Starry Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Starry Hill Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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	14% 10% 3 8% 5% 5% 5% 4 5% 6% 34%	132526
	Night	14% 10% 3 8% 6% 5% 5% 4 5% 6% 33%	128207
Transparency	Day	51% 6% 13% 18% 10% 2	132245
	Night	54% 7% 13% 17% 8% 2	128065
Seeing	Day	24% 12% 22% 26% 12% 3	16463
	Night	27% 9% 15% 26% 20% 3	24976

By Month

Percentage Cloud Cover NW (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	21% 17% 5% 12% 8% 7% 6% 4 5% 5% 10%	3719
	11th to 20th	Day	16% 13% 4 8% 5% 4 5% 4 5% 6% 30%	3696
	21st to end	Day	14% 10% 4 9% 8% 6% 5% 3 5% 4 30%	4298
February	1st to 10th	Day	21% 16% 6% 11% 6% 5% 5% 4 5% 6% 17%	1801
	11th to 20th	Day	22% 19% 4 10% 6% 5% 4 3 4 5% 17%	1755
	21st to end	Day	23% 22% 4 9% 5% 5% 4 3 4 3 17%	1560
March	1st to 10th	Day	11% 12% 4 12% 8% 7% 7% 6% 9% 8% 17%	4646
	11th to 20th	Day	13% 15% 4 11% 7% 6% 6% 4 4 3 25%	5000
	21st to end	Day	15% 14% 5% 13% 9% 8% 7% 5% 7% 7% 11%	5656
April	1st to 10th	Day	22% 17% 5% 10% 7% 6% 6% 4 5% 6% 13%	2400
	11th to 20th	Day	19% 13% 4 10% 6% 5% 4 4 5% 5% 23%	2381
	21st to end	Day	20% 14% 4 11% 6% 5% 5% 4 4 5% 22%	2620
May	1st to 10th	Day	15% 8% 3 7% 5% 5% 5% 4 5% 5% 38%	6320
	11th to 20th	Day	13% 10% 4 12% 8% 7% 7% 4 6% 6% 23%	6277
	21st to end	Day	18% 8% 3 9% 5% 5% 5% 4 6% 7% 27%	6903
June	1st to 10th	Day	23% 10% 3 7% 5% 5% 4 4 6% 6% 27%	2800
	11th to 20th	Day	21% 11% 4 9% 7% 6% 6% 4 7% 5% 20%	2949
	21st to end	Day	12% 8% 4 8% 5% 5% 5% 4 6% 7% 37%	3000
July	1st to 10th	Day	6% 4 1 4 3 2 3 3 4 5% 65%	6739
	11th to 20th	Day	4 3 1 3 2 2 3 2 3 5% 72%	6320
	21st to end	Day	7% 4 1 4 3 3 3 3 5% 6% 62%	6874
August	1st to 10th	Day	5% 4 2 4 3 3 3 3 5% 7% 63%	4400
	11th to 20th	Day	8% 6% 2 7% 4 5% 5% 4 7% 8% 44%	4156
	21st to end	Day	12% 6% 3 7% 4 4 4 3 5% 7% 45%	4450
September	1st to 10th	Day	10% 6% 2 6% 4 4 4 3 6% 7% 48%	2400
	11th to 20th	Day	12% 8% 3 7% 5% 5% 5% 4 5% 7% 40%	2400
	21st to end	Day	14% 8% 3 7% 4 5% 4 3 5% 7% 40%	2112
October	1st to 10th	Day	14% 7% 2 6% 3 4 4 3 5% 9% 43%	3280
	11th to 20th	Day	19% 8% 3 8% 5% 6% 5% 5% 6% 8% 28%	3261
	21st to end	Day	18% 8% 3 9% 6% 5% 6% 5% 6% 7% 24%	3382
November	1st to 10th	Day	23% 14% 4 9% 7% 6% 5% 5% 6% 5% 16%	1714
	11th to 20th	Day	25% 13% 5% 9% 6% 5% 4 3 6% 6% 17%	1680
	21st to end	Day	22% 15% 3 10% 5% 4 5% 3 6% 5% 21%	1680
December	1st to 10th	Day	15% 10% 3 8% 5% 6% 6% 4 4 5% 34%	2700
	11th to 20th	Day	20% 12% 4 10% 7% 6% 7% 5% 6% 6% 17%	3105
	21st to end	Day	20% 15% 4 11% 8% 7% 7% 4 5% 5% 14%	4092
January	1st to 10th	Night	21% 17% 6% 13% 7% 6% 5% 3 5% 4 12%	6446
	11th to 20th	Night	13% 14% 5% 9% 5% 5% 4 4 5% 7% 29%	6422
	21st to end	Night	16% 11% 4 9% 6% 5% 5% 4 5% 5% 30%	6907
February	1st to 10th	Night	22% 14% 5% 10% 7% 5% 5% 3 5% 6% 18%	2600
	11th to 20th	Night	20% 15% 4 11% 6% 5% 4 3 4 5% 22%	2532
	21st to end	Night	20% 18% 4 9% 6% 6% 6% 4 4 5% 19%	2069
March	1st to 10th	Night	11% 13% 4 10% 7% 7% 7% 6% 8% 7% 21%	5244
	11th to 20th	Night	13% 10% 3 9% 6% 6% 5% 5% 6% 7% 30%	4999
	21st to end	Night	19% 15% 4 11% 7% 7% 6% 5% 6% 6% 14%	5319
April	1st to 10th	Night	17% 15% 4 11% 8% 6% 5% 4 5% 5% 20%	2000
	11th to 20th	Night	16% 13% 4 10% 6% 5% 4 4 5% 5% 28%	1969
	21st to end	Night	19% 12% 3 9% 6% 5% 6% 3 5% 6% 25%	1780
May	1st to 10th	Night	14% 9% 3 7% 4 4 4 4 5% 5% 40%	3640
	11th to 20th	Night	12% 9% 3 10% 6% 6% 6% 4 5% 7% 32%	3616
	21st to end	Night	12% 9% 3 8% 6% 6% 6% 5% 7% 8% 31%	3972
June	1st to 10th	Night	17% 8% 3 6% 6% 6% 5% 5% 5% 8% 33%	1600
	11th to 20th	Night	16% 11% 4 8% 6% 5% 4 4 6% 8% 30%	1445
	21st to end	Night	10% 7% 2 5% 5% 4 4 4 5% 8% 46%	1400
July	1st to 10th	Night	3 3 1 2 2 2 3 3 4 6% 72%	3216
	11th to 20th	Night	3 2 1 3 2 2 2 2 3 4 77%	3640
	21st to end	Night	4 4 1 3 2 2 3 2 4 6% 69%	3969
August	1st to 10th	Night	3 3 1 3 3 2 2 2 4 5% 71%	2739
	11th to 20th	Night	9% 6% 2 5% 4 4 5% 4 5% 6% 52%	2902
	21st to end	Night	13% 6% 2 6% 4 4 4 3 4 5% 50%	3466
September	1st to 10th	Night	8% 5% 2 5% 3 3 4 3 5% 5% 56%	2000
	11th to 20th	Night	13% 7% 3 7% 5% 4 4 4 4 7% 43%	2000
	21st to end	Night	11% 6% 2 8% 5% 4 5% 3 5% 7% 44%	2282
October	1st to 10th	Night	12% 7% 3 6% 4 4 4 3 6% 7% 45%	3920
	11th to 20th	Night	16% 9% 4 8% 5% 5% 4 4 5% 6% 34%	3897
	21st to end	Night	14% 8% 3 9% 6% 6% 7% 6% 7% 8% 28%	4410
November	1st to 10th	Night	25% 10% 4 8% 6% 6% 5% 4 5% 6% 19%	2800
	11th to 20th	Night	24% 12% 4 9% 7% 6% 5% 4 4 5% 20%	2940
	21st to end	Night	21% 13% 4 9% 5% 5% 6% 4 5% 6% 22%	2940
December	1st to 10th	Night	13% 10% 3 8% 5% 6% 5% 5% 6% 7% 33%	4700
	11th to 20th	Night	19% 14% 5% 12% 8% 6% 6% 5% 6% 5% 15%	5320
	21st to end	Night	18% 12% 4 11% 8% 7% 7% 5% 6% 6% 16%	7106

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	76% 6% 7% 8% 3 1	3719
	11th to 20th	Day	53% 8% 11% 18% 9% 2	3720
	21st to end	Day	59% 5% 5% 16% 15% 1	4298
February	1st to 10th	Day	69% 8% 8% 11% 5%	1801
	11th to 20th	Day	69% 8% 7% 9% 6% 1	1764
	21st to end	Day	72% 5% 9% 9% 5%	1560
March	1st to 10th	Day	61% 7% 16% 10% 5%	4646
	11th to 20th	Day	65% 4 9% 20% 1	5000
	21st to end	Day	72% 8% 14% 7% 1	5656
April	1st to 10th	Day	71% 7% 13% 8% 1	2400
	11th to 20th	Day	62% 6% 12% 13% 7% 1	2381
	21st to end	Day	65% 7% 11% 12% 3 1	2620
May	1st to 10th	Day	47% 6% 15% 24% 9%	6320
	11th to 20th	Day	61% 8% 17% 12% 2	6280
	21st to end	Day	56% 8% 21% 13% 2	6904
June	1st to 10th	Day	58% 7% 15% 14% 6%	2800
	11th to 20th	Day	64% 7% 14% 11% 4	2949
	21st to end	Day	45% 8% 18% 19% 8% 1	3000
July	1st to 10th	Day	23% 5% 15% 25% 18% 15%	6665
	11th to 20th	Day	17% 4 11% 28% 35% 6%	6245
	21st to end	Day	24% 5% 20% 35% 14% 2	6681
August	1st to 10th	Day	23% 5% 11% 34% 24% 3	4383
	11th to 20th	Day	39% 6% 19% 24% 10% 2	4195
	21st to end	Day	41% 5% 14% 23% 15% 2	4449
September	1st to 10th	Day	34% 6% 15% 25% 15% 5%	2400
	11th to 20th	Day	42% 9% 13% 18% 14% 4	2400
	21st to end	Day	44% 7% 15% 20% 12% 2	2112
October	1st to 10th	Day	40% 5% 12% 31% 12% 1	3280
	11th to 20th	Day	56% 6% 13% 16% 9% 1	3261
	21st to end	Day	61% 5% 12% 11% 10% 1	3382
November	1st to 10th	Day	69% 6% 10% 11% 3 1	1714
	11th to 20th	Day	69% 7% 9% 8% 5% 2	1680
	21st to end	Day	66% 6% 7% 10% 10% 2	1680
December	1st to 10th	Day	54% 5% 8% 16% 16% 2	2700
	11th to 20th	Day	70% 5% 12% 9% 3	3108
	21st to end	Day	72% 7% 10% 8% 2	4092
January	1st to 10th	Night	77% 5% 7% 7% 4 1	6446
	11th to 20th	Night	54% 6% 11% 20% 8% 1	6460
	21st to end	Night	56% 6% 7% 19% 11% 1	6907
February	1st to 10th	Night	69% 7% 9% 10% 5% 1	2600
	11th to 20th	Night	65% 7% 9% 10% 7% 2	2534
	21st to end	Night	68% 6% 11% 10% 5%	2069
March	1st to 10th	Night	62% 7% 13% 12% 5% 1	5244
	11th to 20th	Night	56% 6% 15% 20% 2 1	5000
	21st to end	Night	71% 8% 11% 9%	5319
April	1st to 10th	Night	67% 9% 13% 10% 2	2000
	11th to 20th	Night	59% 7% 15% 13% 7% 1	1969
	21st to end	Night	62% 8% 13% 12% 4 1	1780
May	1st to 10th	Night	47% 7% 13% 25% 8%	3637
	11th to 20th	Night	54% 9% 20% 13% 3 1	3616
	21st to end	Night	54% 12% 20% 13% 1	3972
June	1st to 10th	Night	51% 9% 18% 18% 5%	1600
	11th to 20th	Night	55% 9% 17% 16% 3	1445
	21st to end	Night	39% 9% 20% 22% 8% 2	1400
July	1st to 10th	Night	18% 5% 18% 29% 15% 16%	3181
	11th to 20th	Night	15% 3 10% 32% 34% 7%	3592
	21st to end	Night	19% 5% 21% 36% 16% 3	3857
August	1st to 10th	Night	18% 5% 10% 40% 25% 3	2731
	11th to 20th	Night	36% 6% 24% 22% 9% 3	2929
	21st to end	Night	39% 5% 15% 28% 13% 1	3466
September	1st to 10th	Night	31% 7% 16% 26% 15% 5%	2000
	11th to 20th	Night	43% 7% 11% 20% 15% 4	2000
	21st to end	Night	41% 7% 16% 20% 13% 3	2282
October	1st to 10th	Night	39% 6% 16% 27% 11% 1	3920
	11th to 20th	Night	52% 5% 13% 20% 8% 1	3897
	21st to end	Night	57% 7% 11% 15% 10%	4410
November	1st to 10th	Night	66% 8% 10% 11% 4 1	2800
	11th to 20th	Night	68% 8% 9% 8% 5% 2	2940
	21st to end	Night	64% 8% 8% 10% 10% 1	2940
December	1st to 10th	Night	52% 6% 11% 18% 13% 1	4700
	11th to 20th	Night	73% 6% 10% 8% 3	5316
	21st to end	Night	71% 7% 12% 7% 2	7106

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	39% 12% 19% 16% 10% 4	220
	11th to 20th	Day	29% 17% 17% 22% 13% 3	220
	21st to end	Day	32% 16% 19% 19% 11% 3	331
February	1st to 10th	Day	39% 13% 20% 17% 10% 1	401
	11th to 20th	Day	42% 14% 19% 16% 7% 1	395
	21st to end	Day	40% 16% 20% 19% 5% 1	369
March	1st to 10th	Day	30% 20% 24% 19% 6% 1	500
	11th to 20th	Day	31% 16% 19% 19% 13% 1	500
	21st to end	Day	33% 16% 27% 18% 5% 1	550
April	1st to 10th	Day	34% 17% 21% 19% 8% 2	500
	11th to 20th	Day	27% 21% 24% 19% 6% 3	492
	21st to end	Day	29% 15% 24% 22% 8% 2	500
May	1st to 10th	Day	22% 14% 28% 24% 11% 2	503
	11th to 20th	Day	29% 13% 23% 21% 9% 4	528
	21st to end	Day	26% 12% 22% 27% 11% 3	573
June	1st to 10th	Day	25% 10% 23% 27% 12% 3	530
	11th to 20th	Day	25% 13% 28% 25% 7% 3	679
	21st to end	Day	14% 9% 24% 35% 15% 4	730
July	1st to 10th	Day	13% 7% 23% 38% 16% 3	706
	11th to 20th	Day	8% 7% 24% 38% 19% 3	524
	21st to end	Day	9% 3 19% 41% 22% 6%	575
August	1st to 10th	Day	11% 7% 17% 40% 18% 6%	527
	11th to 20th	Day	8% 3 19% 42% 20% 8%	517
	21st to end	Day	11% 11% 23% 33% 15% 7%	583
September	1st to 10th	Day	14% 7% 23% 37% 15% 5%	530
	11th to 20th	Day	16% 10% 21% 31% 19% 2	530
	21st to end	Day	18% 12% 24% 28% 15% 3	530
October	1st to 10th	Day	22% 14% 20% 22% 19% 3	530
	11th to 20th	Day	28% 14% 19% 21% 14% 5%	530
	21st to end	Day	33% 12% 20% 20% 10% 4	482
November	1st to 10th	Day	33% 19% 17% 19% 9% 2	256
	11th to 20th	Day	35% 17% 13% 16% 12% 6%	220
	21st to end	Day	27% 14% 22% 17% 12% 8%	220
December	1st to 10th	Day	40% 14% 9% 15% 14% 9%	220
	11th to 20th	Day	42% 13% 21% 16% 6% 1	220
	21st to end	Day	40% 12% 16% 19% 11% 2	242
January	1st to 10th	Night	42% 13% 15% 16% 12% 2	840
	11th to 20th	Night	32% 12% 15% 24% 16% 3	840
	21st to end	Night	31% 10% 14% 26% 15% 3	924
February	1st to 10th	Night	35% 12% 16% 26% 10% 1	800
	11th to 20th	Night	35% 11% 13% 27% 13% 1	784
	21st to end	Night	35% 12% 17% 25% 10% 1	660
March	1st to 10th	Night	29% 14% 17% 25% 14% 1	800
	11th to 20th	Night	32% 14% 16% 24% 13% 1	800
	21st to end	Night	36% 11% 20% 22% 9% 1	880
April	1st to 10th	Night	30% 13% 17% 23% 15% 2	800
	11th to 20th	Night	27% 10% 21% 25% 15% 2	781
	21st to end	Night	29% 11% 13% 31% 14% 2	580
May	1st to 10th	Night	24% 9% 15% 33% 19% 1	400
	11th to 20th	Night	21% 9% 12% 28% 27% 2	394
	21st to end	Night	24% 7% 15% 24% 26% 3	433
June	1st to 10th	Night	19% 9% 16% 26% 27% 3	400
	11th to 20th	Night	22% 8% 18% 31% 21% 1	400
	21st to end	Night	15% 5% 14% 31% 33% 3	400
July	1st to 10th	Night	11% 3 9% 34% 39% 4	390
	11th to 20th	Night	9% 3 8% 38% 39% 3	388
	21st to end	Night	6% 2 10% 29% 48% 5%	438
August	1st to 10th	Night	11% 3 8% 25% 44% 9%	476
	11th to 20th	Night	9% 4 5% 26% 45% 11%	594
	21st to end	Night	13% 5% 9% 32% 37% 4	790
September	1st to 10th	Night	12% 4 10% 33% 35% 6%	800
	11th to 20th	Night	16% 6% 12% 25% 34% 7%	800
	21st to end	Night	18% 7% 12% 32% 27% 4	800
October	1st to 10th	Night	21% 7% 12% 30% 25% 4	800
	11th to 20th	Night	27% 9% 15% 26% 20% 4	800
	21st to end	Night	28% 10% 20% 25% 14% 3	880
November	1st to 10th	Night	35% 11% 17% 28% 9% 1	820
	11th to 20th	Night	37% 12% 16% 21% 12% 2	840
	21st to end	Night	34% 9% 18% 22% 14% 4	840
December	1st to 10th	Night	31% 10% 16% 26% 15% 3	840
	11th to 20th	Night	39% 12% 17% 23% 8% 1	840
	21st to end	Night	42% 13% 17% 18% 8%	924

Page generated at 2026-01-28 22:21:44. Version ac07

Copyright © 2026 Attilla Danko.