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 2026-08-15.

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% 34%	143363
	Night	13% 11% 4 9% 5% 5% 4 3 5% 7% 33%	138719
Transparency	Day	48% 4 11% 22% 13% 1	146917
	Night	50% 4 11% 21% 13% 1	143239
Seeing	Day	19% 10% 28% 24% 14% 6%	18191
	Night	19% 8% 29% 30% 12% 1	27973

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	12% 18% 7% 17% 6% 5% 5% 3 4 5% 18%	4691
	11th to 20th	Day	12% 15% 8% 17% 11% 11% 5% 3 4 4 10%	4710
	21st to end	Day	17% 17% 5% 12% 9% 6% 7% 3 3 5% 16%	5401
February	1st to 10th	Day	14% 14% 5% 13% 9% 5% 5% 4 5% 6% 20%	1895
	11th to 20th	Day	14% 13% 6% 15% 8% 5% 5% 3 3 4 24%	2048
	21st to end	Day	15% 15% 5% 12% 6% 4 5% 4 5% 5% 24%	1730
March	1st to 10th	Day	14% 12% 4 11% 5% 5% 4 3 5% 6% 31%	4580
	11th to 20th	Day	11% 11% 4 9% 6% 5% 4 3 5% 7% 35%	5085
	21st to end	Day	16% 15% 5% 9% 5% 4 4 3 5% 6% 27%	5586
April	1st to 10th	Day	18% 14% 4 8% 5% 5% 4 3 5% 6% 29%	2520
	11th to 20th	Day	15% 10% 3 8% 4 4 5% 4 6% 6% 35%	2613
	21st to end	Day	15% 9% 3 7% 5% 4 5% 3 5% 8% 35%	2730
May	1st to 10th	Day	16% 9% 3 7% 5% 4 5% 4 5% 7% 35%	7616
	11th to 20th	Day	13% 8% 3 8% 5% 5% 5% 4 6% 8% 35%	7626
	21st to end	Day	15% 6% 2 6% 4 4 4 3 5% 7% 44%	8681
June	1st to 10th	Day	10% 5% 2 7% 5% 5% 5% 4 6% 9% 43%	2940
	11th to 20th	Day	10% 6% 2 6% 6% 4 5% 4 7% 8% 41%	2940
	21st to end	Day	9% 6% 2 6% 5% 6% 6% 5% 7% 9% 40%	2940
July	1st to 10th	Day	10% 6% 2 6% 5% 5% 5% 4 7% 10% 40%	8280
	11th to 20th	Day	6% 5% 2 6% 4 4 4 5% 7% 10% 49%	8280
	21st to end	Day	7% 6% 2 5% 4 5% 5% 5% 7% 9% 45%	8754
August	1st to 10th	Day	11% 5% 2 6% 4 5% 5% 4 6% 8% 42%	4364
	11th to 20th	Day	11% 6% 2 7% 5% 5% 4 4 6% 8% 42%	4226
	21st to end	Day	7% 6% 2 7% 5% 5% 6% 5% 7% 9% 41%	4493
September	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 4 6% 8% 42%	2310
	11th to 20th	Day	10% 4 3 5% 4 4 4 4 7% 8% 49%	2200
	21st to end	Day	14% 7% 3 8% 5% 4 5% 3 6% 8% 38%	2200
October	1st to 10th	Day	12% 6% 3 7% 5% 4 4 3 5% 7% 43%	2194
	11th to 20th	Day	17% 9% 3 8% 5% 5% 5% 3 6% 7% 32%	1988
	21st to end	Day	19% 11% 3 8% 5% 5% 4 4 5% 6% 28%	1980
November	1st to 10th	Day	11% 12% 3 7% 5% 4 5% 3 6% 6% 39%	1852
	11th to 20th	Day	14% 16% 4 10% 6% 4 4 4 5% 6% 28%	1890
	21st to end	Day	18% 14% 7% 9% 5% 4 3 3 4 6% 26%	1890
December	1st to 10th	Day	11% 15% 9% 16% 6% 5% 5% 3 5% 6% 17%	2972
	11th to 20th	Day	16% 21% 8% 12% 7% 5% 4 3 5% 4 16%	3088
	21st to end	Day	23% 21% 7% 12% 6% 3 4 3 3 5% 12%	4070
January	1st to 10th	Night	14% 16% 7% 17% 6% 4 4 3 4 5% 20%	8232
	11th to 20th	Night	10% 15% 7% 17% 9% 10% 6% 3 5% 5% 13%	8280
	21st to end	Night	14% 16% 7% 12% 7% 7% 6% 3 5% 5% 18%	8786
February	1st to 10th	Night	14% 17% 5% 10% 8% 6% 5% 4 5% 4 22%	2724
	11th to 20th	Night	13% 13% 4 11% 7% 6% 5% 5% 5% 6% 26%	2458
	21st to end	Night	16% 12% 5% 9% 6% 4 4 4 5% 7% 28%	2076
March	1st to 10th	Night	14% 11% 4 8% 4 3 4 3 5% 7% 36%	5493
	11th to 20th	Night	10% 9% 3 6% 4 4 5% 4 5% 8% 43%	5095
	21st to end	Night	16% 11% 3 9% 5% 4 4 3 5% 6% 34%	5588
April	1st to 10th	Night	15% 12% 2 8% 5% 4 4 4 6% 7% 34%	2117
	11th to 20th	Night	17% 9% 3 7% 5% 4 4 4 5% 5% 36%	1956
	21st to end	Night	16% 8% 2 7% 4 4 4 4 5% 7% 37%	1890
May	1st to 10th	Night	16% 10% 3 8% 5% 4 4 4 6% 7% 34%	5237
	11th to 20th	Night	15% 8% 3 7% 4 3 4 3 5% 7% 42%	5234
	21st to end	Night	13% 6% 2 7% 4 4 4 4 5% 8% 44%	5481
June	1st to 10th	Night	11% 5% 2 5% 4 4 4 4 6% 8% 46%	1680
	11th to 20th	Night	11% 6% 2 6% 5% 4 4 5% 6% 9% 43%	1680
	21st to end	Night	9% 5% 2 5% 4 4 4 3 7% 11% 47%	1680
July	1st to 10th	Night	8% 5% 2 6% 4 4 4 4 6% 9% 48%	4708
	11th to 20th	Night	7% 4 2 5% 3 3 5% 4 6% 10% 53%	4707
	21st to end	Night	9% 6% 2 6% 4 4 4 3 6% 8% 47%	5339
August	1st to 10th	Night	10% 5% 2 5% 4 4 4 4 6% 9% 47%	3005
	11th to 20th	Night	10% 6% 2 6% 4 4 4 3 5% 8% 47%	2911
	21st to end	Night	9% 6% 3 6% 4 4 4 4 6% 7% 46%	3388
September	1st to 10th	Night	9% 7% 2 6% 4 4 4 4 5% 8% 48%	2070
	11th to 20th	Night	10% 4 2 4 3 4 4 2 5% 6% 57%	2200
	21st to end	Night	13% 7% 2 6% 5% 4 4 3 6% 7% 42%	2200
October	1st to 10th	Night	14% 7% 3 6% 4 3 4 3 4 6% 46%	2205
	11th to 20th	Night	18% 11% 4 8% 5% 4 4 3 6% 5% 34%	2410
	21st to end	Night	20% 9% 3 7% 4 4 4 3 5% 6% 33%	2860
November	1st to 10th	Night	13% 10% 3 8% 4 4 4 3 5% 6% 41%	2666
	11th to 20th	Night	14% 14% 3 8% 5% 5% 5% 4 5% 7% 29%	2730
	21st to end	Night	18% 16% 5% 9% 5% 3 4 3 5% 6% 26%	2730
December	1st to 10th	Night	12% 15% 8% 16% 6% 5% 5% 3 5% 6% 20%	4414
	11th to 20th	Night	14% 20% 7% 14% 7% 5% 4 3 4 5% 16%	5339
	21st to end	Night	24% 22% 6% 11% 5% 4 3 2 4 5% 13%	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	69% 3 12% 13% 3	4692
	11th to 20th	Day	78% 4 7% 6% 5%	4710
	21st to end	Day	72% 2 10% 10% 5%	5401
February	1st to 10th	Day	64% 4 9% 16% 7%	1895
	11th to 20th	Day	64% 3 7% 15% 9% 1	2058
	21st to end	Day	62% 3 10% 16% 7% 1	1730
March	1st to 10th	Day	55% 3 12% 19% 10% 2	4580
	11th to 20th	Day	48% 4 15% 22% 9% 1	5085
	21st to end	Day	59% 3 9% 16% 11% 2	5586
April	1st to 10th	Day	57% 4 9% 19% 11%	2520
	11th to 20th	Day	49% 4 10% 21% 14% 2	2613
	21st to end	Day	49% 4 12% 20% 14% 2	2730
May	1st to 10th	Day	48% 4 10% 22% 14% 1	7651
	11th to 20th	Day	46% 5% 13% 22% 13% 1	7630
	21st to end	Day	40% 3 9% 22% 22% 4	8695
June	1st to 10th	Day	37% 5% 12% 28% 16% 2	2940
	11th to 20th	Day	38% 5% 13% 25% 17% 1	2940
	21st to end	Day	38% 6% 14% 24% 16% 2	2940
July	1st to 10th	Day	38% 5% 14% 26% 16% 2	8211
	11th to 20th	Day	29% 4 12% 32% 21% 1	8205
	21st to end	Day	33% 6% 13% 25% 22% 1	8561
August	1st to 10th	Day	38% 6% 12% 27% 17% 1	4351
	11th to 20th	Day	38% 5% 11% 32% 12% 1	4265
	21st to end	Day	37% 5% 13% 31% 13% 1	4492
September	1st to 10th	Day	38% 5% 13% 27% 15% 2	2310
	11th to 20th	Day	33% 4 12% 32% 19% 1	2200
	21st to end	Day	45% 4 11% 24% 16%	2200
October	1st to 10th	Day	31% 3 10% 35% 20% 1	3571
	11th to 20th	Day	42% 3 10% 27% 18%	3216
	21st to end	Day	47% 3 12% 25% 11% 1	3180
November	1st to 10th	Day	45% 5% 12% 24% 14% 1	1852
	11th to 20th	Day	57% 3 11% 20% 7% 1	1890
	21st to end	Day	60% 3 13% 17% 6% 1	1890
December	1st to 10th	Day	67% 5% 15% 10% 3	2972
	11th to 20th	Day	72% 3 9% 13% 3	3085
	21st to end	Day	76% 4 7% 9% 4	4070
January	1st to 10th	Night	66% 4 9% 14% 6% 1	8233
	11th to 20th	Night	73% 3 9% 9% 5%	8280
	21st to end	Night	67% 3 10% 14% 5%	8786
February	1st to 10th	Night	64% 4 9% 15% 6% 1	2724
	11th to 20th	Night	59% 4 9% 18% 9% 1	2459
	21st to end	Night	56% 3 13% 16% 9% 2	2076
March	1st to 10th	Night	48% 3 14% 24% 10% 1	5493
	11th to 20th	Night	39% 5% 13% 26% 16% 1	5095
	21st to end	Night	51% 4 11% 19% 13% 2	5588
April	1st to 10th	Night	49% 4 10% 21% 14% 1	2117
	11th to 20th	Night	48% 4 10% 18% 18% 2	1956
	21st to end	Night	45% 4 13% 21% 14% 3	1890
May	1st to 10th	Night	48% 4 11% 18% 15% 3	5264
	11th to 20th	Night	43% 4 10% 27% 15% 2	5235
	21st to end	Night	39% 4 9% 22% 20% 5%	5491
June	1st to 10th	Night	35% 4 9% 29% 20% 3	1680
	11th to 20th	Night	37% 4 14% 25% 17% 2	1680
	21st to end	Night	31% 6% 15% 28% 19% 1	1680
July	1st to 10th	Night	33% 5% 12% 29% 17% 4	4670
	11th to 20th	Night	28% 5% 11% 32% 23% 1	4662
	21st to end	Night	34% 5% 12% 22% 25% 2	5220
August	1st to 10th	Night	33% 5% 14% 29% 19% 1	2994
	11th to 20th	Night	35% 5% 11% 34% 14% 1	2938
	21st to end	Night	36% 6% 10% 31% 16% 1	3388
September	1st to 10th	Night	33% 6% 10% 30% 18% 2	2070
	11th to 20th	Night	29% 4 11% 32% 23% 2	2200
	21st to end	Night	41% 4 12% 27% 16% 1	2200
October	1st to 10th	Night	26% 3 10% 36% 25% 1	3608
	11th to 20th	Night	44% 3 10% 22% 20% 1	3921
	21st to end	Night	40% 3 10% 32% 15%	4610
November	1st to 10th	Night	44% 5% 10% 25% 15% 1	2666
	11th to 20th	Night	54% 4 12% 20% 9% 1	2730
	21st to end	Night	59% 3 11% 19% 6% 1	2730
December	1st to 10th	Night	65% 4 12% 15% 4	4414
	11th to 20th	Night	71% 3 9% 12% 5%	5341
	21st to end	Night	75% 3 8% 8% 5%	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	23% 13% 39% 18% 5% 2	330
	11th to 20th	Day	22% 18% 42% 12% 5% 1	330
	21st to end	Day	30% 17% 29% 20% 4	363
February	1st to 10th	Day	24% 20% 35% 16% 4 1	330
	11th to 20th	Day	25% 18% 39% 15% 3	330
	21st to end	Day	26% 26% 34% 11% 3	273
March	1st to 10th	Day	23% 17% 41% 15% 4	330
	11th to 20th	Day	27% 12% 32% 22% 7% 1	535
	21st to end	Day	26% 15% 33% 20% 5% 1	594
April	1st to 10th	Day	27% 12% 34% 20% 7% 1	540
	11th to 20th	Day	26% 16% 30% 16% 8% 4	532
	21st to end	Day	21% 11% 30% 24% 13% 1	540
May	1st to 10th	Day	25% 13% 30% 21% 9% 2	543
	11th to 20th	Day	19% 10% 27% 24% 15% 5%	567
	21st to end	Day	17% 8% 22% 27% 19% 7%	712
June	1st to 10th	Day	13% 7% 28% 32% 16% 5%	780
	11th to 20th	Day	14% 7% 30% 28% 16% 5%	780
	21st to end	Day	12% 7% 28% 31% 17% 5%	780
July	1st to 10th	Day	10% 4 20% 30% 24% 12%	771
	11th to 20th	Day	10% 5% 25% 29% 21% 10%	768
	21st to end	Day	8% 3 23% 31% 26% 9%	765
August	1st to 10th	Day	9% 3 19% 29% 21% 19%	566
	11th to 20th	Day	15% 2 17% 29% 26% 11%	535
	21st to end	Day	12% 2 19% 29% 25% 13%	583
September	1st to 10th	Day	13% 5% 20% 25% 23% 14%	530
	11th to 20th	Day	10% 2 17% 22% 30% 19%	530
	21st to end	Day	19% 9% 19% 24% 20% 9%	530
October	1st to 10th	Day	18% 6% 25% 24% 19% 7%	530
	11th to 20th	Day	21% 12% 29% 27% 9% 2	518
	21st to end	Day	28% 11% 38% 16% 6% 1	363
November	1st to 10th	Day	18% 17% 35% 22% 7% 1	330
	11th to 20th	Day	27% 18% 33% 16% 5% 2	330
	21st to end	Day	25% 19% 38% 13% 5%	330
December	1st to 10th	Day	25% 20% 31% 17% 7% 1	330
	11th to 20th	Day	26% 20% 36% 15% 3 1	330
	21st to end	Day	29% 20% 31% 15% 4	363
January	1st to 10th	Night	22% 14% 35% 22% 6% 1	1050
	11th to 20th	Night	24% 13% 37% 21% 4 1	1050
	21st to end	Night	20% 12% 35% 27% 5%	1064
February	1st to 10th	Night	25% 12% 33% 27% 3	840
	11th to 20th	Night	22% 15% 34% 25% 3	823
	21st to end	Night	24% 14% 40% 20% 3	692
March	1st to 10th	Night	22% 11% 35% 24% 7%	840
	11th to 20th	Night	20% 13% 35% 25% 5% 1	840
	21st to end	Night	23% 11% 32% 26% 7%	924
April	1st to 10th	Night	20% 12% 38% 25% 5% 1	647
	11th to 20th	Night	22% 11% 34% 24% 8% 1	623
	21st to end	Night	23% 6% 34% 29% 7%	630
May	1st to 10th	Night	20% 7% 31% 32% 9% 1	630
	11th to 20th	Night	23% 5% 29% 32% 11% 1	621
	21st to end	Night	13% 3 25% 38% 19% 2	683
June	1st to 10th	Night	13% 2 24% 43% 16% 1	630
	11th to 20th	Night	13% 3 25% 40% 19%	630
	21st to end	Night	11% 2 23% 41% 21% 2	630
July	1st to 10th	Night	9% 1 16% 43% 28% 3	615
	11th to 20th	Night	10% 2 17% 42% 26% 3	612
	21st to end	Night	7% 1 17% 48% 25% 2	690
August	1st to 10th	Night	8% 1 14% 46% 28% 3	627
	11th to 20th	Night	9% 2 16% 40% 28% 5%	606
	21st to end	Night	11% 3 14% 38% 30% 4	660
September	1st to 10th	Night	13% 3 17% 37% 26% 4	670
	11th to 20th	Night	11% 3 14% 35% 33% 4	800
	21st to end	Night	17% 4 21% 31% 22% 5%	800
October	1st to 10th	Night	16% 5% 23% 35% 18% 2	800
	11th to 20th	Night	23% 10% 28% 29% 9% 1	800
	21st to end	Night	25% 9% 33% 26% 7%	880
November	1st to 10th	Night	20% 9% 32% 29% 10% 1	820
	11th to 20th	Night	22% 14% 34% 23% 7%	840
	21st to end	Night	26% 11% 32% 26% 4	840
December	1st to 10th	Night	25% 13% 35% 21% 7%	861
	11th to 20th	Night	22% 15% 37% 23% 4	1050
	21st to end	Night	32% 11% 34% 20% 2	1155

Page generated at 2026-08-12 03:37:21. Version ac07

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