Gretna City Park Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Gretna City Park Observatory clear sky chart from 2005-11-06 to 2026-07-16.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	10% 6% 2 7% 5% 5% 6% 5% 7% 9% 38%	137628
	Night	11% 6% 2 6% 4 4 4 4 6% 8% 46%	135052
Transparency	Day	38% 9% 14% 18% 15% 6%	141329
	Night	34% 8% 15% 19% 17% 7%	139417
Seeing	Day	12% 7% 20% 26% 23% 13%	16773
	Night	11% 6% 18% 30% 26% 8%	26103

By Month

Percentage Cloud Cover SE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	14% 6% 3 6% 4 4 5% 4 6% 7% 39%	5856
	11th to 20th	Day	11% 7% 2 5% 4 3 4 3 6% 7% 47%	5880
	21st to end	Day	20% 7% 2 7% 4 5% 5% 3 5% 6% 36%	6436
February	1st to 10th	Day	14% 8% 3 7% 4 4 4 3 5% 7% 41%	2100
	11th to 20th	Day	14% 7% 2 6% 4 5% 4 4 5% 9% 41%	2048
	21st to end	Day	15% 7% 2 7% 5% 4 5% 4 5% 6% 39%	1730
March	1st to 10th	Day	18% 8% 2 7% 4 4 4 4 6% 7% 36%	4804
	11th to 20th	Day	11% 5% 2 5% 4 4 4 4 6% 8% 48%	5090
	21st to end	Day	12% 6% 2 7% 5% 5% 4 3 5% 7% 44%	5864
April	1st to 10th	Day	11% 6% 2 5% 4 5% 5% 5% 7% 9% 41%	2520
	11th to 20th	Day	12% 6% 3 7% 5% 4 4 4 6% 8% 42%	2494
	21st to end	Day	8% 5% 2 5% 5% 4 5% 4 8% 9% 46%	2520
May	1st to 10th	Day	17% 6% 2 7% 5% 5% 4 4 7% 9% 34%	7021
	11th to 20th	Day	7% 4 2 6% 5% 6% 6% 5% 7% 9% 44%	7031
	21st to end	Day	7% 5% 2 6% 6% 7% 7% 7% 9% 12% 33%	7736
June	1st to 10th	Day	9% 6% 3 8% 6% 7% 7% 6% 8% 11% 30%	2520
	11th to 20th	Day	5% 6% 2 8% 7% 8% 9% 7% 10% 10% 28%	2573
	21st to end	Day	6% 6% 2 8% 7% 8% 8% 7% 10% 11% 25%	2730
July	1st to 10th	Day	5% 5% 2 8% 6% 7% 9% 8% 10% 12% 26%	6030
	11th to 20th	Day	5% 5% 3 9% 7% 8% 10% 8% 10% 12% 23%	5670
	21st to end	Day	5% 5% 3 9% 7% 8% 9% 7% 10% 13% 24%	5922
August	1st to 10th	Day	4 3 1 5% 4 5% 7% 5% 8% 13% 45%	3905
	11th to 20th	Day	6% 4 2 7% 5% 6% 8% 6% 10% 13% 33%	3849
	21st to end	Day	7% 7% 4 11% 8% 6% 6% 5% 7% 9% 31%	4312
September	1st to 10th	Day	10% 6% 3 8% 6% 6% 7% 6% 8% 10% 30%	2400
	11th to 20th	Day	10% 6% 2 7% 5% 6% 6% 5% 7% 9% 37%	2330
	21st to end	Day	7% 4 2 6% 5% 5% 6% 5% 6% 8% 46%	2200
October	1st to 10th	Day	7% 3 2 5% 4 4 5% 4 6% 9% 52%	2136
	11th to 20th	Day	6% 4 2 5% 3 3 4 4 6% 8% 56%	2000
	21st to end	Day	9% 4 2 5% 4 3 4 3 6% 7% 55%	2200
November	1st to 10th	Day	8% 4 1 5% 3 3 4 4 5% 7% 53%	2058
	11th to 20th	Day	10% 6% 2 4 3 3 3 3 4 6% 55%	2039
	21st to end	Day	12% 6% 2 6% 4 4 5% 5% 6% 7% 43%	1890
December	1st to 10th	Day	18% 6% 2 5% 4 5% 4 4 5% 7% 40%	3020
	11th to 20th	Day	9% 6% 2 5% 4 3 4 4 5% 8% 50%	3632
	21st to end	Day	16% 8% 2 7% 4 4 4 3 5% 6% 43%	5082
January	1st to 10th	Night	14% 8% 3 7% 4 4 4 3 5% 6% 43%	7037
	11th to 20th	Night	16% 7% 2 5% 3 3 3 3 5% 7% 46%	7080
	21st to end	Night	18% 8% 3 7% 4 4 4 3 5% 7% 37%	7745
February	1st to 10th	Night	14% 6% 3 7% 5% 4 4 3 5% 6% 43%	2520
	11th to 20th	Night	17% 8% 3 6% 4 3 3 3 5% 6% 42%	2458
	21st to end	Night	17% 7% 3 5% 5% 4 4 3 4 6% 41%	2076
March	1st to 10th	Night	18% 9% 3 6% 4 3 4 3 4 6% 38%	5310
	11th to 20th	Night	10% 6% 2 5% 4 4 4 4 5% 6% 50%	5090
	21st to end	Night	14% 6% 2 5% 3 4 4 3 5% 6% 48%	5292
April	1st to 10th	Night	14% 5% 2 6% 4 4 4 4 6% 6% 45%	2100
	11th to 20th	Night	12% 5% 2 6% 4 4 4 3 5% 7% 47%	2075
	21st to end	Night	6% 4 2 4 4 3 5% 4 6% 8% 55%	2100
May	1st to 10th	Night	17% 7% 3 7% 5% 5% 4 4 5% 8% 35%	5832
	11th to 20th	Night	12% 5% 2 6% 4 5% 4 4 6% 8% 45%	5829
	21st to end	Night	5% 4 2 6% 5% 5% 5% 5% 8% 12% 42%	6415
June	1st to 10th	Night	7% 6% 2 5% 4 5% 5% 4 7% 11% 44%	2100
	11th to 20th	Night	3 4 1 5% 5% 5% 6% 5% 9% 11% 47%	2036
	21st to end	Night	5% 5% 2 5% 4 4 5% 5% 9% 11% 44%	1890
July	1st to 10th	Night	3 4 1 6% 5% 5% 6% 6% 9% 14% 42%	4168
	11th to 20th	Night	3 3 1 4 3 4 5% 5% 8% 13% 50%	4382
	21st to end	Night	2 3 2 6% 5% 6% 7% 6% 9% 13% 41%	4929
August	1st to 10th	Night	2 2 1 3 3 4 5% 4 8% 12% 57%	3244
	11th to 20th	Night	3 2 1 4 3 5% 5% 6% 9% 13% 48%	3200
	21st to end	Night	5% 4 2 7% 5% 6% 6% 6% 8% 10% 41%	3586
September	1st to 10th	Night	7% 5% 3 5% 4 5% 6% 5% 8% 11% 41%	2000
	11th to 20th	Night	7% 4 2 6% 5% 5% 4 5% 6% 11% 44%	2049
	21st to end	Night	8% 5% 2 5% 3 4 4 4 5% 8% 53%	2200
October	1st to 10th	Night	6% 4 1 5% 3 3 3 3 5% 8% 58%	2258
	11th to 20th	Night	6% 3 1 4 3 3 3 2 5% 7% 62%	2400
	21st to end	Night	9% 4 2 4 3 4 4 4 4 5% 57%	2640
November	1st to 10th	Night	9% 5% 1 5% 3 3 3 3 5% 5% 57%	2460
	11th to 20th	Night	11% 4 1 4 4 3 3 3 5% 6% 56%	2574
	21st to end	Night	9% 5% 2 6% 4 5% 5% 3 6% 7% 49%	2730
December	1st to 10th	Night	12% 7% 3 7% 5% 4 4 4 5% 7% 43%	4380
	11th to 20th	Night	10% 6% 2 6% 4 3 4 3 5% 6% 51%	4751
	21st to end	Night	16% 9% 3 7% 4 4 4 3 3 4 43%	6116

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	40% 6% 12% 17% 19% 5%	5857
	11th to 20th	Day	35% 4 7% 18% 27% 9%	5880
	21st to end	Day	50% 4 8% 12% 16% 10%	6436
February	1st to 10th	Day	43% 6% 9% 14% 19% 10%	2100
	11th to 20th	Day	40% 7% 9% 18% 18% 8%	2058
	21st to end	Day	45% 7% 11% 14% 18% 6%	1730
March	1st to 10th	Day	45% 7% 10% 16% 13% 9%	4804
	11th to 20th	Day	31% 8% 11% 17% 17% 15%	5090
	21st to end	Day	39% 7% 11% 19% 16% 7%	5866
April	1st to 10th	Day	36% 10% 13% 17% 16% 8%	2520
	11th to 20th	Day	38% 6% 10% 20% 18% 7%	2494
	21st to end	Day	30% 9% 14% 22% 20% 5%	2520
May	1st to 10th	Day	44% 10% 15% 18% 10% 3	7056
	11th to 20th	Day	32% 9% 16% 24% 18% 2	7035
	21st to end	Day	36% 13% 20% 23% 8% 1	7749
June	1st to 10th	Day	42% 10% 16% 21% 10% 1	2520
	11th to 20th	Day	40% 13% 19% 19% 9% 1	2573
	21st to end	Day	43% 15% 19% 16% 7%	2730
July	1st to 10th	Day	41% 14% 27% 16% 2	5966
	11th to 20th	Day	44% 17% 31% 7% 1	5603
	21st to end	Day	43% 15% 20% 20% 2	5752
August	1st to 10th	Day	26% 10% 16% 27% 21%	3893
	11th to 20th	Day	35% 12% 24% 23% 6% 1	3885
	21st to end	Day	47% 11% 14% 15% 12% 1	4311
September	1st to 10th	Day	43% 14% 16% 14% 10% 4	2400
	11th to 20th	Day	38% 11% 14% 18% 14% 4	2330
	21st to end	Day	31% 10% 12% 22% 15% 9%	2200
October	1st to 10th	Day	27% 5% 8% 14% 21% 24%	3474
	11th to 20th	Day	16% 4 7% 22% 27% 23%	3240
	21st to end	Day	20% 5% 7% 14% 39% 15%	3538
November	1st to 10th	Day	28% 7% 9% 23% 24% 9%	2058
	11th to 20th	Day	31% 5% 7% 20% 25% 12%	2039
	21st to end	Day	36% 7% 10% 19% 18% 10%	1890
December	1st to 10th	Day	44% 6% 9% 15% 22% 4	3020
	11th to 20th	Day	33% 7% 9% 21% 22% 8%	3630
	21st to end	Day	41% 7% 7% 16% 24% 4	5082
January	1st to 10th	Night	41% 5% 11% 15% 20% 8%	7038
	11th to 20th	Night	39% 4 5% 17% 25% 10%	7080
	21st to end	Night	48% 4 9% 13% 18% 9%	7745
February	1st to 10th	Night	40% 5% 9% 16% 20% 9%	2520
	11th to 20th	Night	42% 5% 8% 15% 18% 10%	2459
	21st to end	Night	41% 8% 11% 15% 14% 10%	2076
March	1st to 10th	Night	45% 7% 10% 12% 17% 10%	5310
	11th to 20th	Night	32% 8% 12% 14% 19% 15%	5090
	21st to end	Night	35% 6% 12% 20% 19% 7%	5292
April	1st to 10th	Night	36% 9% 12% 17% 15% 11%	2100
	11th to 20th	Night	36% 6% 10% 22% 17% 9%	2075
	21st to end	Night	25% 7% 14% 24% 23% 5%	2100
May	1st to 10th	Night	45% 9% 13% 18% 12% 3	5862
	11th to 20th	Night	35% 7% 16% 25% 15% 2	5830
	21st to end	Night	32% 10% 22% 26% 8% 1	6427
June	1st to 10th	Night	31% 12% 19% 26% 13%	2100
	11th to 20th	Night	27% 12% 23% 26% 12%	2036
	21st to end	Night	29% 13% 27% 22% 9%	1890
July	1st to 10th	Night	29% 13% 36% 20% 2	4125
	11th to 20th	Night	23% 16% 49% 11% 1	4330
	21st to end	Night	30% 17% 25% 25% 2	4785
August	1st to 10th	Night	18% 9% 25% 30% 18%	3232
	11th to 20th	Night	23% 13% 32% 29% 3	3230
	21st to end	Night	35% 14% 19% 20% 9% 3	3586
September	1st to 10th	Night	34% 12% 22% 17% 13% 3	2000
	11th to 20th	Night	31% 12% 16% 19% 16% 6%	2049
	21st to end	Night	28% 8% 14% 27% 14% 9%	2200
October	1st to 10th	Night	29% 3 10% 14% 21% 23%	3687
	11th to 20th	Night	16% 3 6% 21% 31% 23%	3899
	21st to end	Night	19% 4 7% 10% 40% 19%	4252
November	1st to 10th	Night	30% 5% 7% 22% 26% 11%	2460
	11th to 20th	Night	29% 5% 8% 19% 26% 13%	2574
	21st to end	Night	34% 6% 9% 21% 21% 10%	2730
December	1st to 10th	Night	42% 4 8% 18% 22% 6%	4380
	11th to 20th	Night	34% 6% 11% 21% 18% 11%	4752
	21st to end	Night	43% 6% 8% 14% 22% 7%	6116

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	19% 16% 37% 22% 5% 1	330
	11th to 20th	Day	14% 17% 37% 25% 6%	330
	21st to end	Day	24% 18% 35% 20% 3	363
February	1st to 10th	Day	20% 17% 36% 20% 6% 1	330
	11th to 20th	Day	21% 18% 35% 19% 6%	330
	21st to end	Day	13% 21% 43% 17% 5%	273
March	1st to 10th	Day	17% 15% 33% 31% 4	330
	11th to 20th	Day	16% 18% 38% 22% 5% 1	330
	21st to end	Day	13% 12% 33% 28% 12% 1	489
April	1st to 10th	Day	15% 13% 32% 28% 11% 1	540
	11th to 20th	Day	15% 14% 31% 25% 12% 2	532
	21st to end	Day	7% 8% 29% 31% 20% 4	540
May	1st to 10th	Day	13% 8% 25% 30% 22% 3	543
	11th to 20th	Day	10% 4 23% 30% 27% 7%	567
	21st to end	Day	7% 2 15% 34% 28% 14%	617
June	1st to 10th	Day	10% 3 10% 29% 32% 16%	570
	11th to 20th	Day	9% 2 12% 26% 32% 19%	623
	21st to end	Day	7% 2 9% 28% 33% 23%	780
July	1st to 10th	Day	8% 2 9% 21% 35% 26%	771
	11th to 20th	Day	8% 1 4 15% 38% 35%	619
	21st to end	Day	9% 6% 17% 36% 32%	576
August	1st to 10th	Day	5% 2 4 17% 35% 38%	526
	11th to 20th	Day	7% 4 13% 34% 41%	519
	21st to end	Day	9% 3 8% 18% 35% 27%	583
September	1st to 10th	Day	12% 2 8% 23% 34% 23%	530
	11th to 20th	Day	9% 2 11% 22% 38% 17%	530
	21st to end	Day	7% 3 9% 29% 38% 13%	530
October	1st to 10th	Day	6% 5% 19% 38% 27% 5%	466
	11th to 20th	Day	5% 4 23% 39% 24% 6%	330
	21st to end	Day	15% 8% 23% 34% 19% 2	363
November	1st to 10th	Day	13% 9% 19% 38% 18% 3	330
	11th to 20th	Day	15% 9% 29% 31% 13% 3	330
	21st to end	Day	12% 14% 35% 30% 8% 1	330
December	1st to 10th	Day	18% 10% 36% 23% 9% 3	330
	11th to 20th	Day	16% 14% 35% 28% 6% 1	330
	21st to end	Day	21% 13% 28% 26% 11% 1	363
January	1st to 10th	Night	17% 11% 31% 34% 7% 1	840
	11th to 20th	Night	17% 12% 25% 36% 9% 1	840
	21st to end	Night	19% 12% 32% 30% 6%	924
February	1st to 10th	Night	16% 11% 30% 34% 8%	840
	11th to 20th	Night	20% 13% 29% 29% 10%	823
	21st to end	Night	19% 13% 33% 30% 6%	692
March	1st to 10th	Night	17% 11% 30% 30% 11%	761
	11th to 20th	Night	14% 7% 28% 34% 16% 1	630
	21st to end	Night	10% 11% 27% 35% 17% 1	693
April	1st to 10th	Night	14% 11% 26% 34% 14% 1	630
	11th to 20th	Night	14% 8% 28% 31% 17% 1	623
	21st to end	Night	8% 7% 22% 39% 22% 2	630
May	1st to 10th	Night	8% 5% 19% 42% 25%	630
	11th to 20th	Night	8% 3 17% 41% 28% 3	621
	21st to end	Night	5% 1 13% 33% 38% 10%	683
June	1st to 10th	Night	8% 2 7% 33% 41% 9%	630
	11th to 20th	Night	4 2 5% 26% 49% 15%	630
	21st to end	Night	7% 6% 21% 46% 21%	630
July	1st to 10th	Night	6% 1 4 13% 51% 26%	615
	11th to 20th	Night	5% 1 2 7% 48% 38%	597
	21st to end	Night	3 3 14% 52% 28%	657
August	1st to 10th	Night	2 2 12% 49% 35%	597
	11th to 20th	Night	6% 2 9% 46% 37%	594
	21st to end	Night	8% 1 4 17% 46% 25%	660
September	1st to 10th	Night	9% 1 5% 17% 45% 24%	600
	11th to 20th	Night	8% 3 6% 23% 46% 14%	649
	21st to end	Night	11% 1 9% 30% 40% 10%	800
October	1st to 10th	Night	8% 3 12% 37% 35% 6%	800
	11th to 20th	Night	7% 4 17% 35% 32% 6%	800
	21st to end	Night	10% 7% 18% 35% 27% 3	880
November	1st to 10th	Night	11% 5% 17% 37% 26% 3	820
	11th to 20th	Night	12% 8% 20% 37% 20% 3	840
	21st to end	Night	12% 9% 26% 35% 17% 2	840
December	1st to 10th	Night	16% 10% 26% 30% 17% 1	840
	11th to 20th	Night	17% 7% 26% 35% 14% 1	840
	21st to end	Night	22% 10% 27% 30% 10% 1	924

Page generated at 2026-07-13 02:14:47. Version ac07

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