Seminary Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Seminary Hill Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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% 4 9% 5% 5% 5% 4 6% 7% 31%	137846
	Night	16% 10% 4 9% 6% 5% 5% 4 5% 6% 31%	134856
Transparency	Day	50% 6% 11% 20% 11% 1	141329
	Night	52% 6% 11% 19% 11% 1	139436
Seeing	Day	25% 17% 28% 19% 9% 2	16958
	Night	23% 14% 27% 27% 9% 1	25868

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	10% 10% 7% 20% 8% 6% 4 3 5% 5% 23%	4661
	11th to 20th	Day	10% 13% 5% 14% 6% 5% 5% 3 5% 6% 26%	4650
	21st to end	Day	9% 12% 5% 10% 6% 6% 5% 4 5% 7% 32%	5124
February	1st to 10th	Day	12% 12% 4 9% 7% 6% 5% 4 5% 7% 29%	1864
	11th to 20th	Day	10% 12% 5% 13% 7% 5% 5% 4 5% 6% 29%	2048
	21st to end	Day	15% 13% 4 10% 6% 5% 3 3 4 7% 30%	1730
March	1st to 10th	Day	17% 13% 5% 13% 6% 5% 4 3 4 4 26%	4576
	11th to 20th	Day	14% 10% 5% 8% 6% 5% 5% 3 6% 6% 33%	4761
	21st to end	Day	21% 12% 4 9% 5% 4 4 3 4 6% 29%	6069
April	1st to 10th	Day	19% 13% 3 8% 4 4 3 3 4 6% 33%	2520
	11th to 20th	Day	17% 10% 3 7% 5% 4 4 4 5% 6% 33%	2494
	21st to end	Day	21% 9% 3 8% 5% 4 3 4 5% 6% 32%	2719
May	1st to 10th	Day	28% 11% 3 8% 5% 4 4 4 5% 6% 22%	7639
	11th to 20th	Day	18% 9% 3 8% 5% 5% 5% 4 6% 7% 30%	8176
	21st to end	Day	19% 9% 3 7% 5% 5% 5% 4 6% 8% 30%	9060
June	1st to 10th	Day	18% 8% 3 7% 6% 6% 5% 4 6% 7% 30%	2940
	11th to 20th	Day	16% 8% 2 6% 5% 5% 5% 4 6% 8% 35%	2940
	21st to end	Day	19% 7% 3 7% 6% 6% 5% 5% 6% 7% 29%	2940
July	1st to 10th	Day	13% 7% 2 8% 5% 6% 5% 5% 7% 9% 34%	6500
	11th to 20th	Day	9% 6% 2 7% 5% 5% 7% 5% 8% 10% 35%	6430
	21st to end	Day	13% 6% 3 6% 4 4 5% 4 7% 8% 39%	6926
August	1st to 10th	Day	15% 6% 3 8% 5% 5% 6% 5% 7% 9% 32%	4466
	11th to 20th	Day	14% 8% 3 8% 5% 5% 5% 4 7% 8% 32%	4062
	21st to end	Day	9% 6% 3 7% 4 5% 5% 5% 7% 10% 41%	4312
September	1st to 10th	Day	12% 5% 1 6% 4 4 5% 4 6% 8% 44%	2400
	11th to 20th	Day	12% 5% 2 6% 3 4 3 3 5% 7% 49%	2246
	21st to end	Day	15% 6% 3 6% 4 4 4 4 5% 8% 42%	2200
October	1st to 10th	Day	20% 7% 2 7% 5% 4 4 3 5% 6% 36%	2179
	11th to 20th	Day	14% 9% 2 8% 5% 4 5% 3 5% 7% 37%	1919
	21st to end	Day	22% 11% 3 7% 5% 4 4 4 5% 5% 29%	1980
November	1st to 10th	Day	13% 9% 3 7% 4 4 4 4 7% 7% 38%	1851
	11th to 20th	Day	15% 12% 2 7% 5% 4 5% 4 5% 6% 35%	1890
	21st to end	Day	18% 12% 4 8% 5% 4 5% 3 5% 6% 29%	1780
December	1st to 10th	Day	15% 13% 5% 12% 7% 6% 5% 3 6% 7% 20%	2680
	11th to 20th	Day	20% 12% 4 9% 5% 4 4 3 5% 6% 29%	3066
	21st to end	Day	19% 13% 5% 10% 5% 4 4 3 4 5% 27%	4048
January	1st to 10th	Night	11% 11% 8% 17% 7% 5% 4 3 4 5% 24%	8234
	11th to 20th	Night	10% 13% 6% 17% 7% 6% 6% 4 6% 6% 20%	8235
	21st to end	Night	11% 10% 4 10% 8% 7% 6% 4 6% 6% 27%	9057
February	1st to 10th	Night	12% 12% 4 10% 6% 6% 6% 4 6% 7% 28%	2751
	11th to 20th	Night	10% 10% 5% 10% 7% 6% 6% 5% 6% 7% 29%	2458
	21st to end	Night	12% 10% 4 11% 6% 4 5% 3 5% 7% 31%	2076
March	1st to 10th	Night	20% 11% 4 9% 6% 4 5% 4 5% 6% 27%	5484
	11th to 20th	Night	14% 13% 3 9% 6% 6% 4 3 4 6% 32%	5418
	21st to end	Night	22% 12% 4 8% 4 4 4 3 4 5% 29%	5101
April	1st to 10th	Night	19% 10% 3 8% 6% 5% 4 2 5% 6% 32%	2100
	11th to 20th	Night	18% 11% 3 7% 5% 4 5% 3 4 5% 34%	2075
	21st to end	Night	19% 9% 4 6% 5% 5% 4 3 5% 7% 35%	1900
May	1st to 10th	Night	28% 12% 3 8% 4 4 4 3 4 6% 23%	5259
	11th to 20th	Night	17% 9% 3 8% 5% 5% 4 4 6% 7% 31%	4739
	21st to end	Night	20% 7% 2 7% 5% 5% 5% 4 6% 8% 31%	5114
June	1st to 10th	Night	19% 6% 3 7% 6% 5% 5% 4 6% 7% 33%	1680
	11th to 20th	Night	19% 6% 2 7% 4 4 5% 5% 5% 7% 37%	1680
	21st to end	Night	16% 7% 3 7% 6% 4 5% 5% 6% 8% 34%	1680
July	1st to 10th	Night	11% 6% 3 6% 4 5% 6% 5% 7% 9% 38%	3680
	11th to 20th	Night	14% 7% 3 7% 5% 5% 5% 4 6% 8% 37%	3640
	21st to end	Night	11% 6% 2 6% 5% 4 4 3 5% 7% 46%	3926
August	1st to 10th	Night	19% 9% 2 8% 4 4 4 3 4 7% 35%	2652
	11th to 20th	Night	13% 8% 3 7% 5% 4 4 4 6% 9% 38%	2969
	21st to end	Night	9% 6% 2 5% 4 4 4 4 7% 10% 46%	3586
September	1st to 10th	Night	13% 6% 2 5% 3 3 4 3 6% 8% 48%	2000
	11th to 20th	Night	12% 6% 2 6% 4 3 4 3 4 6% 51%	2153
	21st to end	Night	14% 6% 2 5% 4 4 4 3 6% 7% 44%	2200
October	1st to 10th	Night	20% 7% 2 7% 5% 4 4 3 5% 6% 37%	2219
	11th to 20th	Night	16% 9% 3 6% 5% 4 5% 3 6% 7% 36%	2480
	21st to end	Night	22% 11% 3 6% 4 5% 5% 3 5% 7% 30%	2860
November	1st to 10th	Night	14% 9% 3 8% 5% 4 5% 4 5% 6% 37%	2667
	11th to 20th	Night	15% 10% 3 7% 6% 4 5% 3 5% 6% 36%	2730
	21st to end	Night	15% 11% 5% 9% 5% 4 4 4 6% 7% 30%	2835
December	1st to 10th	Night	15% 14% 5% 15% 7% 5% 5% 4 4 6% 20%	4720
	11th to 20th	Night	22% 12% 4 8% 5% 4 4 3 4 5% 28%	5348
	21st to end	Night	19% 14% 5% 11% 6% 4 4 3 6% 5% 23%	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	60% 5% 9% 20% 5%	4662
	11th to 20th	Day	57% 6% 10% 16% 10% 2	4680
	21st to end	Day	50% 5% 9% 21% 13% 2	5124
February	1st to 10th	Day	54% 6% 12% 17% 9% 2	1864
	11th to 20th	Day	53% 6% 10% 19% 10% 2	2057
	21st to end	Day	55% 5% 10% 17% 12% 1	1730
March	1st to 10th	Day	62% 5% 7% 12% 9% 6%	4579
	11th to 20th	Day	47% 8% 14% 18% 9% 3	4761
	21st to end	Day	55% 5% 5% 14% 17% 3	6069
April	1st to 10th	Day	54% 4 9% 19% 12% 3	2520
	11th to 20th	Day	49% 7% 9% 18% 15% 2	2494
	21st to end	Day	53% 5% 8% 18% 13% 2	2719
May	1st to 10th	Day	62% 6% 9% 14% 8% 1	7651
	11th to 20th	Day	52% 7% 13% 18% 9% 1	8180
	21st to end	Day	50% 6% 11% 22% 9% 1	9061
June	1st to 10th	Day	52% 7% 11% 19% 10%	2940
	11th to 20th	Day	47% 7% 12% 19% 14% 2	2940
	21st to end	Day	51% 7% 12% 18% 10% 1	2940
July	1st to 10th	Day	45% 6% 15% 21% 13%	6430
	11th to 20th	Day	41% 7% 16% 23% 12% 1	6355
	21st to end	Day	42% 6% 13% 29% 11% 1	6728
August	1st to 10th	Day	47% 7% 15% 20% 11%	4452
	11th to 20th	Day	48% 7% 15% 23% 7%	4101
	21st to end	Day	38% 6% 15% 26% 13% 1	4312
September	1st to 10th	Day	36% 7% 13% 28% 15% 1	2400
	11th to 20th	Day	36% 5% 10% 28% 20% 1	2246
	21st to end	Day	39% 7% 12% 26% 14% 2	2200
October	1st to 10th	Day	47% 5% 9% 22% 16% 1	3543
	11th to 20th	Day	44% 4 13% 25% 14% 1	3096
	21st to end	Day	43% 4 9% 30% 12% 2	3180
November	1st to 10th	Day	42% 7% 14% 21% 14% 1	1851
	11th to 20th	Day	49% 5% 11% 16% 17% 2	1890
	21st to end	Day	55% 6% 16% 16% 7% 1	1780
December	1st to 10th	Day	62% 5% 13% 15% 5%	2680
	11th to 20th	Day	56% 5% 8% 16% 13% 1	3066
	21st to end	Day	58% 4 7% 15% 13% 3	4048
January	1st to 10th	Night	61% 5% 10% 15% 7% 1	8234
	11th to 20th	Night	61% 7% 10% 11% 9% 2	8280
	21st to end	Night	55% 6% 8% 15% 13% 3	9057
February	1st to 10th	Night	53% 7% 10% 19% 10% 1	2751
	11th to 20th	Night	51% 8% 11% 20% 10% 1	2460
	21st to end	Night	50% 7% 11% 20% 9% 3	2076
March	1st to 10th	Night	56% 4 9% 14% 13% 3	5494
	11th to 20th	Night	52% 7% 15% 15% 10% 2	5419
	21st to end	Night	55% 6% 6% 14% 18% 3	5101
April	1st to 10th	Night	54% 5% 9% 17% 14% 1	2100
	11th to 20th	Night	50% 6% 9% 20% 14% 1	2075
	21st to end	Night	49% 5% 10% 17% 16% 3	1900
May	1st to 10th	Night	62% 5% 10% 13% 9% 1	5265
	11th to 20th	Night	51% 7% 12% 20% 8% 1	4740
	21st to end	Night	50% 7% 12% 23% 9% 1	5115
June	1st to 10th	Night	48% 8% 11% 20% 12%	1680
	11th to 20th	Night	46% 8% 14% 19% 12% 1	1680
	21st to end	Night	47% 8% 13% 21% 11% 1	1680
July	1st to 10th	Night	41% 6% 18% 24% 11%	3640
	11th to 20th	Night	45% 6% 14% 22% 13%	3592
	21st to end	Night	38% 5% 13% 33% 10% 1	3809
August	1st to 10th	Night	50% 6% 13% 21% 10%	2642
	11th to 20th	Night	44% 6% 19% 21% 10%	2996
	21st to end	Night	32% 6% 19% 29% 13% 1	3586
September	1st to 10th	Night	35% 6% 15% 28% 16% 1	2000
	11th to 20th	Night	34% 5% 12% 29% 19% 1	2153
	21st to end	Night	38% 7% 12% 27% 16% 2	2200
October	1st to 10th	Night	48% 5% 13% 22% 10% 1	3622
	11th to 20th	Night	43% 4 12% 24% 15% 2	4029
	21st to end	Night	45% 5% 10% 29% 10% 1	4610
November	1st to 10th	Night	47% 6% 11% 22% 12% 2	2667
	11th to 20th	Night	47% 6% 11% 21% 14% 1	2730
	21st to end	Night	52% 7% 14% 20% 8%	2835
December	1st to 10th	Night	65% 5% 11% 16% 3	4720
	11th to 20th	Night	58% 5% 9% 13% 15% 1	5348
	21st to end	Night	61% 5% 7% 15% 8% 3	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	26% 23% 26% 18% 6%	220
	11th to 20th	Day	31% 25% 30% 11% 1 1	220
	21st to end	Day	26% 26% 30% 13% 6%	242
February	1st to 10th	Day	30% 28% 30% 11% 2	334
	11th to 20th	Day	28% 26% 30% 14% 2	536
	21st to end	Day	29% 23% 30% 15% 3	446
March	1st to 10th	Day	31% 20% 32% 15% 3	540
	11th to 20th	Day	33% 23% 28% 11% 4 1	540
	21st to end	Day	35% 21% 25% 11% 6% 2	594
April	1st to 10th	Day	37% 20% 25% 11% 6% 1	540
	11th to 20th	Day	34% 22% 24% 12% 6% 1	532
	21st to end	Day	31% 22% 27% 11% 7% 1	540
May	1st to 10th	Day	36% 21% 24% 12% 5% 2	543
	11th to 20th	Day	24% 18% 33% 18% 5% 2	567
	21st to end	Day	22% 15% 33% 20% 8% 1	617
June	1st to 10th	Day	23% 13% 34% 19% 10% 1	570
	11th to 20th	Day	21% 16% 32% 20% 9% 2	570
	21st to end	Day	22% 16% 32% 18% 9% 2	570
July	1st to 10th	Day	17% 12% 33% 23% 12% 3	565
	11th to 20th	Day	13% 10% 33% 31% 11% 3	544
	21st to end	Day	16% 8% 38% 26% 10% 2	576
August	1st to 10th	Day	13% 11% 28% 27% 18% 3	526
	11th to 20th	Day	17% 9% 27% 25% 14% 8%	519
	21st to end	Day	13% 14% 26% 26% 16% 5%	583
September	1st to 10th	Day	16% 12% 25% 25% 15% 7%	530
	11th to 20th	Day	15% 13% 23% 22% 17% 11%	530
	21st to end	Day	16% 14% 29% 22% 12% 7%	530
October	1st to 10th	Day	25% 13% 23% 20% 13% 6%	530
	11th to 20th	Day	23% 17% 22% 26% 9% 2	485
	21st to end	Day	35% 13% 24% 20% 7%	462
November	1st to 10th	Day	27% 18% 23% 20% 12% 1	425
	11th to 20th	Day	32% 18% 24% 18% 7% 1	430
	21st to end	Day	35% 18% 26% 15% 5% 1	320
December	1st to 10th	Day	30% 22% 26% 16% 5%	220
	11th to 20th	Day	34% 24% 30% 8% 4	220
	21st to end	Day	33% 19% 28% 14% 5%	242
January	1st to 10th	Night	23% 19% 33% 22% 3	840
	11th to 20th	Night	25% 19% 29% 23% 3	840
	21st to end	Night	21% 19% 35% 21% 4 1	924
February	1st to 10th	Night	23% 18% 34% 22% 3	840
	11th to 20th	Night	21% 19% 36% 22% 2	823
	21st to end	Night	24% 18% 32% 23% 3	692
March	1st to 10th	Night	26% 17% 31% 23% 3	840
	11th to 20th	Night	26% 19% 28% 23% 5%	840
	21st to end	Night	29% 17% 26% 21% 7% 1	924
April	1st to 10th	Night	27% 20% 26% 22% 5%	840
	11th to 20th	Night	26% 21% 24% 22% 6%	831
	21st to end	Night	26% 16% 30% 22% 6%	640
May	1st to 10th	Night	30% 12% 29% 23% 6%	630
	11th to 20th	Night	19% 13% 31% 30% 7%	451
	21st to end	Night	20% 9% 26% 34% 11% 1	455
June	1st to 10th	Night	21% 8% 27% 34% 10%	420
	11th to 20th	Night	20% 11% 26% 32% 11%	420
	21st to end	Night	17% 7% 24% 41% 10%	420
July	1st to 10th	Night	14% 6% 21% 44% 14%	410
	11th to 20th	Night	15% 4 21% 42% 17% 2	398
	21st to end	Night	13% 5% 21% 44% 16% 1	438
August	1st to 10th	Night	15% 4 17% 42% 22% 1	438
	11th to 20th	Night	14% 6% 24% 35% 19% 3	650
	21st to end	Night	15% 6% 21% 35% 22% 2	880
September	1st to 10th	Night	14% 9% 24% 32% 19% 2	800
	11th to 20th	Night	16% 7% 20% 31% 22% 4	800
	21st to end	Night	18% 8% 23% 31% 19% 2	800
October	1st to 10th	Night	26% 9% 18% 29% 17% 2	800
	11th to 20th	Night	23% 12% 23% 31% 11% 1	800
	21st to end	Night	29% 14% 25% 24% 8%	880
November	1st to 10th	Night	22% 17% 25% 27% 9%	820
	11th to 20th	Night	24% 16% 28% 23% 8% 1	840
	21st to end	Night	25% 17% 27% 25% 6%	840
December	1st to 10th	Night	24% 18% 32% 22% 4	840
	11th to 20th	Night	27% 19% 29% 21% 4	840
	21st to end	Night	29% 19% 31% 18% 4	924

Page generated at 2026-07-13 03:14:51. Version ac07

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