneral/documents/ops/2025/op25-006.pdf

Original opinion text

STATE OF TENNESSEE
OFFICE OF THE ATTORNEY GENERAL
February 28, 2025
Opinion No. 25-006
Enforceability of Prohibition on Weather Modification Activities

Question 1
Does Tenn. Code Ann. § 68-201-112 allow Tennessee to enforce Tenn. Code Ann. § 68- 201-122 when there is injection, release, or dispersion of the prohibited material into federally controlled airspace, or airspace that is otherwise under the jurisdiction of the federal government?

Opinion 1
It depends. A court would likely find some attempts to enforce Tenn. Code Ann. § 68- 201-122 expressly or impliedly preempted, but not others.

Question 2
If not, in what other manner, if any, can Tennessee enforce Tenn. Code Ann. § 68-201-122 when there is injection, release, or dispersion of the prohibited material into federally controlled airspace, or airspace that is otherwise under the jurisdiction of the federal government, for the purposes of the release of chemicals, chemical compounds, substances, or other apparatus?

Opinion 2
See Response to Question 1. Tennessee can likely enforce Tenn. Code Ann. § 68-201-122 to ground-based activities that involve injection, release, or dispersion of material into the air. But to the extent the question contemplates enforcement of § 68-201-122 to emissions, the federal Clean Air Act may preempt enforcement. And for releases occurring in navigable airspace, a court may find § 68-201-122 impliedly preempted depending on the facts.

ANALYSIS

A recent amendment to the Tennessee Air Quality Act ("the Act"), codified in Tenn. Code Ann. § 68-201-122, provides that "[t]he intentional injection, release, or dispersion, by any means, of chemicals, chemical compounds, substances, or apparatus within the borders of this state into the atmosphere with the express purpose of affecting temperature, weather, or the intensity of the sunlight is prohibited." Tenn. Code Ann. § 68-201-122. Like other violations of the Act, violations of § 68-201-122 trigger the enforcement mechanisms outlined in Tenn. Code Ann. § 68- 201-112. Under that statutory section, a person who knowingly violates a provision of the Act commits a Class C misdemeanor and is subject to a fine "not to exceed ten thousand dollars ($10,000) per day per violation." Tenn. Code Ann. § 68-201-112(a). "In addition to the criminal penalties of § 68-201-112," any person who violates a provision of the Act "shall be subject to a civil penalty of up to twenty-five thousand dollars ($25,000) per day for each day of violation." Tenn. Code Ann. § 68-201-116(b)(1). Moreover, if the Air Pollution Control Board or the Commissioner of Environment and Conservation finds that a provision of the Act has been violated and that the violation constitutes a public nuisance, "the board or commissioner has authority to abate any such public nuisance in the manner provided by the general law relating to the abatement of public nuisances." Tenn. Code Ann. § 68-201-112(d).

Your question about enforcing § 68-201-122, namely your concern that certain airspace is "under the jurisdiction of the federal government," implicates the doctrine of federal preemption.

Preemption Doctrine. The Supremacy Clause provides that the "Constitution, and the Laws of the United States which shall be made in Pursuance thereof . . . shall be the supreme Law of the Land." U.S. Const., art. VI, cl. 2. That Clause provides "a rule of decision" for determining whether federal or state law applies in a particular situation. Armstrong v. Exceptional Child Ctr., Inc., 575 U.S. 320, 324 (2015). If "a state law confers rights or imposes restrictions that conflict with the federal law," "the federal law takes precedence and the state law is preempted." Murphy v. NCAA, 584 U.S. 453, 477 (2018).

Any preemption of state law must "stem from either the Constitution itself or a valid statute enacted by Congress." Kansas v. Garcia, 589 U.S. 191, 202 (2020). In some cases, a federal statute "may preempt state authority by so stating in express terms," express preemption. Pac. Gas & Elec. Co. v. State Energy Res. Conservation & Dev. Comm'n, 461 U.S. 190, 203 (1983). But the Supreme Court has long recognized that "preemption may also occur by virtue of restrictions or rights that are inferred from statutory law." Garcia, 589 U.S. at 203. That implied preemption takes two forms: conflict preemption and field preemption.

The conflict preemption doctrine has evolved over time. At the founding, courts looked only to "whether the ordinary meaning of federal and state law directly conflict." Id. at 214 (Thomas, J., concurring) (quotations omitted). Over time, though, the Supreme Court loosened the conflict inquiry and held state laws preempted if they stood "as an obstacle to the accomplishment and execution of the full purposes and objectives of Congress." Hines v. Davidowitz, 312 U.S. 52, 67 (1941); see also Matthews v. Centrus Energy Corp., 15 F.4th 714, 720 (6th Cir. 2021). That purposes-and-objectives inquiry has been widely criticized because "[a] free wheeling judicial inquiry into whether a state statute is in tension with federal objectives would undercut the principle that it is Congress rather than the courts that pre-empts state law." See, e.g., Gade v. Nat'l Solid Wastes Mgmt. Ass'n, 505 U.S. 88, 111 (1992) (Kennedy, J., concurring in part and concurring in judgment). And in recent years, the Court has reined in the doctrine. In Garcia, for example, the Court emphasized that "[t]he Supremacy Clause gives priority to 'the Laws of the United States,' not the criminal law enforcement priorities or preferences of federal officers." 589 U.S. at 212. And it noted that preemption cannot be based on "a 'freewheeling judicial inquiry into whether a state statute is in tension with federal objectives.'" Id. at 202 (quoting Chamber of Com. of U.S. v. Whiting, 563 U.S. 582, 607 (2011) (plurality)).

Field preemption, another species of implied preemption, likewise rests on a shaky foundation. Under the field preemption doctrine, courts determine "if the federal law so thoroughly occupies the legislative field 'as to make reasonable the inference that Congress left no room for the States to supplement it.'" Riggs v. Burson, 941 S.W.2d 44, 49 (Tenn. 1997) (quoting Fid. Fed. Sav. & Loan Ass'n v. de la Cuesta, 458 U.S. 141, 153 (1982)). But that just begs the question of "what field" and why preempt the field if Congress provided no instruction to do so. In this vein, it is noteworthy that the jurisprudential framework for analyzing federal preemption has been roundly criticized or questioned by judges and commentators alike. See, e.g., Garcia, 589 U.S. at 214 n.* (Thomas, J., concurring) ("I am also skeptical of field pre-emption . . . ."). And unsurprisingly, "[f]ield preemption of state law is disfavored." Nat'l Press Photographers Ass'n v. McCraw, 90 F.4th 770, 796 (5th Cir. 2024).

The enforcement of § 68-201-122 implicates each of these preemption doctrines. And, given the state of federal law, the analysis differs depending on where the base of the prohibited weather modification activity occurs. As explained below, although we believe that a court is likely to conclude that Tennessee can generally enforce the statute as applied to activities conducted from a base on the ground, there is a risk that a court could hold that enforcement is preempted as applied to activities occurring in navigable airspace.

Enforcement of § 68-201-122 to activities in navigable airspace. Numerous federal statutes address issues implicating activities in navigable airspace. A court would likely find at least some applications of enforcing § 68-201-122 preempted by these laws. But no federal law cleanly preempts enforcement of § 68-201-122 wholesale to activities in navigable airspace.

Start with 49 U.S.C. § 40103(a)(1). That provision states that "[t]he United States Government has exclusive sovereignty of airspace of the United States." 49 U.S.C. § 40103(a)(1). On its face, this declaration of sovereignty might appear to suggest that the federal government exercises exclusive dominion over the entirety of the skies. In reality, though, § 40103(a)(1) does much more limited work. The congressional acts on which § 40103(a)(1) is based, including the Air Commerce Act of 1926, make clear that "Congress used this section to declare sovereignty only internationally" and "did not intend to trample on the sovereignty of states' airspace rights." Stephen J. Migala, UAS: Understanding the Airspace of States, 82 J. Air. L. & Com. 3, 15 (2017). Indeed, when the United States Supreme Court engaged with the Air Commerce Act of 1926 in Braniff Airways, Inc. v. Nebraska State Board of Equalization & Assessment, it recognized that the congressional declaration of sovereignty "was an assertion of exclusive national sovereignty" and that the legislation "did not expressly exclude the sovereign powers of the states." Braniff Airways, Inc. v. Neb. State Bd. of Equalization & Assessment, 347 U.S. 590, 595 (1954). See also Tenn. Code Ann. § 42-1-102 ("Sovereignty in the space above the lands and waters of this state is declared to rest in the state, except where granted to and assumed by the United States pursuant to a constitutional grant from the people of this state.").

Other federal statutes implicating the use of navigable airspace do expressly preempt state law. Under 42 U.S.C. § 7573, "[n]o State or political subdivision thereof may adopt or attempt to enforce any standard respecting emissions of any air pollutant from any aircraft or engine thereof unless such standard is identical to a standard applicable to such aircraft under this part." 42 U.S.C.
§ 7573. Although precedent on this section is "sparse," Codoni v. Port of Seattle, No. 2:23-cv- 795-JNW, 2024 WL 4882015, at 11 (W.D. Wash. Nov. 25, 2024), preemption concerns would be particularly acute if a state regulation, as applied in a given case, would necessitate aircraft alterations. Id. at 12; see also California v. Dep't of the Navy, 624 F.2d 885, 888 (9th Cir. 1980) (opining that 42 U.S.C. § 7573 "was not intended to be preclusive of all state regulation of the field of aircraft engines" and agreeing that it is "concerned with direct state regulation of aircraft or aircraft engines or with other state regulation which would affect the aircraft or engine"). This statute, though, may not be implicated in most contemplated enforcement efforts pertaining to the prohibition in § 68-201-122. As one commentator has discussed, "[t]he Clean Air Act's mobile source program . . . largely targets emissions from the operation of engines rather than intentional releases conveyed by the mobile sources themselves." Tracy D. Hester, Remaking the World to Save It: Applying U.S. Environmental Laws to Climate Engineering Projects, 38 Ecology L.Q. 851, 879 (2011).

The area of greatest uncertainty regarding federal preemption in navigable airspace comes from the Federal Aviation Act. Under 49 U.S.C. § 40103(b)(1), "[t]he Administrator of the Federal Aviation Administration shall develop plans and policy for the use of the navigable airspace and assign by regulation or order the use of the airspace necessary to ensure the safety of aircraft and the efficient use of airspace." 49 U.S.C. § 40103(b)(1). Section 40103(b)(2) provides that "[t]he Administrator shall prescribe air traffic regulations on the flight of aircraft" for, among other things, "protecting individuals and property on the ground," 49 U.S.C. § 40103(b)(2)(B), and "using the navigable airspace efficiently." 49 U.S.C. § 40103(b)(2)(C).

These provisions have served as the foundation for a well-entrenched judicial understanding that state and local control is preempted as it concerns the areas of airspace use and the regulation of aircraft in flight. In Gustafson v. City of Lake Angelus, for instance, the Sixth Circuit cited to these provisions in connection with its acknowledgment that "the FAA has exclusive authority in regulating the airspace over the United States." Gustafson v. City of Lake Angelus, 76 F.3d 778, 783 (6th Cir. 1996). Similarly, in the previously cited Riggs decision, the Tennessee Supreme Court referenced 49 U.S.C. § 40103(b)(1) in support of its observation that there was "congressional intent to place exclusive authority for regulating the airspace above the United States with the Federal Aviation Administration." Riggs, 941 S.W.2d at 49.

But accepting that the Federal Aviation Act likely preempts some sphere of state law, that does not mean that all state or local regulation is prohibited by the mere fact that it could have some effect on the use of airspace. As a recent fact sheet issued by the Federal Aviation Administration explains, federal field preemption of airspace matters is itself specifically limited to the fields of aviation safety and efficient use of the airspace. State and Local Regulation of Unmanned Aircraft Systems (UAS) Fact Sheet ("FAA Fact Sheet") (July 14, 2023). Laws "aimed at objectives other than aviation safety or airspace efficiency that do not impair the reasonable use by [unmanned aircraft systems] of the airspace" are likely not to be subject to field or conflict preemption. The FAA Fact Sheet even states that "[r]estrictions on how [unmanned aircraft systems] are utilized (i.e., conduct) instead of where they may operate in the airspace would more likely be consistent with Federal preemption principles." In other words, the exclusivity of the Federal Aviation Administration's jurisdiction is cabined to "certain regulatory fields, not over certain airspace."

Applying those principles here leaves little doubt that federal law does not altogether preempt § 68-201-122. Tennessee Code Annotated § 68-201-122 does not directly involve the regulation of aircraft or navigable airspace. It broadly addresses weather modification in broad terms. And as explained infra, that field as a whole, regulation of weather modification, is not federally preempted.

Any preemption analysis of § 68-201-122 will turn on particular applications of the law. It is certainly foreseeable that a court might find enforcement of § 68-201-122 preempted as applied to certain attempts to regulate operations of aircraft in navigable airspace. But unless a court adopts a broad view of field preemption (broader than even the approach adopted by the federal government), some regulation of weather modification activities in navigable airspace would likely be permitted. And any preemption challenge would have to overcome the assumption that a state's police powers are not to be superseded absent clear congressional purpose, Riggs, 941 S.W.2d at 48, and the reality that "states have traditionally had primary carriage of weather- modification regulations."

Enforcement of § 68-201-122 to ground-based activities. The prospect of preemption differs for the enforcement of § 68-201-122 to ground-based activities.

States exercise broad authority with respect to activities occurring on land, even if those ground-based activities implicate the usage of navigable airspace. This Office has previously concluded that States "retain regulatory authority over the decision of whether land may be used as an airport or heliport." Tenn. Att'y Gen. Op. 93-04 (Jan. 13, 1993). And courts have generally distinguished the "United States' sovereign regulation of the airspace over the United States and the regulation of aircraft in flight" from regulation involving local control of land. Gustafson, 76 F.3d at 783. As one court stated, there is "no conflict between . . . [non-federal] regulatory power over land use, and the federal regulation of airspace." Condor Corp. v. City of St. Paul, 912 F.2d 215, 219 (8th Cir. 1990). So the federal laws addressing navigable airspace seem unlikely to pose any preemption problem for enforcing § 68-201-122 with respect to ground-based weather modification activities, such as ground-based cloud seeding.

And no federal laws that we are aware of substantively regulate weather modification efforts. Reporting requirements exist, see 15 U.S.C. § 330a, but "[a]ttempts to pass a comprehensive weather modification policy at the federal level have failed." MacKenzie L. Hertz, It's Raining, It's Pouring, Weather Modification Regulation is Snoring: A Proposal to Fill the Gap in Weather Modification Governance, 96 N.D. L. Rev. 31, 46 (2021). "The federal government has . . . left substantive weather modification regulation to state authorities."

In light of the foregoing, it is this Office's opinion that enforcement of § 68-201-122 relative to ground-based weather modification activities would be entirely consistent with the historical liberty states have exercised, virtually unimpeded by the federal government, to regulate in this area.

Nor does there appear to be any preemption issue under the federal Clean Air Act concerning § 68-201-122's prohibition on the release of material involved in ground-based weather modification efforts. First, it is not entirely clear that the framework of the Clean Air Act would even apply to trigger any potential preemption concern. As one commentator has observed, "[t]he Clean Air Act only applies to releases of 'pollutants' that meet statutory and regulatory criteria," and "[h]istorical attempts to modify weather through cloud seeding or other rain-making technology have fallen under separate state regulatory regimes rather than the Clean Air Act." Yet, even if we assume that an "air pollutant" would be implicated by a given weather modification activity, the prohibition represented by § 68-201-122 would still not appear to create any type of preemption issue. Indeed, enforcing § 68-201-122 would be in furtherance of a statutorily imposed ban on such emitted "pollutants" being released in a particular setting, and except for preemption of certain state regulation pertaining to pollutant emissions from moving sources, "nothing in the Clean Air Act precludes or denies the right of any state . . . to adopt or enforce any standard or limitation respecting emissions of air pollutants, or any requirement respecting control or abatement of air pollution." 61B Am. Jur. 2d Pollution Control § 147. The only statutory qualification is that a State's exercise of such authority must simply be reflected in a policy that is not less stringent than the Clean Air Act "if an emission standard or limitation is in effect under an applicable implementation plan or under section 7411 or section 7412 of this title." 42 U.S.C.
§ 7416. Even if "air pollutants" are involved, there is thus leeway for Tennessee to operate legislatively.

To recap our analysis, it is our opinion that the prospect of enforcing § 68-201-122 is dependent on where the base of the prohibited weather modification activity occurs and what activity is being regulated. Although a court is likely to conclude that Tennessee can generally enforce the statute as applied to activities based on the ground, there is a risk that a court could hold that enforcement is impliedly preempted for some attempts to regulate activities occurring in navigable airspace. Moreover, to the extent there might be contemplated enforcement of the statute relative to actual emissions produced by aircraft from flight operations, such enforcement is susceptible to express preemption under the federal Clean Air Act.

JONATHAN SKRMETTI
Attorney General and Reporter

J. MATTHEW RICE
Solicitor General

MATTHEW KERNODLE
Assistant Attorney General

Requested by:
The Honorable Rusty Crowe
State Senator
Cordell Hull Building, Suite 720
425 Rep. John Lewis Way North
Nashville, Tennessee 37243

The Honorable Page Walley
State Senator
Cordell Hull Building, Suite 750
425 Rep. John Lewis Way North
Nashville, Tennessee 37243

The Honorable Janice Bowling
State Senator
Cordell Hull Building, Suite 718
425 Rep. John Lewis Way North
Nashville, Tennessee 37243

The Honorable Mark Pody
State Senator
Cordell Hull Building, Suite 754
425 Rep. John Lewis Way North
Nashville, Tennessee 37243