Atmospheric moisture content as a potential factor in fast vs slow states of positive lightning leaders

Editor/Photographer

The purpose of this study is to investigate the hypothesis that atmospheric moisture content (represented by dewpoint temperature) plays a role in whether positive lightning leaders are slow (highly branched) versus fast (branchless) below cloud base. This page is a work-in-progress and will be updated as more data is collected.

Background information

High-speed video of lightning reveals that positive lightning leaders have two distinct states of propagation: fast and slow [1]. A fast positive leader propagates at greater speeds and rarely branches, while a slow positive leader branches extensively. Positive lightning leaders have also been observed transitioning in state from slow to fast or from fast to slow. Many of the branches of slow positive leaders are weakly luminous and have a tendency to produce recoil leaders that re-illuminate the branch. Occasionally, these recoil leaders will result in the initiation of a dart leader that travels from the branch either partway or all the way down the parent channel. In the case of a cloud-to-ground or ground-to-cloud lightning flash with the negative end connected to the ground, a dart leader that propagates all the way to the ground will result in a subsequent return stroke.

With their numerous recoil leaders, multiple return strokes and large areas of sky covered, lightning flashes consisting of slow positive leaders below cloud base are typically the most visually spectacular - both to the in-person observer and when captured on still photographs or video.

Upward lightning flash showing the highly-branched nature of slow positive leaders.

It is currently not known what conditions determine whether a positive lightning leader will be fast or slow.

Hypothesis

Photographic and video evidence suggests that positive leaders that propagate through air with lower moisture content (surface dewpoints at or below 60°F) are more likely to be of the "slow" variety (slower propagation and highly-branched). The converse of this hypothesis is that positive leaders propagating through air with higher moisture content (dewpoints higher than 60°F) are more likely to be of the "fast" variety and therefore unbranched.

The hypothesis suggests that slow positive leaders below cloud base are more likely to occur with events:

During the cool season (late fall through early spring)
At higher altitudes
In more arid locations (desert, high plains, US west coast)

The hypothesis suggests that fast positive leaders below cloud base are more likely to occur with events:

During the warm season (late spring through early fall)
At lower altitudes
In more humid locations (US Midwest, Gulf Coast, Florida)

Methodology

The primary methodology for this study involves:

Collecting photographic and video examples of lightning flashes where positive leaders propagate below cloud base and where the approximate date, time and location of the event can be determined.
Retrieving surface temperature, surface dewpoint temperature and atmospheric skew-t profile records from the nearest ASOS, AWOS, upper air sounding and/or model-derived data source at (or as close as possible to) the time of the event.

Most observed upward lightning occurs with man-made structures that are taller than 200m. As such, a vertical temperature/dewpoint profile of at least the lowest 2km of atmosphere will be necessary to represent the conditions in the volume of air the leaders are propagating through. The surface dewpoint temperature value, while not necessarily representative of the conditions at leader altitude, can serve as a proxy where sounding data is not available. During most warm-season thunderstorm environments, the surface dewpoint temperature is at its maximum at the surface, meaning the leader-altitude dewpoint temperature will usually be lower.

The goal of this study is to investigate if a correlation exists between fast vs slow leader state and dewpoint temperature, and if such a correlation exists, determine the threshold dewpoint temperature where fast vs slow transition occurs.

This page will serve as an ongoing repository for the collection of observed upward lightning flash events and the corresponding meterological data.

Flash type definitions

This study will focus primarily on upward positive leaders, but other flashes with below-cloud-base positive leaders will also be analyzed.

Upward (U): Upward positive leader from tall objects or terrain.
Downward (D): Downward positive leader during a ground flash.
Cloud (C): Cloud flash leader.
Low-altitude cloud (L): A cloud flash where a leader propagates more than halfway to the ground from cloud base, but does not contact the ground.

Leader type definitions

Highly branched leader (H): Exhibits at least 3 branching points in the lower third of the visible flash below cloud base.
Moderately branched leader (M): Exhibits only one or two branching points in the lower third of the visible channel below cloud base.
Unbranched leader (N): Exhibits no branching below cloud base.
Transitioning leader (T): Exhibits a transition from unbranched to highly branched (or vice versa) at a point less than two-thirds below cloud base.

Examples:

Charts of 70 collected events to date

Below-Cloud-Base Positive Lightning Leader Events vs Surface Dewpoint Temperature and Surface Relative Humidity

Below-Cloud-Base Positive Lightning Leader Events vs Surface Dewpoint Temperature

Below-Cloud-Base Positive Lightning Leader Events vs Surface Relative Humidity

Highly-Branched Positive Leader Event List

Date & Link	Location	Approx. Time	SFC TMP °F	SFC DP °F	SFC RH %	Flash Type	Notes
7/4/03	Sioux City, IA	12:00AM	73	64	73	U
4/3/07	St. Albans, WV	11:51PM	58	56	93	U
1/7/08	Chicago, IL	8:19PM	55	54	94	U
4/10/09	St. Albans, WV	6:53PM	55	52	88	U
4/26/09	Oklahoma City, OK	11:21PM	63	57	82	U	(3)
8/20/11	St. Louis, MO	5:51AM	76	66	87	U
5/23/14	New York, NY	8:51PM	60	52	75	U
11/11/15	Chicago, IL	7:53PM	61	41	48	U
3/15/16	Chicago, IL	10:53PM	50	46	86	U
4/27/16	Bruceville-Eddy, TX	12:48AM	64	61	88	U
5/17/16	Clinton, OK	2:18AM	57	55	94	U
4/29/17	Barry County, MI	11:40PM	47	37	68	U
5/18/17	Oklahoma City, OK	8:52PM	70	56	61	U
6/3/17	Kansas City, MO	9:21PM	54	45	72	U
3/22/19	Amarillo, TX	8:53PM	46	40	79	U
1/10/20	Coweta, OK	5:42PM	52	52	100	U
5/30/20	Castle Valley, UT	10:53PM	69	52	54	U	(2)
6/17/20	Lubbock, TX	7:53PM	72	61	68	U
7/12/20	Oklahoma City, OK	12:52AM	77	63	62	U
5/17/21	Abilene, TX	10:52PM	70	58	71	D
5/19/21	San Antonio, TX	8:04AM?	66	62	97	U
10/4/21	Hollywood, CA	5:53PM	75	44	33	U	(1)
3/29/22	Wichita, KS	9:53PM	58	52	81	U
4/29/22	Kansas City, MO	11:54PM	57	54	89	U
10/2/22	Moab, UT	6:55PM	53			U	(2)(4)
4/1/23	New York, NY	7:51PM	57	44	62	U
4/15/23	St. Louis, MO	8:03PM	60	56	86	U	(3)
4/18/23	Houston, TX					U
4/19/23	Chicago, IL	6:53PM	54	45	72	U
5/7/23	Oglesby, IL	9:21PM	66	61	84	U
5/29/23	Sierra Buttes, CA	3:56PM				U	(1)(2)
6/7/23	Lubbock, TX	11:53PM	66	57	73	U

Moderately-Branched Positive Leader Event List

Date & Link	Location	Approx. Time	SFC TMP °F	SFC DP °F	SFC RH %	Flash Type	Notes
5/30/01	Oklahoma City, OK	2:05AM	63	63	100	U
5/7/07	Oklahoma City, OK	2:59AM	64	59	83	U
6/3/08	St. Albans, WV	5:51PM	73	69	87	U
6/19/11	St. Louis, MO	3:00AM	70	64	83	U	(3)
4/3/18	Pittsburgh, PA	11:00PM	56	52	87	U
8/25/19	Wichita, KS	4:53AM	65	61	87	U
9/8/19	St. Louis, MO	6:51AM	67	63	87	U	(3)
6/6/22	Tulsa, OK	5:00AM	66	65	96	U

Unbranched Positive Leader Event List

Date & Link	Location	Approx. Time	SFC TMP °F	SFC DP °F	SFC RH %	Flash Type	Notes
5/30/01	Oklahoma City, OK	2:05AM	63	63	100	U
4/10/04	Raleigh, NC	5:51PM	88	58	70	U
2/6/08	Lexington, KY	2:28AM	63	57	82	U
7/20/06	Chicago, IL	6:11AM	72	66	83	U
6/19/09	Chicago, IL	6:32AM	66	64	94	U
6/19/11	St. Louis, MO	3:00AM	70	64	83	U	(3)
6/30/14	Chicago, IL	8:02PM	71	68	90	U
6/30/14	Chicago, IL	11:04PM	69	68	96	U
3/14/16	Pittsburgh, PA	9:51PM	53	49	86	U
7/21/17	Chicago, IL	9:53PM	75	73	94	U
6/2/18	Eagle, NE	2:51AM	64	61	88	U
6/30/19	Chicago, IL	2:53PM	74	69	85	U
8/25/19	Wichita, KS	4:53AM	65	61	87	U
8/29/19	Kansas City, MO	10:54PM	69	64	84	U
9/15/19	Chicago, IL	1:20AM	70	61	73	U
8/10/20	Chicago, IL	4:53PM	71	67	87	U
7/9/21	St. Louis, MO	9:00PM	77	73	88	U
8/10/21	New York, NY	8:08PM	76	70	82	U
10/11/21	Chicago, IL	4:35PM	68	64	87	U
5/17/22	Wichita, KS	3:53AM	62	55	78	U
4/30/23	New York, NY	5:51PM	58	58	100	U
7/12/23	St. Louis, MO	9:00PM	82	73	74	U
7/29/23	Chicago, IL	12:53AM	73	67	81	U
8/2/23	St. Louis, MO	6:51AM	72	68	87	U
8/9/23	Huntsville, AL	4:57PM	71	70	96	U
8/12/23	St. Louis, MO	5:51AM	74	68	76	U

Transitioning Positive Leader Event List

Date & Link	Location	Approx. Time	SFC TMP °F	SFC DP °F	SFC RH %	Flash Type	Notes
3/29/17	Austin, TX	3:53AM	62	62	100	U
5/27/17	St. Louis, MO	5:04PM	74	67	79	U
7/9/21	St. Louis, MO	9:00PM	77	73	88	U
9/13/21	New York, NY	9:51PM	78	59	52	U
5/21/22	St. Louis, MO	6:00AM	58	55	90	U
4/4/23	Industry, IL	5:54PM	78	64	62	D

Notes key

Occurred at a high elevation or mountain peak at a significantly higher altitude than the nearest surface observation
No official surface observations available within a 50 mile radius. Observation is at the closest ASOS/AWOS station available.
The storm produced both branched and unbranched positive leader flashes before and/or after the recorded event.
Some ASOS/AWOS data was faulty or missing for this event.

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