Observe the Heavens Astronomy Camp
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observe the Heavens Astronomy Camp clear sky chart from 2005-11-06 to 2026-01-17.

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	7% 8% 3 7% 5% 5% 5% 4 6% 7% 44%	130918
	Night	9% 10% 3 8% 5% 4 5% 4 6% 7% 40%	129465
Transparency	Day	38% 5% 11% 23% 16% 6%	130644
	Night	43% 5% 12% 22% 14% 5%	129316
Seeing	Day	13% 9% 23% 29% 19% 6%	16166
	Night	16% 4 14% 39% 24% 3	25391

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	13% 18% 9% 14% 8% 7% 5% 3 4 4 14%	3719
	11th to 20th	Day	13% 9% 3 7% 5% 6% 6% 4 5% 6% 35%	3664
	21st to end	Day	18% 18% 4 10% 4 4 5% 3 4 5% 24%	4004
February	1st to 10th	Day	11% 14% 4 10% 7% 7% 5% 4 5% 8% 25%	1624
	11th to 20th	Day	11% 14% 3 8% 6% 5% 5% 5% 7% 7% 29%	1825
	21st to end	Day	12% 13% 4 9% 5% 6% 5% 5% 6% 7% 29%	1650
March	1st to 10th	Day	6% 10% 3 6% 5% 5% 7% 6% 9% 9% 35%	4518
	11th to 20th	Day	6% 11% 3 7% 4 5% 4 3 5% 8% 44%	4662
	21st to end	Day	10% 11% 4 9% 7% 6% 6% 5% 8% 9% 27%	5969
April	1st to 10th	Day	10% 11% 3 9% 7% 6% 6% 6% 8% 9% 27%	2400
	11th to 20th	Day	8% 8% 2 9% 7% 6% 5% 5% 7% 8% 37%	2416
	21st to end	Day	8% 7% 3 9% 6% 6% 6% 4 7% 9% 36%	2613
May	1st to 10th	Day	5% 6% 2 7% 4 4 5% 4 7% 9% 46%	6320
	11th to 20th	Day	6% 7% 4 9% 6% 6% 6% 5% 7% 10% 34%	6277
	21st to end	Day	6% 6% 3 8% 5% 5% 6% 5% 6% 8% 42%	6903
June	1st to 10th	Day	12% 7% 3 7% 6% 5% 5% 3 6% 7% 40%	2976
	11th to 20th	Day	9% 6% 3 8% 7% 6% 6% 5% 7% 7% 37%	3000
	21st to end	Day	5% 4 2 5% 4 4 5% 5% 6% 8% 53%	2993
July	1st to 10th	Day	2 2 1 3 2 3 3 2 4 6% 71%	6320
	11th to 20th	Day	1 2 1 2 2 2 2 2 3 4 78%	6320
	21st to end	Day	3 2 1 3 2 2 3 3 5% 6% 71%	6874
August	1st to 10th	Day	3 2 1 4 3 3 4 3 5% 7% 66%	4541
	11th to 20th	Day	2 2 1 3 3 4 3 3 5% 8% 66%	4360
	21st to end	Day	2 3 2 4 4 4 5% 3 5% 6% 61%	4347
September	1st to 10th	Day	3 3 1 3 3 3 4 3 5% 6% 66%	2400
	11th to 20th	Day	5% 5% 2 5% 5% 4 5% 5% 6% 7% 51%	2322
	21st to end	Day	6% 5% 2 6% 4 5% 4 3 6% 6% 52%	2200
October	1st to 10th	Day	5% 4 2 6% 3 4 3 4 5% 7% 56%	3384
	11th to 20th	Day	6% 5% 2 7% 5% 5% 6% 5% 8% 8% 43%	3105
	21st to end	Day	11% 8% 3 6% 6% 5% 6% 5% 6% 6% 39%	3200
November	1st to 10th	Day	16% 12% 4 9% 6% 4 5% 4 6% 7% 28%	1854
	11th to 20th	Day	15% 14% 4 10% 7% 5% 5% 4 4 5% 27%	1714
	21st to end	Day	14% 20% 6% 10% 7% 5% 5% 4 4 4 20%	1566
December	1st to 10th	Day	10% 11% 4 12% 7% 6% 6% 5% 6% 6% 26%	2360
	11th to 20th	Day	17% 23% 7% 12% 6% 5% 4 3 5% 4 14%	2712
	21st to end	Day	13% 23% 8% 15% 9% 5% 5% 3 4 4 10%	3806
January	1st to 10th	Night	17% 19% 7% 14% 8% 5% 4 3 3 4 15%	6446
	11th to 20th	Night	12% 10% 3 8% 6% 5% 5% 4 6% 9% 32%	6366
	21st to end	Night	17% 19% 5% 9% 5% 4 3 3 5% 4 26%	6952
February	1st to 10th	Night	11% 17% 5% 10% 6% 4 5% 4 6% 7% 26%	2775
	11th to 20th	Night	9% 13% 4 10% 5% 6% 4 5% 7% 6% 31%	2461
	21st to end	Night	9% 11% 4 8% 5% 5% 5% 5% 7% 8% 34%	1980
March	1st to 10th	Night	5% 7% 3 8% 5% 6% 6% 4 7% 9% 41%	5376
	11th to 20th	Night	9% 11% 3 7% 4 4 4 3 4 6% 46%	5321
	21st to end	Night	9% 9% 3 9% 7% 7% 8% 6% 8% 10% 25%	5006
April	1st to 10th	Night	8% 8% 2 9% 6% 5% 6% 4 7% 8% 37%	2000
	11th to 20th	Night	6% 8% 3 8% 7% 5% 5% 3 5% 7% 42%	1934
	21st to end	Night	5% 7% 4 7% 6% 5% 6% 4 7% 8% 43%	1785
May	1st to 10th	Night	5% 5% 2 6% 4 4 5% 4 7% 9% 47%	3640
	11th to 20th	Night	6% 6% 3 7% 5% 5% 5% 5% 7% 8% 43%	3616
	21st to end	Night	6% 5% 2 6% 4 4 4 4 6% 10% 48%	3972
June	1st to 10th	Night	8% 6% 2 6% 4 4 5% 4 7% 9% 44%	1420
	11th to 20th	Night	5% 5% 2 6% 5% 5% 6% 5% 7% 9% 43%	1400
	21st to end	Night	3 4 1 5% 4 4 5% 3 6% 7% 58%	1401
July	1st to 10th	Night	2 2 1 2 2 2 2 3 4 6% 75%	3640
	11th to 20th	Night	2 1 1 2 2 2 2 2 3 5% 80%	3640
	21st to end	Night	2 1 1 2 2 2 2 2 4 6% 75%	3969
August	1st to 10th	Night	2 3 1 3 2 2 2 3 4 7% 70%	2609
	11th to 20th	Night	2 3 1 4 3 3 3 4 6% 8% 64%	2690
	21st to end	Night	2 3 1 4 4 3 5% 4 5% 7% 63%	3570
September	1st to 10th	Night	2 2 1 3 2 3 4 3 5% 9% 66%	2000
	11th to 20th	Night	5% 4 2 5% 3 3 5% 4 7% 7% 55%	2077
	21st to end	Night	6% 4 2 5% 3 4 3 3 7% 7% 55%	2200
October	1st to 10th	Night	5% 3 1 4 3 3 4 3 6% 8% 60%	3814
	11th to 20th	Night	7% 5% 2 6% 3 4 5% 4 7% 8% 48%	4047
	21st to end	Night	11% 7% 2 6% 4 5% 5% 4 6% 7% 43%	4600
November	1st to 10th	Night	17% 9% 4 8% 5% 5% 5% 5% 7% 8% 28%	2665
	11th to 20th	Night	16% 13% 4 8% 6% 6% 6% 4 6% 5% 26%	2903
	21st to end	Night	15% 16% 6% 11% 7% 5% 5% 3 5% 5% 23%	3029
December	1st to 10th	Night	8% 10% 4 11% 7% 5% 5% 5% 6% 7% 33%	5040
	11th to 20th	Night	16% 20% 7% 13% 7% 5% 5% 3 4 5% 15%	5700
	21st to end	Night	17% 22% 6% 12% 7% 6% 5% 4 4 3 13%	7421

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	73% 6% 8% 11% 3	3719
	11th to 20th	Day	48% 6% 21% 18% 5% 2	3688
	21st to end	Day	63% 6% 10% 16% 4	4004
February	1st to 10th	Day	57% 8% 14% 11% 9% 1	1624
	11th to 20th	Day	52% 7% 15% 17% 8%	1834
	21st to end	Day	53% 8% 13% 16% 9% 1	1650
March	1st to 10th	Day	42% 6% 15% 21% 16%	4518
	11th to 20th	Day	39% 5% 17% 29% 11%	4662
	21st to end	Day	50% 10% 17% 16% 7%	5969
April	1st to 10th	Day	50% 7% 15% 20% 7% 1	2400
	11th to 20th	Day	43% 6% 14% 22% 12% 3	2416
	21st to end	Day	43% 7% 12% 25% 11% 3	2613
May	1st to 10th	Day	33% 4 7% 28% 26% 2	6320
	11th to 20th	Day	39% 7% 11% 28% 14% 1	6280
	21st to end	Day	36% 9% 14% 30% 11% 2	6904
June	1st to 10th	Day	42% 5% 9% 21% 18% 5%	2976
	11th to 20th	Day	43% 6% 9% 21% 16% 5%	3000
	21st to end	Day	28% 4 12% 28% 22% 6%	2993
July	1st to 10th	Day	16% 3 5% 22% 27% 27%	6251
	11th to 20th	Day	12% 2 4 22% 30% 31%	6245
	21st to end	Day	16% 2 4 30% 30% 19%	6681
August	1st to 10th	Day	18% 3 5% 26% 35% 14%	4524
	11th to 20th	Day	19% 2 8% 41% 25% 5%	4402
	21st to end	Day	24% 3 11% 32% 25% 5%	4346
September	1st to 10th	Day	19% 4 9% 32% 28% 8%	2400
	11th to 20th	Day	30% 5% 13% 26% 22% 4	2322
	21st to end	Day	32% 5% 17% 30% 13% 4	2200
October	1st to 10th	Day	28% 4 15% 36% 15% 1	3384
	11th to 20th	Day	37% 5% 18% 23% 16%	3105
	21st to end	Day	44% 5% 17% 21% 12% 1	3200
November	1st to 10th	Day	56% 8% 13% 16% 6% 1	1854
	11th to 20th	Day	60% 7% 14% 13% 5% 2	1714
	21st to end	Day	65% 7% 11% 11% 4	1566
December	1st to 10th	Day	57% 8% 20% 13% 3	2360
	11th to 20th	Day	73% 6% 9% 8% 3	2714
	21st to end	Day	73% 9% 8% 6% 3	3806
January	1st to 10th	Night	72% 6% 9% 11% 2	6446
	11th to 20th	Night	48% 6% 17% 20% 7% 1	6404
	21st to end	Night	60% 6% 14% 16% 4	6952
February	1st to 10th	Night	57% 7% 13% 13% 7% 2	2775
	11th to 20th	Night	51% 8% 14% 18% 9% 1	2463
	21st to end	Night	45% 8% 16% 19% 12% 1	1980
March	1st to 10th	Night	38% 5% 15% 26% 16% 1	5376
	11th to 20th	Night	41% 3 15% 27% 14% 1	5322
	21st to end	Night	50% 9% 15% 19% 6%	5006
April	1st to 10th	Night	43% 6% 15% 25% 10% 1	2000
	11th to 20th	Night	42% 5% 9% 25% 17% 2	1934
	21st to end	Night	37% 5% 12% 25% 18% 2	1785
May	1st to 10th	Night	31% 6% 6% 27% 27% 3	3637
	11th to 20th	Night	35% 6% 10% 32% 15% 2	3616
	21st to end	Night	30% 9% 13% 35% 12% 1	3972
June	1st to 10th	Night	35% 7% 9% 25% 19% 7%	1420
	11th to 20th	Night	34% 6% 12% 22% 20% 6%	1400
	21st to end	Night	24% 4 9% 29% 26% 8%	1401
July	1st to 10th	Night	12% 3 3 20% 33% 28%	3600
	11th to 20th	Night	10% 2 3 24% 25% 36%	3592
	21st to end	Night	13% 1 3 26% 33% 24%	3857
August	1st to 10th	Night	16% 2 4 26% 31% 21%	2601
	11th to 20th	Night	18% 2 6% 38% 30% 6%	2714
	21st to end	Night	22% 2 10% 30% 27% 9%	3570
September	1st to 10th	Night	16% 3 8% 30% 30% 13%	2000
	11th to 20th	Night	27% 5% 10% 26% 24% 7%	2077
	21st to end	Night	28% 3 14% 33% 16% 6%	2200
October	1st to 10th	Night	24% 3 12% 42% 18% 1	3814
	11th to 20th	Night	34% 4 16% 27% 18% 1	4047
	21st to end	Night	41% 5% 16% 27% 10% 2	4600
November	1st to 10th	Night	53% 8% 17% 15% 7% 1	2665
	11th to 20th	Night	56% 9% 15% 13% 6% 2	2903
	21st to end	Night	62% 7% 13% 12% 6%	3029
December	1st to 10th	Night	49% 9% 17% 22% 2	5040
	11th to 20th	Night	72% 6% 12% 8% 3	5697
	21st to end	Night	71% 7% 9% 9% 3	7421

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	29% 7% 20% 29% 13% 4	220
	11th to 20th	Day	26% 2 20% 29% 19% 4	212
	21st to end	Day	27% 5% 18% 31% 15% 4	220
February	1st to 10th	Day	17% 7% 26% 35% 13% 1	224
	11th to 20th	Day	20% 8% 22% 32% 15% 3	430
	21st to end	Day	19% 9% 26% 30% 14% 2	414
March	1st to 10th	Day	16% 14% 26% 26% 13% 5%	500
	11th to 20th	Day	19% 10% 20% 27% 19% 5%	500
	21st to end	Day	19% 12% 26% 28% 11% 4	550
April	1st to 10th	Day	18% 12% 27% 27% 11% 5%	500
	11th to 20th	Day	13% 19% 29% 22% 12% 5%	492
	21st to end	Day	12% 15% 24% 27% 17% 5%	500
May	1st to 10th	Day	10% 11% 29% 28% 17% 5%	503
	11th to 20th	Day	15% 9% 24% 27% 19% 6%	528
	21st to end	Day	12% 10% 22% 26% 21% 9%	573
June	1st to 10th	Day	15% 9% 20% 28% 22% 6%	706
	11th to 20th	Day	12% 11% 26% 29% 18% 4	730
	21st to end	Day	6% 7% 20% 32% 27% 8%	723
July	1st to 10th	Day	5% 6% 27% 37% 21% 5%	526
	11th to 20th	Day	5% 10% 24% 34% 20% 7%	524
	21st to end	Day	3 7% 23% 35% 22% 10%	575
August	1st to 10th	Day	6% 5% 17% 35% 26% 11%	527
	11th to 20th	Day	5% 4 21% 30% 27% 13%	517
	21st to end	Day	7% 6% 25% 26% 23% 11%	583
September	1st to 10th	Day	7% 5% 24% 26% 27% 11%	530
	11th to 20th	Day	8% 8% 20% 33% 23% 8%	530
	21st to end	Day	10% 12% 25% 28% 18% 7%	530
October	1st to 10th	Day	10% 10% 23% 30% 21% 5%	530
	11th to 20th	Day	15% 8% 21% 32% 18% 6%	438
	21st to end	Day	19% 9% 26% 31% 13% 3	363
November	1st to 10th	Day	23% 8% 22% 28% 18% 1	330
	11th to 20th	Day	25% 9% 19% 27% 17% 4	236
	21st to end	Day	22% 7% 19% 32% 16% 3	220
December	1st to 10th	Day	23% 5% 21% 23% 21% 6%	220
	11th to 20th	Day	26% 10% 24% 28% 11% 2	220
	21st to end	Day	27% 6% 18% 36% 11% 2	242
January	1st to 10th	Night	30% 5% 18% 29% 15% 2	840
	11th to 20th	Night	26% 4 15% 38% 15% 3	824
	21st to end	Night	23% 4 11% 40% 19% 2	880
February	1st to 10th	Night	22% 4 18% 40% 14% 2	800
	11th to 20th	Night	17% 5% 19% 38% 20% 2	784
	21st to end	Night	16% 4 17% 40% 19% 4	660
March	1st to 10th	Night	15% 5% 15% 40% 23% 2	800
	11th to 20th	Night	18% 5% 19% 33% 24% 2	800
	21st to end	Night	15% 4 20% 39% 18% 3	880
April	1st to 10th	Night	13% 5% 14% 40% 25% 4	800
	11th to 20th	Night	10% 4 19% 43% 20% 3	746
	21st to end	Night	9% 5% 18% 42% 24% 2	585
May	1st to 10th	Night	8% 6% 15% 41% 27% 5%	400
	11th to 20th	Night	10% 4 10% 36% 36% 6%	394
	21st to end	Night	11% 4 8% 37% 33% 6%	433
June	1st to 10th	Night	10% 2 12% 44% 30% 3	400
	11th to 20th	Night	8% 3 14% 46% 26% 3	400
	21st to end	Night	5% 3 9% 42% 38% 3	400
July	1st to 10th	Night	3 2 9% 38% 44% 5%	390
	11th to 20th	Night	3 1 9% 51% 35% 2	388
	21st to end	Night	1 3 7% 39% 45% 5%	438
August	1st to 10th	Night	4 1 5% 34% 47% 9%	398
	11th to 20th	Night	3 2 8% 35% 44% 9%	457
	21st to end	Night	4 3 9% 40% 39% 5%	855
September	1st to 10th	Night	3 1 11% 41% 37% 7%	800
	11th to 20th	Night	7% 3 8% 36% 37% 9%	800
	21st to end	Night	10% 4 11% 41% 30% 5%	800
October	1st to 10th	Night	12% 2 12% 40% 30% 3	800
	11th to 20th	Night	14% 5% 14% 37% 26% 4	800
	21st to end	Night	18% 4 16% 42% 18% 3	880
November	1st to 10th	Night	23% 4 13% 44% 15%	820
	11th to 20th	Night	24% 6% 18% 33% 16% 3	840
	21st to end	Night	24% 4 14% 35% 19% 3	929
December	1st to 10th	Night	21% 5% 15% 39% 17% 3	1050
	11th to 20th	Night	28% 6% 15% 35% 14% 1	1050
	21st to end	Night	28% 5% 15% 39% 12% 1	1070

Page generated at 2026-01-13 22:14:26. Version ac07

Copyright © 2026 Attilla Danko.