Girard Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Girard Observatory clear sky chart from 2005-11-06 to 2026-06-01.

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	18% 12% 4 10% 5% 5% 5% 4 5% 6% 26%	132068
	Night	17% 15% 5% 11% 6% 5% 4 3 4 5% 25%	131093
Transparency	Day	59% 5% 11% 18% 7% 1	135536
	Night	62% 5% 10% 16% 8% 1	135712
Seeing	Day	28% 17% 29% 17% 7% 2	16594
	Night	26% 15% 27% 23% 8% 1	25569

By Month

Percentage Cloud Cover NE (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	13% 21% 9% 19% 8% 6% 4 2 3 3 12%	4661
	11th to 20th	Day	8% 16% 9% 23% 12% 7% 5% 3 4 3 9%	4650
	21st to end	Day	9% 16% 7% 14% 10% 12% 6% 3 5% 5% 13%	5124
February	1st to 10th	Day	14% 17% 6% 15% 9% 6% 5% 3 3 4 17%	1790
	11th to 20th	Day	13% 19% 8% 15% 8% 5% 5% 3 4 4 17%	2022
	21st to end	Day	13% 17% 6% 13% 8% 7% 4 3 4 5% 20%	1730
March	1st to 10th	Day	19% 16% 8% 14% 5% 5% 5% 3 3 4 19%	4576
	11th to 20th	Day	13% 18% 5% 11% 5% 4 4 3 5% 5% 26%	4746
	21st to end	Day	20% 15% 6% 11% 5% 4 3 3 3 4 26%	6102
April	1st to 10th	Day	22% 17% 4 10% 4 4 4 2 4 5% 25%	2520
	11th to 20th	Day	20% 15% 4 8% 4 4 4 3 3 4 32%	2503
	21st to end	Day	25% 12% 3 7% 5% 4 4 4 4 6% 27%	2730
May	1st to 10th	Day	29% 13% 3 8% 4 5% 4 4 4 6% 20%	6194
	11th to 20th	Day	22% 8% 3 9% 5% 5% 6% 4 6% 7% 27%	6458
	21st to end	Day	22% 9% 3 6% 4 4 5% 4 6% 6% 30%	7103
June	1st to 10th	Day	27% 9% 2 6% 4 4 4 5% 5% 7% 28%	2800
	11th to 20th	Day	19% 7% 2 6% 4 4 4 4 7% 8% 35%	2800
	21st to end	Day	25% 8% 2 6% 4 5% 5% 4 5% 6% 28%	2800
July	1st to 10th	Day	16% 7% 3 7% 5% 5% 6% 6% 7% 9% 28%	6360
	11th to 20th	Day	15% 6% 2 6% 5% 5% 5% 5% 7% 9% 35%	6360
	21st to end	Day	15% 7% 2 5% 4 4 5% 4 6% 9% 40%	6926
August	1st to 10th	Day	19% 8% 3 7% 5% 5% 5% 4 7% 8% 29%	4563
	11th to 20th	Day	19% 7% 3 7% 5% 5% 5% 3 7% 8% 31%	4124
	21st to end	Day	20% 6% 2 5% 3 4 4 4 6% 7% 38%	4312
September	1st to 10th	Day	16% 6% 3 6% 3 4 4 4 5% 7% 41%	2400
	11th to 20th	Day	18% 6% 2 4 4 3 4 3 4 7% 45%	2266
	21st to end	Day	20% 6% 3 5% 4 4 4 5% 6% 8% 36%	2200
October	1st to 10th	Day	23% 9% 3 7% 4 4 4 4 5% 6% 31%	2140
	11th to 20th	Day	22% 12% 3 7% 5% 4 5% 3 5% 5% 28%	1910
	21st to end	Day	27% 17% 4 8% 3 3 4 2 4 3 24%	1980
November	1st to 10th	Day	20% 18% 4 7% 4 4 3 4 5% 5% 26%	1851
	11th to 20th	Day	17% 21% 7% 11% 4 4 4 3 4 4 23%	1890
	21st to end	Day	18% 22% 7% 12% 5% 4 4 3 5% 4 17%	1683
December	1st to 10th	Day	16% 23% 8% 19% 7% 4 4 2 4 3 10%	2680
	11th to 20th	Day	18% 19% 7% 15% 5% 4 4 2 4 4 19%	3066
	21st to end	Day	18% 18% 5% 14% 6% 5% 4 3 4 4 18%	4048
January	1st to 10th	Night	12% 22% 8% 18% 8% 5% 4 2 3 3 14%	8234
	11th to 20th	Night	9% 21% 9% 21% 9% 6% 4 3 3 3 10%	8235
	21st to end	Night	9% 16% 6% 15% 12% 9% 5% 4 4 4 17%	9057
February	1st to 10th	Night	14% 18% 6% 16% 7% 5% 4 3 4 4 18%	2826
	11th to 20th	Night	13% 17% 7% 15% 8% 6% 4 3 4 4 19%	2484
	21st to end	Night	13% 19% 5% 12% 6% 5% 4 3 4 4 24%	2076
March	1st to 10th	Night	18% 16% 6% 10% 6% 5% 3 3 4 4 25%	5484
	11th to 20th	Night	14% 17% 5% 11% 5% 5% 4 3 5% 6% 24%	5415
	21st to end	Night	16% 20% 6% 12% 5% 4 4 3 3 4 25%	5072
April	1st to 10th	Night	23% 14% 5% 9% 4 4 3 3 5% 4 26%	2100
	11th to 20th	Night	24% 13% 3 7% 5% 4 4 2 3 5% 31%	2065
	21st to end	Night	21% 12% 3 8% 4 3 3 3 5% 5% 32%	1890
May	1st to 10th	Night	27% 12% 4 9% 4 4 4 3 4 6% 23%	4008
	11th to 20th	Night	24% 8% 3 7% 5% 5% 4 3 5% 6% 30%	3655
	21st to end	Night	21% 10% 3 7% 5% 4 4 3 4 6% 32%	4014
June	1st to 10th	Night	27% 8% 3 5% 4 3 4 4 5% 8% 29%	1600
	11th to 20th	Night	20% 6% 3 6% 3 4 5% 3 6% 7% 36%	1600
	21st to end	Night	20% 9% 2 7% 4 4 4 5% 7% 7% 31%	1600
July	1st to 10th	Night	15% 8% 3 8% 6% 5% 6% 4 7% 8% 30%	3600
	11th to 20th	Night	19% 7% 2 6% 4 4 4 3 5% 7% 38%	3600
	21st to end	Night	14% 6% 2 7% 5% 5% 4 4 6% 7% 39%	3926
August	1st to 10th	Night	23% 8% 3 7% 4 3 4 3 6% 6% 34%	2586
	11th to 20th	Night	17% 7% 3 8% 5% 5% 6% 4 6% 7% 33%	2890
	21st to end	Night	18% 7% 2 5% 4 4 4 4 5% 6% 42%	3586
September	1st to 10th	Night	17% 7% 2 6% 4 4 4 3 4 6% 43%	2000
	11th to 20th	Night	16% 5% 2 6% 3 3 4 4 5% 6% 47%	2133
	21st to end	Night	21% 6% 2 5% 4 3 4 3 4 6% 41%	2200
October	1st to 10th	Night	21% 9% 3 7% 4 4 4 3 5% 5% 36%	2257
	11th to 20th	Night	22% 11% 3 8% 5% 4 4 3 4 5% 30%	2490
	21st to end	Night	26% 15% 3 7% 4 4 4 3 4 5% 25%	2860
November	1st to 10th	Night	22% 15% 4 7% 5% 4 5% 3 5% 6% 26%	2667
	11th to 20th	Night	19% 17% 6% 9% 5% 4 4 3 4 5% 26%	2730
	21st to end	Night	19% 20% 7% 11% 5% 3 3 3 4 4 21%	2935
December	1st to 10th	Night	15% 26% 11% 17% 6% 4 3 2 3 3 10%	4720
	11th to 20th	Night	19% 22% 7% 16% 5% 4 3 2 3 3 16%	5348
	21st to end	Night	18% 19% 5% 12% 6% 4 4 2 3 4 21%	7150

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	79% 3 8% 8% 1	4662
	11th to 20th	Day	80% 4 6% 6% 3	4680
	21st to end	Day	74% 3 6% 9% 7%	5124
February	1st to 10th	Day	72% 5% 6% 12% 5% 1	1790
	11th to 20th	Day	72% 4 8% 11% 4 1	2031
	21st to end	Day	66% 6% 8% 12% 6% 1	1730
March	1st to 10th	Day	70% 4 7% 9% 7% 3	4579
	11th to 20th	Day	60% 5% 13% 12% 9% 1	4746
	21st to end	Day	63% 3 6% 13% 13% 1	6102
April	1st to 10th	Day	63% 4 7% 17% 8% 1	2520
	11th to 20th	Day	57% 4 7% 18% 12% 1	2503
	21st to end	Day	59% 5% 8% 17% 9% 2	2730
May	1st to 10th	Day	65% 6% 9% 15% 5% 1	6205
	11th to 20th	Day	54% 8% 13% 16% 7% 1	6462
	21st to end	Day	52% 7% 12% 20% 9% 1	7103
June	1st to 10th	Day	54% 7% 12% 18% 9% 1	2800
	11th to 20th	Day	46% 7% 12% 23% 11% 2	2800
	21st to end	Day	55% 7% 10% 18% 8% 1	2800
July	1st to 10th	Day	51% 4 15% 23% 7%	6290
	11th to 20th	Day	46% 4 14% 29% 7%	6285
	21st to end	Day	42% 5% 12% 34% 7%	6728
August	1st to 10th	Day	53% 5% 16% 20% 6%	4549
	11th to 20th	Day	51% 5% 18% 21% 5%	4163
	21st to end	Day	45% 4 17% 20% 12% 1	4312
September	1st to 10th	Day	41% 7% 13% 28% 10% 1	2400
	11th to 20th	Day	40% 4 12% 31% 13%	2266
	21st to end	Day	45% 7% 14% 23% 10% 1	2200
October	1st to 10th	Day	49% 4 13% 26% 8%	3491
	11th to 20th	Day	54% 4 13% 22% 7%	3087
	21st to end	Day	55% 3 14% 20% 7% 1	3180
November	1st to 10th	Day	60% 6% 10% 16% 7%	1851
	11th to 20th	Day	66% 6% 8% 12% 7% 1	1890
	21st to end	Day	70% 6% 11% 9% 3	1683
December	1st to 10th	Day	81% 3 7% 7% 1	2680
	11th to 20th	Day	71% 3 7% 14% 6%	3066
	21st to end	Day	71% 4 6% 10% 8% 1	4048
January	1st to 10th	Night	76% 4 8% 9% 2	8234
	11th to 20th	Night	80% 4 6% 7% 3	8280
	21st to end	Night	72% 3 6% 9% 9%	9057
February	1st to 10th	Night	71% 5% 6% 13% 5%	2826
	11th to 20th	Night	69% 5% 8% 13% 5%	2486
	21st to end	Night	64% 4 8% 14% 9% 1	2076
March	1st to 10th	Night	65% 4 8% 13% 9% 2	5494
	11th to 20th	Night	61% 5% 12% 13% 8% 1	5416
	21st to end	Night	65% 3 5% 17% 9% 1	5072
April	1st to 10th	Night	61% 4 8% 16% 10% 1	2100
	11th to 20th	Night	57% 5% 8% 18% 12%	2065
	21st to end	Night	54% 5% 9% 18% 12% 2	1890
May	1st to 10th	Night	63% 6% 9% 14% 6% 1	4014
	11th to 20th	Night	54% 7% 12% 19% 8% 1	3656
	21st to end	Night	53% 6% 10% 19% 11% 1	4015
June	1st to 10th	Night	53% 6% 10% 21% 9% 1	1600
	11th to 20th	Night	45% 7% 13% 21% 11% 3	1600
	21st to end	Night	50% 9% 13% 18% 9% 1	1600
July	1st to 10th	Night	50% 4 18% 20% 8%	3560
	11th to 20th	Night	47% 4 13% 28% 8%	3552
	21st to end	Night	44% 4 15% 27% 9%	3809
August	1st to 10th	Night	51% 6% 14% 23% 6%	2576
	11th to 20th	Night	50% 6% 17% 19% 7%	2917
	21st to end	Night	44% 5% 13% 24% 13% 1	3586
September	1st to 10th	Night	42% 6% 12% 28% 11% 1	2000
	11th to 20th	Night	38% 5% 15% 29% 12%	2133
	21st to end	Night	44% 6% 11% 25% 13% 1	2200
October	1st to 10th	Night	53% 4 10% 24% 10%	3686
	11th to 20th	Night	52% 4 10% 23% 10%	4052
	21st to end	Night	55% 4 13% 17% 10% 1	4610
November	1st to 10th	Night	59% 5% 11% 17% 6% 1	2667
	11th to 20th	Night	61% 5% 9% 14% 8% 2	2730
	21st to end	Night	68% 5% 11% 12% 3	2935
December	1st to 10th	Night	82% 3 7% 6% 2	4720
	11th to 20th	Night	75% 3 6% 10% 6%	5348
	21st to end	Night	69% 5% 6% 12% 8% 1	7150

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	31% 19% 34% 11% 5%	220
	11th to 20th	Day	30% 27% 30% 11% 2	220
	21st to end	Day	29% 21% 33% 12% 4 1	242
February	1st to 10th	Day	32% 30% 28% 8% 1	278
	11th to 20th	Day	32% 25% 32% 10% 2	510
	21st to end	Day	32% 20% 32% 14% 1	446
March	1st to 10th	Day	34% 19% 31% 14% 2	540
	11th to 20th	Day	33% 21% 30% 11% 3 1	540
	21st to end	Day	36% 22% 24% 12% 5% 1	594
April	1st to 10th	Day	37% 18% 27% 13% 4	540
	11th to 20th	Day	37% 19% 24% 13% 4 2	532
	21st to end	Day	33% 18% 29% 12% 6% 2	540
May	1st to 10th	Day	36% 18% 26% 14% 5% 2	543
	11th to 20th	Day	25% 18% 30% 19% 6% 2	567
	21st to end	Day	23% 20% 31% 20% 5% 1	617
June	1st to 10th	Day	30% 14% 31% 17% 7% 1	530
	11th to 20th	Day	21% 15% 33% 22% 9% 1	530
	21st to end	Day	29% 12% 36% 15% 7% 2	530
July	1st to 10th	Day	19% 10% 33% 26% 10% 1	525
	11th to 20th	Day	16% 10% 38% 26% 8% 2	524
	21st to end	Day	19% 11% 35% 27% 8% 1	576
August	1st to 10th	Day	18% 10% 29% 28% 13% 2	526
	11th to 20th	Day	20% 8% 28% 24% 16% 4	519
	21st to end	Day	21% 14% 25% 24% 12% 5%	583
September	1st to 10th	Day	19% 13% 29% 21% 14% 4	530
	11th to 20th	Day	18% 10% 23% 22% 19% 7%	530
	21st to end	Day	20% 14% 26% 25% 12% 4	530
October	1st to 10th	Day	26% 16% 25% 19% 12% 3	530
	11th to 20th	Day	30% 20% 22% 19% 8% 1	480
	21st to end	Day	40% 16% 24% 16% 4	462
November	1st to 10th	Day	38% 16% 28% 14% 4	425
	11th to 20th	Day	31% 22% 27% 14% 5% 2	430
	21st to end	Day	35% 21% 32% 12% 1	223
December	1st to 10th	Day	33% 20% 30% 13% 4 1	220
	11th to 20th	Day	31% 30% 30% 8% 1	220
	21st to end	Day	36% 21% 30% 10% 3	242
January	1st to 10th	Night	28% 22% 32% 15% 3	840
	11th to 20th	Night	28% 23% 28% 16% 4	840
	21st to end	Night	21% 21% 34% 19% 5% 1	924
February	1st to 10th	Night	25% 21% 31% 19% 4	840
	11th to 20th	Night	23% 24% 33% 17% 3	823
	21st to end	Night	26% 18% 33% 20% 2	692
March	1st to 10th	Night	30% 18% 30% 19% 3	840
	11th to 20th	Night	30% 19% 26% 19% 6%	840
	21st to end	Night	28% 19% 27% 18% 7% 1	924
April	1st to 10th	Night	32% 17% 28% 20% 4	840
	11th to 20th	Night	32% 16% 26% 19% 7%	821
	21st to end	Night	29% 16% 29% 20% 6%	630
May	1st to 10th	Night	29% 11% 30% 23% 7%	572
	11th to 20th	Night	23% 12% 31% 26% 7%	414
	21st to end	Night	22% 12% 29% 30% 7% 1	455
June	1st to 10th	Night	30% 8% 27% 27% 8% 1	400
	11th to 20th	Night	22% 8% 26% 32% 12%	400
	21st to end	Night	20% 9% 26% 36% 9% 1	400
July	1st to 10th	Night	16% 5% 30% 36% 12% 1	390
	11th to 20th	Night	13% 4 26% 39% 17% 1	388
	21st to end	Night	15% 5% 25% 41% 13%	438
August	1st to 10th	Night	18% 7% 17% 37% 21% 1	398
	11th to 20th	Night	16% 7% 21% 35% 20% 2	596
	21st to end	Night	17% 8% 25% 31% 18% 1	880
September	1st to 10th	Night	19% 7% 25% 33% 13% 2	800
	11th to 20th	Night	19% 7% 18% 30% 24% 2	800
	21st to end	Night	23% 8% 27% 24% 16% 1	800
October	1st to 10th	Night	26% 12% 24% 26% 11% 1	800
	11th to 20th	Night	29% 13% 26% 22% 9% 1	800
	21st to end	Night	35% 15% 27% 20% 4	880
November	1st to 10th	Night	32% 16% 28% 18% 5% 1	820
	11th to 20th	Night	27% 20% 27% 16% 8% 1	840
	21st to end	Night	31% 23% 25% 16% 4 1	840
December	1st to 10th	Night	33% 23% 26% 15% 4	840
	11th to 20th	Night	31% 22% 27% 15% 4 1	840
	21st to end	Night	33% 21% 27% 17% 3	924

Page generated at 2026-05-29 01:53:30. Version ac07

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