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 2025-03-12.

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	11% 6% 2 7% 5% 5% 6% 5% 7% 9% 38%	109611
	Night	11% 6% 2 5% 4 4 4 4 6% 8% 46%	109372
Transparency	Day	38% 9% 13% 18% 16% 6%	113348
	Night	34% 8% 14% 19% 18% 7%	113762
Seeing	Day	12% 7% 20% 25% 23% 13%	14938
	Night	12% 6% 18% 30% 26% 8%	24462

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	16% 5% 3 6% 4 4 5% 4 6% 7% 41%	4516
	11th to 20th	Day	14% 8% 2 6% 4 3 4 3 5% 5% 47%	4520
	21st to end	Day	20% 7% 2 7% 4 5% 5% 4 5% 6% 35%	4972
February	1st to 10th	Day	15% 8% 3 7% 4 4 5% 3 5% 7% 39%	2000
	11th to 20th	Day	14% 7% 2 6% 4 5% 3 4 5% 8% 42%	1948
	21st to end	Day	16% 7% 2 7% 5% 4 5% 4 5% 6% 38%	1650
March	1st to 10th	Day	21% 9% 3 8% 4 5% 5% 4 7% 7% 28%	3375
	11th to 20th	Day	14% 6% 2 7% 5% 4 5% 5% 7% 9% 36%	3491
	21st to end	Day	11% 6% 2 5% 4 5% 5% 4 6% 8% 45%	4005
April	1st to 10th	Day	10% 6% 2 5% 4 5% 5% 5% 7% 9% 42%	2280
	11th to 20th	Day	13% 6% 3 7% 5% 4 4 4 6% 9% 40%	2254
	21st to end	Day	8% 5% 2 5% 5% 4 4 4 7% 8% 47%	2280
May	1st to 10th	Day	8% 4 1 7% 5% 6% 5% 5% 8% 11% 38%	3765
	11th to 20th	Day	11% 5% 2 7% 5% 5% 5% 5% 6% 8% 41%	3764
	21st to end	Day	5% 4 2 5% 5% 6% 7% 5% 7% 10% 45%	4129
June	1st to 10th	Day	9% 6% 3 8% 6% 6% 7% 5% 8% 11% 30%	2280
	11th to 20th	Day	5% 5% 2 7% 7% 7% 8% 7% 10% 10% 29%	2327
	21st to end	Day	6% 7% 3 9% 7% 8% 8% 7% 10% 11% 24%	2470
July	1st to 10th	Day	7% 7% 3 9% 7% 7% 9% 8% 10% 12% 21%	4120
	11th to 20th	Day	5% 5% 3 10% 7% 8% 10% 7% 10% 12% 21%	3910
	21st to end	Day	7% 6% 3 9% 8% 8% 9% 8% 10% 12% 20%	4118
August	1st to 10th	Day	4 3 1 5% 4 4 6% 5% 8% 13% 46%	3785
	11th to 20th	Day	6% 4 2 7% 5% 7% 8% 6% 10% 12% 33%	3729
	21st to end	Day	7% 7% 4 11% 8% 6% 6% 5% 7% 9% 31%	4180
September	1st to 10th	Day	10% 7% 3 8% 6% 6% 7% 6% 8% 9% 29%	2280
	11th to 20th	Day	10% 6% 2 7% 5% 6% 6% 6% 7% 9% 35%	2214
	21st to end	Day	7% 4 2 6% 5% 5% 6% 5% 6% 8% 46%	2090
October	1st to 10th	Day	7% 3 1 4 4 4 5% 4 6% 9% 52%	2029
	11th to 20th	Day	6% 4 2 5% 4 3 4 4 6% 8% 55%	1900
	21st to end	Day	9% 4 2 5% 4 3 4 3 6% 7% 54%	2090
November	1st to 10th	Day	8% 5% 2 5% 3 3 4 4 5% 8% 53%	1958
	11th to 20th	Day	11% 6% 2 4 3 4 3 3 4 6% 54%	1942
	21st to end	Day	12% 6% 2 6% 4 5% 5% 5% 6% 7% 43%	1800
December	1st to 10th	Day	17% 6% 2 5% 4 5% 4 4 5% 7% 41%	2930
	11th to 20th	Day	9% 6% 2 5% 4 4 4 4 5% 8% 49%	3538
	21st to end	Day	16% 8% 2 7% 4 4 4 3 5% 6% 43%	4972
January	1st to 10th	Night	15% 8% 3 7% 4 4 3 3 4 5% 45%	5429
	11th to 20th	Night	19% 8% 2 5% 2 3 3 2 4 6% 46%	5440
	21st to end	Night	19% 8% 2 4 4 4 4 3 5% 7% 39%	5984
February	1st to 10th	Night	14% 6% 3 7% 5% 4 4 3 5% 6% 42%	2400
	11th to 20th	Night	17% 8% 3 6% 4 3 3 3 5% 6% 42%	2338
	21st to end	Night	17% 7% 3 5% 5% 4 4 3 4 6% 41%	1980
March	1st to 10th	Night	22% 11% 3 7% 4 3 4 3 4 7% 31%	3726
	11th to 20th	Night	12% 8% 2 6% 5% 5% 5% 4 7% 7% 39%	3497
	21st to end	Night	14% 6% 2 5% 3 4 4 4 6% 7% 47%	3611
April	1st to 10th	Night	13% 5% 2 6% 4 4 4 4 6% 7% 46%	1900
	11th to 20th	Night	13% 5% 3 7% 4 4 4 3 5% 7% 45%	1875
	21st to end	Night	6% 4 1 4 3 3 5% 4 6% 8% 56%	1900
May	1st to 10th	Night	12% 5% 2 7% 5% 5% 4 4 5% 8% 43%	3130
	11th to 20th	Night	14% 5% 2 6% 4 4 4 4 5% 7% 43%	3129
	21st to end	Night	5% 3 1 4 4 4 4 4 8% 13% 50%	3426
June	1st to 10th	Night	7% 6% 2 5% 3 4 5% 4 7% 11% 45%	1900
	11th to 20th	Night	3 4 1 5% 5% 4 6% 5% 9% 11% 47%	1842
	21st to end	Night	6% 6% 2 5% 4 4 5% 5% 8% 11% 44%	1710
July	1st to 10th	Night	4 5% 1 6% 5% 5% 6% 6% 9% 14% 39%	2850
	11th to 20th	Night	3 3 1 4 4 4 5% 5% 9% 14% 50%	3031
	21st to end	Night	3 4 2 6% 5% 6% 7% 6% 10% 15% 37%	3435
August	1st to 10th	Night	1 2 1 3 3 4 5% 4 8% 12% 57%	3144
	11th to 20th	Night	4 2 1 4 3 5% 5% 6% 9% 13% 48%	3100
	21st to end	Night	5% 4 2 7% 5% 6% 6% 6% 8% 10% 41%	3476
September	1st to 10th	Night	7% 6% 3 6% 4 5% 6% 5% 8% 11% 41%	1900
	11th to 20th	Night	7% 4 2 6% 5% 5% 4 5% 6% 11% 43%	1946
	21st to end	Night	8% 5% 1 5% 3 4 4 4 5% 8% 53%	2090
October	1st to 10th	Night	6% 4 1 5% 3 3 3 3 5% 8% 59%	2145
	11th to 20th	Night	6% 3 1 4 3 3 3 2 5% 8% 61%	2280
	21st to end	Night	9% 4 2 5% 3 4 4 4 4 5% 57%	2508
November	1st to 10th	Night	9% 5% 2 5% 3 3 3 3 5% 5% 56%	2340
	11th to 20th	Night	11% 5% 1 4 4 3 4 3 5% 6% 54%	2451
	21st to end	Night	9% 6% 2 6% 4 5% 4 3 6% 6% 49%	2600
December	1st to 10th	Night	11% 7% 3 7% 5% 4 4 4 5% 7% 43%	4250
	11th to 20th	Night	10% 6% 2 6% 4 4 4 3 5% 6% 51%	4625
	21st to end	Night	16% 9% 3 7% 4 4 4 3 3 4 44%	5984

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% 8% 17% 21% 7%	4517
	11th to 20th	Day	38% 5% 7% 15% 28% 7%	4520
	21st to end	Day	50% 5% 8% 13% 16% 8%	4972
February	1st to 10th	Day	45% 6% 9% 13% 17% 10%	2000
	11th to 20th	Day	40% 7% 9% 17% 19% 8%	1958
	21st to end	Day	45% 7% 11% 14% 18% 6%	1650
March	1st to 10th	Day	50% 9% 12% 15% 11% 4	3375
	11th to 20th	Day	39% 11% 14% 19% 13% 4	3491
	21st to end	Day	34% 9% 13% 16% 19% 8%	4005
April	1st to 10th	Day	35% 10% 13% 16% 17% 9%	2280
	11th to 20th	Day	39% 7% 11% 20% 17% 7%	2254
	21st to end	Day	30% 9% 12% 21% 22% 6%	2280
May	1st to 10th	Day	33% 14% 18% 20% 11% 4	3800
	11th to 20th	Day	36% 10% 12% 18% 21% 3	3768
	21st to end	Day	27% 12% 17% 33% 10% 1	4141
June	1st to 10th	Day	42% 10% 16% 21% 9% 1	2280
	11th to 20th	Day	39% 13% 18% 20% 10% 1	2327
	21st to end	Day	45% 15% 18% 14% 7%	2470
July	1st to 10th	Day	46% 15% 23% 14% 1	4056
	11th to 20th	Day	46% 17% 28% 7% 1	3843
	21st to end	Day	49% 15% 20% 13% 2	3987
August	1st to 10th	Day	25% 9% 16% 27% 22% 1	3773
	11th to 20th	Day	35% 12% 23% 23% 6% 1	3765
	21st to end	Day	48% 11% 13% 15% 12% 1	4179
September	1st to 10th	Day	44% 14% 16% 13% 10% 4	2280
	11th to 20th	Day	40% 12% 14% 16% 13% 5%	2214
	21st to end	Day	32% 10% 12% 22% 15% 9%	2090
October	1st to 10th	Day	27% 5% 8% 13% 21% 25%	3367
	11th to 20th	Day	17% 4 7% 23% 27% 23%	3140
	21st to end	Day	20% 5% 8% 13% 39% 16%	3428
November	1st to 10th	Day	28% 7% 9% 23% 24% 8%	1958
	11th to 20th	Day	32% 5% 7% 20% 25% 12%	1942
	21st to end	Day	36% 7% 9% 19% 18% 10%	1800
December	1st to 10th	Day	43% 6% 9% 15% 22% 4	2930
	11th to 20th	Day	33% 7% 9% 21% 22% 8%	3536
	21st to end	Day	41% 7% 8% 15% 25% 4	4972
January	1st to 10th	Night	41% 5% 7% 15% 23% 8%	5429
	11th to 20th	Night	40% 4 6% 16% 28% 7%	5440
	21st to end	Night	44% 4 10% 14% 17% 10%	5984
February	1st to 10th	Night	42% 5% 9% 17% 19% 9%	2400
	11th to 20th	Night	42% 5% 8% 16% 19% 11%	2339
	21st to end	Night	41% 8% 11% 15% 14% 11%	1980
March	1st to 10th	Night	50% 9% 11% 12% 12% 6%	3726
	11th to 20th	Night	38% 11% 15% 15% 14% 7%	3497
	21st to end	Night	33% 8% 12% 21% 19% 7%	3611
April	1st to 10th	Night	35% 9% 12% 18% 15% 11%	1900
	11th to 20th	Night	38% 6% 11% 21% 16% 9%	1875
	21st to end	Night	24% 7% 12% 25% 25% 6%	1900
May	1st to 10th	Night	34% 12% 15% 21% 14% 4	3160
	11th to 20th	Night	38% 9% 13% 20% 19% 1	3130
	21st to end	Night	23% 10% 20% 33% 13% 1	3435
June	1st to 10th	Night	30% 11% 19% 27% 12%	1900
	11th to 20th	Night	26% 12% 22% 27% 13%	1842
	21st to end	Night	30% 13% 27% 20% 10%	1710
July	1st to 10th	Night	31% 14% 36% 17% 2	2805
	11th to 20th	Night	21% 17% 48% 11% 2	2976
	21st to end	Night	32% 20% 28% 18% 2	3325
August	1st to 10th	Night	17% 10% 24% 30% 19%	3132
	11th to 20th	Night	23% 13% 32% 29% 3	3130
	21st to end	Night	36% 13% 19% 20% 9% 3	3476
September	1st to 10th	Night	34% 12% 21% 17% 12% 3	1900
	11th to 20th	Night	33% 13% 16% 18% 15% 6%	1946
	21st to end	Night	28% 9% 13% 27% 14% 9%	2090
October	1st to 10th	Night	28% 3 10% 14% 21% 23%	3574
	11th to 20th	Night	16% 3 6% 21% 31% 23%	3779
	21st to end	Night	19% 4 7% 10% 40% 20%	4120
November	1st to 10th	Night	30% 5% 8% 22% 24% 11%	2340
	11th to 20th	Night	31% 5% 8% 18% 26% 13%	2451
	21st to end	Night	34% 6% 8% 21% 21% 10%	2600
December	1st to 10th	Night	41% 5% 8% 19% 22% 6%	4250
	11th to 20th	Night	34% 6% 11% 21% 18% 11%	4626
	21st to end	Night	43% 6% 8% 14% 22% 7%	5984

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	20% 14% 39% 20% 5% 1	300
	11th to 20th	Day	15% 17% 36% 26% 6%	300
	21st to end	Day	21% 18% 37% 21% 3	330
February	1st to 10th	Day	21% 18% 33% 21% 6% 1	300
	11th to 20th	Day	21% 17% 35% 21% 6%	300
	21st to end	Day	14% 19% 44% 18% 6%	249
March	1st to 10th	Day	18% 16% 32% 29% 3	300
	11th to 20th	Day	15% 17% 40% 22% 5% 1	273
	21st to end	Day	14% 13% 34% 28% 11% 1	411
April	1st to 10th	Day	14% 12% 32% 29% 11% 2	460
	11th to 20th	Day	17% 15% 29% 25% 12% 2	452
	21st to end	Day	8% 8% 29% 30% 22% 4	460
May	1st to 10th	Day	10% 6% 24% 33% 24% 3	463
	11th to 20th	Day	11% 3 19% 30% 29% 8%	487
	21st to end	Day	6% 1 13% 35% 30% 16%	529
June	1st to 10th	Day	11% 2 9% 27% 35% 17%	490
	11th to 20th	Day	9% 2 11% 25% 34% 19%	537
	21st to end	Day	8% 2 9% 27% 33% 22%	680
July	1st to 10th	Day	9% 2 9% 20% 34% 26%	671
	11th to 20th	Day	7% 1 4 14% 38% 37%	551
	21st to end	Day	9% 5% 16% 35% 33%	532
August	1st to 10th	Day	5% 1 4 17% 35% 39%	486
	11th to 20th	Day	8% 5% 12% 33% 42%	479
	21st to end	Day	10% 3 9% 18% 34% 27%	539
September	1st to 10th	Day	13% 1 7% 20% 35% 24%	490
	11th to 20th	Day	10% 2 12% 21% 38% 17%	490
	21st to end	Day	7% 3 9% 28% 38% 14%	490
October	1st to 10th	Day	7% 5% 18% 38% 27% 5%	429
	11th to 20th	Day	5% 4 22% 39% 24% 6%	300
	21st to end	Day	15% 8% 21% 34% 20% 2	330
November	1st to 10th	Day	13% 9% 18% 37% 19% 3	300
	11th to 20th	Day	16% 10% 30% 31% 11% 2	300
	21st to end	Day	13% 13% 34% 31% 8% 1	300
December	1st to 10th	Day	16% 9% 36% 25% 10% 3	300
	11th to 20th	Day	16% 16% 34% 28% 6%	300
	21st to end	Day	23% 12% 30% 25% 9% 1	330
January	1st to 10th	Night	17% 11% 31% 33% 7% 1	800
	11th to 20th	Night	17% 12% 26% 36% 9% 1	800
	21st to end	Night	19% 11% 33% 30% 7%	880
February	1st to 10th	Night	17% 10% 30% 34% 8% 1	800
	11th to 20th	Night	20% 13% 28% 29% 10%	783
	21st to end	Night	19% 12% 33% 30% 6%	660
March	1st to 10th	Night	16% 11% 31% 30% 11%	725
	11th to 20th	Night	14% 7% 27% 35% 16% 1	575
	21st to end	Night	11% 11% 27% 34% 16% 1	627
April	1st to 10th	Night	14% 11% 26% 33% 15% 1	570
	11th to 20th	Night	15% 8% 28% 31% 17% 1	563
	21st to end	Night	8% 6% 22% 39% 22% 2	570
May	1st to 10th	Night	6% 5% 19% 42% 27% 1	570
	11th to 20th	Night	7% 3 16% 41% 29% 4	561
	21st to end	Night	5% 1 12% 33% 39% 10%	617
June	1st to 10th	Night	8% 1 6% 32% 43% 10%	570
	11th to 20th	Night	3 2 5% 26% 49% 15%	570
	21st to end	Night	7% 1 6% 22% 43% 21%	570
July	1st to 10th	Night	6% 1 4 14% 50% 26%	555
	11th to 20th	Night	4 1 2 7% 48% 38%	552
	21st to end	Night	3 3 13% 52% 29%	624
August	1st to 10th	Night	2 2 12% 49% 35%	567
	11th to 20th	Night	6% 2 9% 45% 37%	564
	21st to end	Night	8% 1 4 17% 45% 25%	627
September	1st to 10th	Night	10% 1 5% 16% 45% 25%	570
	11th to 20th	Night	8% 3 6% 23% 46% 14%	616
	21st to end	Night	11% 1 9% 29% 39% 10%	760
October	1st to 10th	Night	8% 2 12% 36% 36% 6%	760
	11th to 20th	Night	7% 4 17% 34% 32% 6%	760
	21st to end	Night	10% 7% 18% 35% 27% 3	836
November	1st to 10th	Night	11% 6% 18% 37% 26% 3	780
	11th to 20th	Night	13% 8% 20% 37% 20% 3	800
	21st to end	Night	12% 8% 25% 36% 17% 2	800
December	1st to 10th	Night	15% 10% 25% 31% 18% 2	800
	11th to 20th	Night	17% 7% 27% 36% 14% 1	800
	21st to end	Night	23% 10% 27% 30% 10% 1	880

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