Hirsch Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hirsch Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	16% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	126501
	Night	17% 10% 4 9% 5% 5% 4 4 5% 6% 31%	122885
Transparency	Day	48% 7% 12% 20% 11% 1	129990
	Night	51% 7% 12% 18% 11% 1	127470
Seeing	Day	22% 16% 28% 21% 10% 3	16002
	Night	22% 14% 30% 25% 8% 1	24998

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% 12% 9% 17% 6% 4 5% 3 4 6% 23%	3597
	11th to 20th	Day	13% 13% 4 13% 6% 5% 5% 4 5% 6% 26%	3570
	21st to end	Day	16% 13% 4 8% 5% 5% 5% 4 5% 6% 31%	4079
February	1st to 10th	Day	16% 12% 4 9% 7% 5% 6% 5% 6% 6% 24%	1800
	11th to 20th	Day	12% 15% 4 10% 6% 5% 5% 4 5% 6% 27%	1928
	21st to end	Day	18% 12% 3 11% 6% 4 4 4 5% 6% 28%	1650
March	1st to 10th	Day	22% 12% 5% 8% 4 4 4 3 4 4 28%	4476
	11th to 20th	Day	16% 13% 4 7% 5% 4 3 3 4 6% 36%	4699
	21st to end	Day	20% 9% 3 9% 6% 5% 4 3 4 5% 32%	5968
April	1st to 10th	Day	19% 10% 3 8% 5% 4 4 3 4 7% 32%	2400
	11th to 20th	Day	17% 9% 3 7% 5% 5% 5% 4 5% 6% 35%	2374
	21st to end	Day	21% 8% 3 7% 4 4 5% 3 6% 6% 32%	2496
May	1st to 10th	Day	30% 9% 3 7% 5% 5% 4 3 5% 6% 24%	5977
	11th to 20th	Day	17% 8% 3 9% 5% 5% 5% 4 6% 7% 30%	6318
	21st to end	Day	19% 7% 3 7% 5% 4 5% 4 6% 8% 32%	6949
June	1st to 10th	Day	15% 7% 2 7% 5% 6% 5% 5% 7% 8% 32%	2800
	11th to 20th	Day	16% 6% 3 7% 4 5% 5% 5% 7% 11% 32%	2800
	21st to end	Day	16% 6% 3 8% 4 5% 5% 5% 6% 9% 35%	2800
July	1st to 10th	Day	10% 6% 3 7% 5% 5% 6% 5% 7% 9% 35%	6360
	11th to 20th	Day	10% 6% 3 7% 5% 6% 6% 5% 7% 10% 36%	6360
	21st to end	Day	13% 5% 2 6% 5% 5% 4 4 6% 8% 42%	6926
August	1st to 10th	Day	15% 7% 3 8% 5% 5% 5% 4 6% 8% 34%	4541
	11th to 20th	Day	13% 7% 2 7% 5% 5% 6% 4 7% 9% 34%	3854
	21st to end	Day	11% 6% 2 6% 4 5% 6% 5% 7% 9% 39%	4312
September	1st to 10th	Day	13% 6% 2 5% 4 4 5% 4 6% 9% 44%	2400
	11th to 20th	Day	13% 6% 2 5% 3 4 4 3 5% 8% 48%	2303
	21st to end	Day	16% 5% 3 6% 4 4 4 4 7% 7% 40%	2200
October	1st to 10th	Day	19% 9% 3 9% 4 4 4 4 4 4 36%	2023
	11th to 20th	Day	15% 7% 3 7% 4 3 4 4 6% 8% 38%	1900
	21st to end	Day	23% 9% 3 6% 5% 4 4 4 5% 7% 29%	1884
November	1st to 10th	Day	14% 8% 3 6% 3 4 4 4 6% 7% 41%	1761
	11th to 20th	Day	16% 9% 3 8% 4 4 4 5% 4 6% 37%	1800
	21st to end	Day	18% 11% 3 6% 5% 4 5% 3 5% 6% 31%	1650
December	1st to 10th	Day	17% 14% 5% 11% 6% 4 5% 4 4 6% 24%	2600
	11th to 20th	Day	21% 14% 4 8% 4 4 3 3 4 5% 30%	2986
	21st to end	Day	21% 16% 5% 9% 4 4 4 3 4 5% 25%	3960
January	1st to 10th	Night	14% 14% 8% 15% 5% 5% 4 3 4 4 24%	6349
	11th to 20th	Night	11% 15% 6% 14% 7% 5% 4 3 4 7% 23%	6315
	21st to end	Night	16% 10% 4 9% 6% 5% 5% 4 5% 7% 29%	6876
February	1st to 10th	Night	15% 12% 6% 10% 7% 5% 4 4 5% 5% 26%	2600
	11th to 20th	Night	13% 12% 5% 11% 8% 6% 4 4 5% 6% 26%	2358
	21st to end	Night	16% 12% 4 10% 6% 4 5% 3 6% 6% 27%	1980
March	1st to 10th	Night	20% 10% 5% 9% 6% 5% 5% 4 5% 6% 26%	5364
	11th to 20th	Night	17% 12% 3 8% 5% 4 4 3 5% 6% 33%	5258
	21st to end	Night	22% 12% 4 10% 4 4 3 3 4 4 29%	4963
April	1st to 10th	Night	19% 12% 3 8% 4 5% 4 4 5% 6% 29%	2000
	11th to 20th	Night	21% 8% 3 8% 4 5% 3 3 5% 6% 33%	1975
	21st to end	Night	20% 8% 3 6% 4 3 4 4 5% 6% 36%	1902
May	1st to 10th	Night	30% 10% 3 8% 4 4 4 3 4 5% 24%	3993
	11th to 20th	Night	18% 9% 3 9% 6% 5% 5% 4 5% 7% 28%	3575
	21st to end	Night	21% 9% 3 8% 5% 5% 4 4 5% 7% 30%	3926
June	1st to 10th	Night	18% 7% 2 6% 4 5% 4 4 5% 8% 37%	1600
	11th to 20th	Night	16% 6% 2 5% 4 5% 5% 3 7% 8% 39%	1600
	21st to end	Night	13% 5% 2 6% 5% 4 5% 5% 7% 7% 42%	1600
July	1st to 10th	Night	9% 7% 3 8% 6% 5% 4 4 7% 9% 38%	3600
	11th to 20th	Night	13% 6% 3 8% 5% 6% 6% 4 6% 8% 35%	3600
	21st to end	Night	11% 7% 2 6% 4 4 4 3 5% 8% 45%	3926
August	1st to 10th	Night	22% 7% 3 7% 4 4 4 3 5% 8% 34%	2586
	11th to 20th	Night	14% 8% 2 8% 5% 5% 5% 3 6% 7% 38%	3181
	21st to end	Night	11% 7% 3 7% 4 4 4 4 5% 8% 43%	3586
September	1st to 10th	Night	14% 8% 3 6% 4 3 4 4 5% 6% 43%	2000
	11th to 20th	Night	14% 5% 2 3 4 3 4 3 4 8% 50%	2095
	21st to end	Night	15% 7% 2 6% 5% 4 4 4 5% 7% 41%	2200
October	1st to 10th	Night	18% 8% 2 7% 5% 5% 5% 3 5% 6% 35%	2219
	11th to 20th	Night	16% 7% 2 7% 4 4 5% 4 5% 6% 39%	2280
	21st to end	Night	23% 9% 3 7% 4 4 3 4 6% 7% 30%	2712
November	1st to 10th	Night	15% 10% 3 6% 4 4 4 4 5% 5% 41%	2537
	11th to 20th	Night	16% 9% 3 8% 5% 5% 5% 4 5% 6% 35%	2600
	21st to end	Night	16% 11% 3 8% 5% 5% 4 4 5% 6% 32%	2745
December	1st to 10th	Night	18% 15% 6% 10% 5% 4 4 4 5% 7% 22%	4580
	11th to 20th	Night	25% 13% 4 7% 4 4 3 3 5% 5% 27%	5208
	21st to end	Night	22% 16% 5% 8% 4 4 4 3 4 4 24%	6996

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	62% 6% 10% 17% 4	3598
	11th to 20th	Day	57% 7% 11% 14% 10% 2	3600
	21st to end	Day	52% 7% 10% 19% 11% 1	4079
February	1st to 10th	Day	57% 8% 13% 14% 8% 1	1800
	11th to 20th	Day	57% 7% 11% 16% 8% 1	1937
	21st to end	Day	57% 6% 11% 15% 10% 1	1650
March	1st to 10th	Day	58% 6% 11% 13% 8% 3	4479
	11th to 20th	Day	48% 6% 17% 19% 7% 3	4699
	21st to end	Day	54% 6% 7% 13% 18% 2	5968
April	1st to 10th	Day	54% 5% 9% 20% 11% 2	2400
	11th to 20th	Day	49% 6% 10% 18% 15% 2	2374
	21st to end	Day	51% 6% 9% 17% 13% 3	2496
May	1st to 10th	Day	61% 7% 9% 15% 8% 1	5988
	11th to 20th	Day	50% 8% 14% 19% 8% 1	6322
	21st to end	Day	48% 9% 11% 21% 10% 1	6949
June	1st to 10th	Day	46% 10% 13% 20% 11% 1	2800
	11th to 20th	Day	45% 9% 14% 20% 11% 1	2800
	21st to end	Day	45% 9% 14% 20% 11% 1	2800
July	1st to 10th	Day	43% 9% 13% 23% 12% 1	6290
	11th to 20th	Day	40% 10% 15% 22% 12% 1	6285
	21st to end	Day	39% 7% 14% 27% 13%	6728
August	1st to 10th	Day	47% 9% 14% 20% 9% 1	4527
	11th to 20th	Day	43% 9% 18% 24% 6% 1	3890
	21st to end	Day	36% 9% 17% 27% 10% 1	4312
September	1st to 10th	Day	37% 8% 13% 26% 15% 1	2400
	11th to 20th	Day	35% 7% 12% 25% 21% 1	2303
	21st to end	Day	40% 8% 12% 24% 14% 2	2200
October	1st to 10th	Day	44% 8% 10% 21% 15% 1	3335
	11th to 20th	Day	39% 6% 14% 25% 16% 1	3140
	21st to end	Day	38% 7% 15% 27% 12% 2	3084
November	1st to 10th	Day	41% 7% 15% 24% 12% 1	1761
	11th to 20th	Day	48% 6% 11% 18% 14% 2	1800
	21st to end	Day	52% 7% 16% 17% 6% 1	1650
December	1st to 10th	Day	60% 7% 13% 15% 4	2600
	11th to 20th	Day	56% 6% 13% 16% 10%	2986
	21st to end	Day	60% 5% 8% 16% 10% 1	3960
January	1st to 10th	Night	62% 7% 12% 13% 7% 1	6349
	11th to 20th	Night	60% 7% 13% 11% 8% 1	6360
	21st to end	Night	54% 6% 11% 13% 12% 3	6876
February	1st to 10th	Night	58% 7% 11% 15% 7% 1	2600
	11th to 20th	Night	56% 6% 14% 14% 8% 1	2360
	21st to end	Night	56% 7% 12% 16% 8% 2	1980
March	1st to 10th	Night	56% 7% 12% 13% 11% 1	5374
	11th to 20th	Night	51% 6% 17% 18% 7% 2	5259
	21st to end	Night	57% 5% 6% 14% 15% 2	4963
April	1st to 10th	Night	55% 6% 10% 18% 10% 1	2000
	11th to 20th	Night	50% 6% 10% 18% 14% 1	1975
	21st to end	Night	47% 8% 10% 17% 15% 3	1902
May	1st to 10th	Night	61% 7% 10% 12% 8% 1	3999
	11th to 20th	Night	54% 8% 11% 17% 8% 1	3576
	21st to end	Night	53% 8% 10% 17% 11% 1	3927
June	1st to 10th	Night	45% 7% 12% 23% 13% 1	1600
	11th to 20th	Night	42% 8% 15% 22% 12%	1600
	21st to end	Night	39% 9% 15% 23% 13% 1	1600
July	1st to 10th	Night	42% 8% 15% 21% 15% 1	3560
	11th to 20th	Night	43% 10% 13% 20% 13%	3552
	21st to end	Night	36% 6% 14% 30% 13% 1	3809
August	1st to 10th	Night	48% 7% 15% 21% 8% 1	2576
	11th to 20th	Night	43% 9% 18% 22% 9%	3211
	21st to end	Night	36% 8% 15% 30% 11% 1	3586
September	1st to 10th	Night	41% 7% 11% 26% 15% 1	2000
	11th to 20th	Night	33% 5% 13% 27% 22% 1	2095
	21st to end	Night	42% 7% 10% 23% 16% 1	2200
October	1st to 10th	Night	49% 6% 12% 19% 12% 1	3674
	11th to 20th	Night	37% 6% 12% 23% 20% 1	3779
	21st to end	Night	42% 6% 14% 28% 10%	4462
November	1st to 10th	Night	43% 7% 12% 20% 16% 2	2537
	11th to 20th	Night	49% 6% 13% 18% 13% 2	2600
	21st to end	Night	51% 7% 15% 18% 8%	2745
December	1st to 10th	Night	61% 8% 12% 15% 3	4580
	11th to 20th	Night	57% 6% 13% 12% 11%	5208
	21st to end	Night	61% 6% 9% 13% 10% 2	6996

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	25% 32% 25% 15% 4 1	200
	11th to 20th	Day	31% 28% 25% 13% 3 1	200
	21st to end	Day	24% 24% 32% 14% 5% 1	247
February	1st to 10th	Day	33% 27% 31% 8% 1	300
	11th to 20th	Day	30% 28% 30% 10% 2	476
	21st to end	Day	29% 24% 30% 16% 2	414
March	1st to 10th	Day	29% 20% 31% 16% 3	500
	11th to 20th	Day	29% 24% 28% 15% 4	500
	21st to end	Day	33% 23% 24% 14% 6% 1	550
April	1st to 10th	Day	35% 22% 21% 17% 5% 1	500
	11th to 20th	Day	29% 24% 25% 14% 6% 1	492
	21st to end	Day	29% 18% 30% 16% 5% 2	500
May	1st to 10th	Day	31% 16% 29% 14% 8% 2	503
	11th to 20th	Day	20% 17% 33% 21% 7% 2	527
	21st to end	Day	19% 11% 31% 25% 10% 3	573
June	1st to 10th	Day	17% 14% 32% 22% 13% 2	530
	11th to 20th	Day	18% 12% 29% 27% 12% 2	530
	21st to end	Day	17% 14% 32% 24% 11% 2	530
July	1st to 10th	Day	12% 10% 30% 30% 14% 4	525
	11th to 20th	Day	13% 7% 30% 32% 16% 3	524
	21st to end	Day	13% 9% 31% 29% 15% 2	576
August	1st to 10th	Day	13% 8% 24% 32% 17% 6%	526
	11th to 20th	Day	15% 6% 26% 29% 17% 7%	519
	21st to end	Day	11% 13% 28% 27% 16% 6%	583
September	1st to 10th	Day	17% 10% 24% 25% 16% 8%	530
	11th to 20th	Day	14% 9% 23% 23% 17% 13%	530
	21st to end	Day	19% 10% 25% 25% 14% 6%	530
October	1st to 10th	Day	23% 13% 24% 21% 13% 6%	502
	11th to 20th	Day	20% 18% 27% 21% 11% 2	490
	21st to end	Day	30% 16% 26% 22% 6%	430
November	1st to 10th	Day	24% 17% 24% 20% 12% 2	395
	11th to 20th	Day	26% 20% 31% 17% 6% 1	400
	21st to end	Day	29% 21% 26% 19% 4	250
December	1st to 10th	Day	24% 25% 33% 12% 6% 1	200
	11th to 20th	Day	30% 24% 29% 16% 2	200
	21st to end	Day	26% 25% 30% 15% 2 1	220
January	1st to 10th	Night	23% 21% 36% 18% 2	800
	11th to 20th	Night	27% 20% 30% 20% 3	800
	21st to end	Night	20% 20% 37% 20% 3	880
February	1st to 10th	Night	25% 20% 36% 17% 2	800
	11th to 20th	Night	23% 21% 36% 18% 1	783
	21st to end	Night	27% 20% 33% 18% 3	660
March	1st to 10th	Night	26% 21% 33% 17% 2	800
	11th to 20th	Night	25% 22% 32% 19% 2	800
	21st to end	Night	26% 21% 29% 20% 5%	880
April	1st to 10th	Night	28% 20% 28% 22% 3	800
	11th to 20th	Night	26% 19% 30% 19% 5%	791
	21st to end	Night	24% 18% 31% 22% 5%	702
May	1st to 10th	Night	27% 17% 28% 23% 6%	578
	11th to 20th	Night	19% 13% 35% 27% 6% 1	394
	21st to end	Night	16% 8% 30% 36% 8% 1	433
June	1st to 10th	Night	20% 7% 33% 30% 11% 1	400
	11th to 20th	Night	17% 7% 29% 36% 12%	400
	21st to end	Night	13% 6% 30% 38% 12% 1	400
July	1st to 10th	Night	13% 5% 25% 43% 13% 1	390
	11th to 20th	Night	13% 4 25% 40% 17% 1	388
	21st to end	Night	12% 5% 22% 48% 12% 1	438
August	1st to 10th	Night	14% 3 21% 45% 16% 1	398
	11th to 20th	Night	14% 5% 22% 36% 19% 4	774
	21st to end	Night	15% 6% 24% 35% 18% 1	880
September	1st to 10th	Night	17% 7% 27% 33% 15% 2	800
	11th to 20th	Night	17% 6% 23% 30% 20% 4	800
	21st to end	Night	17% 8% 28% 31% 14% 2	800
October	1st to 10th	Night	23% 9% 24% 29% 12% 2	773
	11th to 20th	Night	20% 12% 29% 29% 9%	760
	21st to end	Night	29% 13% 30% 23% 5%	836
November	1st to 10th	Night	22% 15% 32% 24% 7%	780
	11th to 20th	Night	22% 15% 33% 21% 7% 1	800
	21st to end	Night	24% 18% 31% 23% 5%	800
December	1st to 10th	Night	27% 20% 33% 17% 4	800
	11th to 20th	Night	28% 18% 29% 23% 3	800
	21st to end	Night	31% 17% 31% 19% 2	880

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