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

By DAN ROBINSON
Editor/Photographer
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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:
  1. 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.
  2. 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

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

References

  1. Positive and negative lightning leaders by Tom Warner

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