August 1999 Lightning Study for the State of Colorado

Stephen Hodanish
National Weather Service, Pueblo CO

Introduction

During the month of August 1999, the staff at NWS Pueblo assisted in gathering lightning data for a study of cloud to ground lightning over the state of Colorado.  A macro was created on the AWIPS workstation which saved 32 frames of hourly lightning data, with this data being plotted over the state of Colorado and adjacent nearby areas. Hourly CG data, both positive and negative, were then written down and logged. This short document discusses some of the findings.

Reason for Study

The primary reason for this study was to observe the amount and variability of CG lightning over the state of Colorado on an hourly basis. Approximately 1 month of data was documented. Both positive and negative flashes were counted. Only cloud to ground lightning was recorded. The area in which data was collected for was for the state of Colorado and adjacent areas. Areas immediately adjacent to Colorado were included due to mapping limitations of the AWIPS workstation. Most of the adjacent areas were extreme southern Wyoming and extreme northern New Mexico.

Some data for this study is missing. Data on August 25 and 26 are missing. Two hours of August 9th are missing (01 and 02 UTC). On August 17th, the time between 13 UTC and 23 UTC were missing. On 27 Aug., the time between 01 and 13 UTC were missing. On august 1st, the first 6 hours (01-06 UTC) of data is missing.

Data Analysis

    Hourly CG data

A total of 143,499 negative CG flashes and  10,103 positive flashes were documented over the state of Colorado, for a total of  153602 CG flashes. This breaks down to a percentage of 93% negative flashes and 7% positive flashes. Figure 1 shows the flash data plotted for each hour during the month of August. Time in this figure is Universal Time (6pm MDT = 00Z). The maximum hourly flash rate occurred on the 11th of August during the 1 hour period ending at 01 UTC (10th of August, 7 pm MDT)  when a total of 3038 flashes occurred across the state.

On average, just under 5000 flashes (4990) occurred on any given day. However, as seen in figure 1, some days were much more lightning active than others.
 

Fig 1. Lightning plotted hour by hour during August 1999 over the state of Colorado. Yellow is positive flashes, red is negative
flashes, and blue is the sum of both positive flashes. Some data is missing (see text).


Hourly CG data:

Figure 2 shows the average amount of total lightning plotted for each hour for the month of August. The time interval which had the most lightning was between 23 and 24 UTC (5 and 6 pm MDT), when, on average, 735 flashes occurred. Times during the least amount of lighting occurred during the morning hours between 12 and 17 UTC (6 am and 11 am MDT).

 Figure 2. Average amount of lightning plotted for each hour during August 1999.


Figure 3 shows the average amount of lightning per day for August 1999. Data on this chart was missing on the 25 and 26 and parts of the 27th, 9th and 17th. Figure 4 is the same as figure 3, except it shows the total amount of lightning for each day.
 

Fig 3. Average amount of lightning for each day. Data on the 9th, 17th, 25th, 26th and 27th must be used with caution as some or all of the data was missing on these dates.

Fig 3. Total amount of lightning for each day. Data on the 9th, 17th, 25th, 26th and 27th must be used with caution as some or all of the data was missing on these dates.

Discussion

This brief study analyzed cloud to ground lightning which occurred over the state of Colorado and nearby adjacent states during the month of August 1999. During this month, it was observed on average that nearly 5000 CG flashes occurred daily. Lightning over the state was quite variable, as some days had only a hundred or so flashes while other days had over 10,000 flashes. The standard deviation of the daily lighting data, not including days in which data was missing, was 3963. Although no spatial aspects were looked into, it can be inferred that lightning begins to become a threat in Colorado after 12 noon. The maximum amount of lightning occurs between 5 and 6 pm local time.

 

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