Hogsback Observatory
Clear Sky Chart
History

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

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	13% 10% 4 9% 5% 5% 5% 4 5% 7% 33%	113271
	Night	14% 12% 4 9% 5% 4 4 3 5% 7% 32%	115136
Transparency	Day	49% 4 12% 21% 12% 1	116872
	Night	50% 4 11% 21% 12% 1	119676
Seeing	Day	19% 10% 28% 24% 14% 6%	16245
	Night	20% 8% 29% 30% 12% 1	26247

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	11% 18% 7% 17% 6% 5% 4 3 4 5% 20%	3617
	11th to 20th	Day	14% 15% 7% 17% 9% 10% 6% 3 4 5% 9%	3620
	21st to end	Day	22% 19% 5% 12% 5% 4 6% 2 3 5% 16%	4175
February	1st to 10th	Day	14% 14% 5% 13% 9% 5% 5% 4 5% 6% 20%	1805
	11th to 20th	Day	14% 13% 6% 15% 8% 5% 5% 3 3 4 24%	1948
	21st to end	Day	15% 15% 5% 12% 6% 4 5% 4 4 5% 24%	1650
March	1st to 10th	Day	14% 12% 4 11% 6% 5% 5% 3 5% 6% 31%	4480
	11th to 20th	Day	11% 11% 4 9% 6% 5% 4 3 5% 7% 35%	4975
	21st to end	Day	17% 15% 5% 9% 5% 4 4 3 5% 6% 28%	5465
April	1st to 10th	Day	18% 13% 4 8% 5% 5% 4 3 5% 6% 29%	2400
	11th to 20th	Day	16% 10% 3 8% 4 4 5% 4 6% 6% 35%	2361
	21st to end	Day	16% 9% 3 8% 6% 4 5% 3 5% 7% 34%	2470
May	1st to 10th	Day	17% 9% 3 7% 4 4 5% 4 5% 6% 35%	4082
	11th to 20th	Day	16% 7% 2 6% 4 5% 4 5% 6% 8% 37%	4081
	21st to end	Day	11% 6% 2 6% 4 4 4 3 5% 8% 45%	4626
June	1st to 10th	Day	10% 5% 2 7% 5% 5% 5% 4 6% 8% 43%	2660
	11th to 20th	Day	11% 6% 2 6% 5% 4 5% 4 7% 7% 42%	2660
	21st to end	Day	9% 6% 2 7% 5% 6% 6% 5% 7% 9% 39%	2660
July	1st to 10th	Day	11% 6% 3 7% 5% 5% 5% 5% 7% 9% 36%	4440
	11th to 20th	Day	7% 5% 1 5% 4 4 4 5% 7% 10% 48%	4440
	21st to end	Day	6% 6% 2 5% 5% 5% 6% 5% 7% 9% 43%	4688
August	1st to 10th	Day	12% 5% 2 6% 4 5% 5% 4 6% 8% 42%	4104
	11th to 20th	Day	11% 6% 2 7% 5% 5% 4 4 6% 8% 43%	4044
	21st to end	Day	8% 6% 2 7% 5% 5% 6% 5% 7% 9% 42%	4355
September	1st to 10th	Day	10% 6% 2 7% 5% 6% 5% 4 6% 8% 42%	2195
	11th to 20th	Day	10% 4 3 5% 4 4 4 3 7% 7% 48%	2090
	21st to end	Day	14% 7% 2 8% 5% 4 5% 3 6% 8% 38%	2090
October	1st to 10th	Day	13% 7% 3 7% 5% 4 4 3 5% 8% 42%	2085
	11th to 20th	Day	18% 9% 3 8% 5% 5% 5% 3 6% 7% 32%	1889
	21st to end	Day	20% 11% 4 8% 5% 4 4 4 5% 6% 28%	1881
November	1st to 10th	Day	11% 12% 3 7% 5% 4 5% 3 5% 6% 39%	1762
	11th to 20th	Day	14% 16% 4 10% 6% 4 4 5% 5% 6% 27%	1800
	21st to end	Day	19% 14% 6% 9% 5% 4 3 3 4 6% 27%	1800
December	1st to 10th	Day	11% 16% 9% 16% 6% 5% 5% 3 5% 6% 18%	2883
	11th to 20th	Day	16% 21% 8% 11% 8% 5% 4 3 4 4 16%	3008
	21st to end	Day	24% 21% 7% 12% 6% 3 4 3 3 5% 12%	3982
January	1st to 10th	Night	13% 15% 7% 16% 7% 5% 4 4 4 5% 20%	6348
	11th to 20th	Night	13% 15% 7% 17% 8% 8% 6% 3 5% 6% 13%	6360
	21st to end	Night	18% 17% 7% 11% 5% 6% 5% 3 4 5% 18%	6793
February	1st to 10th	Night	15% 17% 5% 11% 8% 6% 5% 4 5% 4 21%	2595
	11th to 20th	Night	13% 14% 4 11% 7% 6% 5% 5% 5% 6% 25%	2338
	21st to end	Night	17% 11% 4 9% 6% 4 4 4 5% 7% 27%	1980
March	1st to 10th	Night	14% 11% 4 8% 4 3 4 3 5% 7% 36%	5373
	11th to 20th	Night	10% 9% 3 6% 4 4 5% 4 5% 8% 44%	4985
	21st to end	Night	17% 11% 3 9% 5% 4 4 3 5% 6% 34%	5467
April	1st to 10th	Night	15% 12% 2 7% 5% 4 4 4 6% 7% 35%	2016
	11th to 20th	Night	18% 9% 3 7% 4 4 4 4 5% 6% 36%	1774
	21st to end	Night	17% 8% 2 7% 4 4 4 4 5% 8% 37%	1710
May	1st to 10th	Night	14% 9% 3 8% 5% 4 5% 4 6% 8% 34%	2813
	11th to 20th	Night	16% 7% 3 6% 4 3 3 3 5% 6% 43%	2812
	21st to end	Night	10% 6% 3 6% 4 3 4 4 5% 9% 45%	2929
June	1st to 10th	Night	11% 5% 2 5% 4 4 4 4 6% 8% 46%	1520
	11th to 20th	Night	11% 6% 2 6% 5% 4 4 4 5% 9% 45%	1520
	21st to end	Night	9% 5% 2 5% 4 5% 4 3 7% 10% 46%	1520
July	1st to 10th	Night	8% 4 2 6% 4 4 5% 5% 7% 10% 45%	2530
	11th to 20th	Night	9% 4 1 4 3 3 5% 4 5% 8% 55%	2530
	21st to end	Night	6% 5% 2 6% 5% 5% 5% 4 7% 10% 47%	2868
August	1st to 10th	Night	10% 5% 2 5% 3 4 4 4 6% 9% 47%	2825
	11th to 20th	Night	10% 6% 2 5% 4 4 4 4 5% 8% 47%	2785
	21st to end	Night	9% 6% 3 6% 4 4 4 4 6% 7% 46%	3284
September	1st to 10th	Night	9% 7% 2 6% 4 4 4 4 5% 7% 48%	1966
	11th to 20th	Night	10% 4 2 5% 3 4 3 2 5% 6% 56%	2090
	21st to end	Night	13% 7% 2 6% 5% 4 4 3 6% 7% 42%	2090
October	1st to 10th	Night	14% 7% 3 6% 4 3 4 3 4 6% 46%	2095
	11th to 20th	Night	18% 11% 4 8% 5% 4 3 3 6% 5% 33%	2289
	21st to end	Night	20% 10% 3 7% 4 4 4 3 5% 6% 32%	2717
November	1st to 10th	Night	13% 10% 3 8% 4 4 4 3 4 5% 41%	2536
	11th to 20th	Night	14% 14% 3 9% 5% 5% 5% 4 5% 7% 29%	2600
	21st to end	Night	18% 15% 5% 9% 5% 3 4 3 5% 6% 27%	2600
December	1st to 10th	Night	12% 15% 8% 16% 6% 5% 5% 3 5% 6% 20%	4283
	11th to 20th	Night	15% 21% 7% 14% 7% 5% 4 3 4 5% 16%	5199
	21st to end	Night	24% 22% 6% 11% 5% 4 3 2 4 5% 13%	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	67% 3 11% 14% 4	3618
	11th to 20th	Day	77% 4 9% 6% 3	3620
	21st to end	Day	73% 2 11% 10% 4	4175
February	1st to 10th	Day	64% 5% 9% 16% 7%	1805
	11th to 20th	Day	65% 3 7% 16% 9% 1	1958
	21st to end	Day	62% 3 11% 15% 7% 1	1650
March	1st to 10th	Day	55% 3 12% 18% 10% 2	4480
	11th to 20th	Day	48% 4 15% 23% 9% 1	4975
	21st to end	Day	59% 3 9% 16% 11% 2	5465
April	1st to 10th	Day	56% 4 9% 20% 11%	2400
	11th to 20th	Day	49% 4 10% 21% 14% 2	2361
	21st to end	Day	50% 4 12% 18% 15% 2	2470
May	1st to 10th	Day	48% 5% 10% 22% 13% 1	4120
	11th to 20th	Day	44% 5% 15% 25% 10% 1	4085
	21st to end	Day	37% 4 12% 21% 23% 3	4639
June	1st to 10th	Day	38% 5% 12% 28% 16% 2	2660
	11th to 20th	Day	37% 6% 12% 25% 18% 2	2660
	21st to end	Day	39% 6% 14% 23% 16% 2	2660
July	1st to 10th	Day	42% 5% 13% 21% 15% 3	4371
	11th to 20th	Day	29% 4 14% 33% 18% 1	4365
	21st to end	Day	35% 7% 14% 21% 21% 2	4540
August	1st to 10th	Day	38% 6% 12% 27% 17% 1	4091
	11th to 20th	Day	38% 5% 10% 33% 13% 1	4083
	21st to end	Day	37% 5% 13% 30% 13% 1	4354
September	1st to 10th	Day	39% 5% 13% 25% 15% 2	2195
	11th to 20th	Day	34% 4 11% 32% 18% 1	2090
	21st to end	Day	45% 4 12% 24% 15%	2090
October	1st to 10th	Day	31% 3 10% 35% 20% 1	3462
	11th to 20th	Day	42% 3 10% 26% 18%	3117
	21st to end	Day	48% 3 12% 25% 10% 1	3081
November	1st to 10th	Day	45% 5% 12% 23% 14% 1	1762
	11th to 20th	Day	58% 3 11% 20% 7% 1	1800
	21st to end	Day	59% 4 14% 16% 6% 1	1800
December	1st to 10th	Day	67% 5% 15% 10% 3	2883
	11th to 20th	Day	73% 3 9% 13% 2	3005
	21st to end	Day	76% 4 7% 9% 4	3982
January	1st to 10th	Night	66% 4 9% 15% 6% 1	6348
	11th to 20th	Night	71% 4 10% 9% 6%	6360
	21st to end	Night	68% 3 11% 12% 5%	6793
February	1st to 10th	Night	65% 4 9% 14% 6% 1	2595
	11th to 20th	Night	60% 4 9% 18% 9% 1	2339
	21st to end	Night	56% 3 14% 16% 9% 2	1980
March	1st to 10th	Night	48% 3 13% 24% 10% 1	5373
	11th to 20th	Night	38% 5% 13% 26% 16% 1	4985
	21st to end	Night	51% 4 11% 19% 13% 1	5467
April	1st to 10th	Night	49% 4 10% 21% 15% 1	2016
	11th to 20th	Night	48% 4 11% 18% 18% 2	1774
	21st to end	Night	45% 4 12% 20% 15% 3	1710
May	1st to 10th	Night	47% 5% 11% 22% 13% 3	2840
	11th to 20th	Night	42% 4 10% 27% 16% 2	2813
	21st to end	Night	35% 5% 11% 23% 20% 5%	2937
June	1st to 10th	Night	35% 4 10% 30% 18% 3	1520
	11th to 20th	Night	37% 4 14% 25% 18% 3	1520
	21st to end	Night	32% 6% 14% 28% 19% 1	1520
July	1st to 10th	Night	33% 5% 13% 28% 14% 6%	2490
	11th to 20th	Night	28% 5% 12% 32% 21% 2	2482
	21st to end	Night	32% 7% 18% 18% 23% 2	2777
August	1st to 10th	Night	34% 5% 13% 30% 18% 1	2814
	11th to 20th	Night	34% 5% 10% 35% 14% 2	2812
	21st to end	Night	37% 6% 10% 30% 16% 1	3284
September	1st to 10th	Night	33% 6% 11% 29% 19% 2	1966
	11th to 20th	Night	30% 4 10% 31% 23% 2	2090
	21st to end	Night	41% 4 12% 27% 16% 1	2090
October	1st to 10th	Night	26% 3 10% 36% 24% 1	3498
	11th to 20th	Night	44% 3 10% 22% 20%	3800
	21st to end	Night	40% 3 10% 32% 14% 1	4467
November	1st to 10th	Night	44% 5% 10% 25% 15% 1	2536
	11th to 20th	Night	54% 4 12% 19% 9% 1	2600
	21st to end	Night	59% 3 12% 19% 6% 1	2600
December	1st to 10th	Night	65% 4 13% 14% 4	4283
	11th to 20th	Night	71% 3 9% 12% 5%	5201
	21st to end	Night	76% 3 8% 8% 5%	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	22% 12% 40% 19% 5% 2	300
	11th to 20th	Day	22% 14% 44% 14% 5% 1	300
	21st to end	Day	31% 15% 29% 20% 4	330
February	1st to 10th	Day	26% 20% 34% 16% 3 1	300
	11th to 20th	Day	26% 17% 39% 16% 2	300
	21st to end	Day	27% 24% 34% 12% 3	249
March	1st to 10th	Day	23% 17% 42% 15% 4	300
	11th to 20th	Day	25% 11% 33% 23% 7% 1	495
	21st to end	Day	27% 14% 32% 21% 5% 1	550
April	1st to 10th	Day	26% 12% 33% 21% 7% 1	500
	11th to 20th	Day	27% 15% 30% 16% 8% 5%	452
	21st to end	Day	22% 10% 31% 23% 12% 2	460
May	1st to 10th	Day	25% 11% 29% 22% 10% 3	463
	11th to 20th	Day	20% 8% 24% 26% 16% 6%	487
	21st to end	Day	16% 7% 21% 27% 21% 8%	614
June	1st to 10th	Day	13% 7% 27% 31% 16% 6%	680
	11th to 20th	Day	14% 5% 29% 29% 18% 5%	680
	21st to end	Day	13% 7% 28% 29% 17% 6%	680
July	1st to 10th	Day	9% 4 20% 29% 24% 13%	671
	11th to 20th	Day	11% 5% 24% 28% 21% 11%	668
	21st to end	Day	8% 2 23% 31% 26% 10%	663
August	1st to 10th	Day	9% 2 19% 29% 19% 21%	486
	11th to 20th	Day	15% 2 16% 28% 27% 12%	479
	21st to end	Day	13% 2 19% 30% 24% 13%	539
September	1st to 10th	Day	14% 4 20% 26% 24% 13%	490
	11th to 20th	Day	10% 2 18% 23% 29% 17%	490
	21st to end	Day	18% 9% 20% 24% 19% 9%	490
October	1st to 10th	Day	19% 7% 26% 24% 17% 7%	490
	11th to 20th	Day	21% 13% 28% 27% 9% 1	479
	21st to end	Day	30% 10% 37% 15% 5% 1	330
November	1st to 10th	Day	16% 17% 36% 22% 8% 1	300
	11th to 20th	Day	25% 18% 34% 17% 5% 2	300
	21st to end	Day	25% 17% 39% 14% 6%	300
December	1st to 10th	Day	26% 20% 30% 17% 6% 1	300
	11th to 20th	Day	25% 19% 36% 15% 3 1	300
	21st to end	Day	28% 20% 31% 16% 5%	330
January	1st to 10th	Night	22% 13% 35% 23% 6% 1	1000
	11th to 20th	Night	25% 12% 36% 22% 4 1	1000
	21st to end	Night	21% 11% 35% 28% 5%	1014
February	1st to 10th	Night	26% 13% 33% 26% 3	800
	11th to 20th	Night	23% 15% 34% 25% 4	783
	21st to end	Night	24% 14% 40% 20% 3	660
March	1st to 10th	Night	22% 11% 35% 25% 7% 1	800
	11th to 20th	Night	19% 13% 36% 26% 6% 1	800
	21st to end	Night	23% 10% 32% 27% 8%	880
April	1st to 10th	Night	19% 11% 38% 26% 5% 1	616
	11th to 20th	Night	24% 9% 34% 24% 8% 1	565
	21st to end	Night	23% 6% 34% 29% 7%	570
May	1st to 10th	Night	20% 6% 30% 33% 10% 1	570
	11th to 20th	Night	24% 3 28% 33% 11% 1	561
	21st to end	Night	13% 3 24% 38% 21% 2	617
June	1st to 10th	Night	13% 2 24% 44% 16% 1	570
	11th to 20th	Night	13% 3 23% 41% 20%	570
	21st to end	Night	11% 2 22% 41% 21% 2	570
July	1st to 10th	Night	9% 1 16% 41% 29% 3	555
	11th to 20th	Night	11% 1 17% 41% 26% 3	552
	21st to end	Night	7% 1 16% 48% 25% 2	624
August	1st to 10th	Night	8% 1 14% 46% 28% 3	567
	11th to 20th	Night	9% 1 16% 40% 29% 5%	564
	21st to end	Night	11% 3 14% 37% 30% 4	627
September	1st to 10th	Night	13% 3 17% 38% 25% 4	636
	11th to 20th	Night	12% 3 14% 36% 31% 4	760
	21st to end	Night	18% 4 21% 31% 21% 5%	760
October	1st to 10th	Night	17% 6% 24% 34% 17% 3	760
	11th to 20th	Night	23% 10% 28% 28% 9% 1	760
	21st to end	Night	26% 9% 33% 26% 7%	836
November	1st to 10th	Night	20% 9% 31% 29% 11% 1	780
	11th to 20th	Night	23% 14% 34% 22% 7% 1	800
	21st to end	Night	26% 11% 33% 27% 4	800
December	1st to 10th	Night	25% 13% 34% 21% 7%	820
	11th to 20th	Night	22% 14% 36% 24% 4	1000
	21st to end	Night	32% 11% 34% 20% 3	1100

Page generated at 2025-04-08 09:17:24. Version ac07

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