Steve's Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Steve's Observatory clear sky chart from 2005-11-06 to 2026-07-31.

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	12% 7% 3 7% 5% 5% 5% 4 6% 7% 40%	136400
	Night	13% 7% 2 6% 4 4 4 4 5% 7% 43%	136678
Transparency	Day	41% 5% 11% 19% 18% 7%	140040
	Night	39% 4 11% 18% 19% 8%	141117
Seeing	Day	15% 9% 21% 25% 21% 9%	17248
	Night	15% 8% 23% 29% 20% 4	26889

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	16% 10% 3 8% 3 3 4 3 4 6% 40%	5289
	11th to 20th	Day	11% 5% 3 6% 3 3 4 3 4 6% 52%	5310
	21st to end	Day	22% 10% 3 6% 4 4 4 3 4 4 36%	6015
February	1st to 10th	Day	16% 9% 3 6% 4 4 4 3 4 6% 41%	2100
	11th to 20th	Day	18% 7% 3 6% 4 4 4 3 5% 6% 39%	2048
	21st to end	Day	15% 8% 4 8% 5% 3 4 4 4 6% 38%	1730
March	1st to 10th	Day	19% 10% 3 7% 5% 5% 4 4 5% 6% 33%	4862
	11th to 20th	Day	14% 7% 2 7% 4 4 4 4 5% 5% 45%	5090
	21st to end	Day	14% 8% 3 8% 6% 4 5% 3 4 5% 41%	5812
April	1st to 10th	Day	11% 8% 2 7% 5% 5% 5% 5% 6% 7% 40%	2520
	11th to 20th	Day	15% 7% 2 6% 5% 4 5% 4 5% 6% 41%	2494
	21st to end	Day	8% 5% 2 6% 6% 5% 5% 5% 6% 8% 43%	2520
May	1st to 10th	Day	21% 8% 3 7% 4 5% 5% 4 5% 7% 32%	7021
	11th to 20th	Day	10% 6% 2 7% 5% 5% 6% 4 7% 8% 40%	7031
	21st to end	Day	10% 7% 3 8% 6% 6% 7% 5% 7% 10% 32%	7736
June	1st to 10th	Day	9% 6% 3 7% 6% 7% 7% 7% 8% 9% 33%	2703
	11th to 20th	Day	7% 5% 2 8% 7% 7% 7% 5% 8% 9% 36%	2801
	21st to end	Day	6% 6% 2 8% 8% 7% 6% 7% 9% 10% 31%	2730
July	1st to 10th	Day	5% 5% 3 8% 7% 8% 8% 6% 8% 11% 31%	6030
	11th to 20th	Day	6% 5% 2 7% 7% 7% 8% 6% 9% 12% 32%	6030
	21st to end	Day	8% 7% 3 9% 7% 6% 6% 5% 8% 12% 29%	6276
August	1st to 10th	Day	3 3 1 5% 4 4 5% 4 5% 8% 58%	3905
	11th to 20th	Day	5% 4 2 6% 5% 6% 6% 5% 6% 10% 46%	3849
	21st to end	Day	11% 5% 2 7% 5% 6% 5% 5% 7% 8% 39%	4312
September	1st to 10th	Day	10% 6% 3 7% 6% 6% 7% 5% 6% 9% 36%	2400
	11th to 20th	Day	12% 6% 2 6% 5% 5% 6% 4 5% 6% 43%	2365
	21st to end	Day	10% 5% 2 5% 4 4 4 3 6% 7% 51%	2200
October	1st to 10th	Day	7% 4 2 5% 4 3 4 4 6% 7% 55%	2052
	11th to 20th	Day	8% 4 2 6% 4 3 3 2 5% 6% 57%	2000
	21st to end	Day	11% 5% 2 4 3 4 4 2 3 5% 56%	2200
November	1st to 10th	Day	11% 5% 2 5% 4 3 4 3 5% 6% 52%	2058
	11th to 20th	Day	11% 6% 2 5% 3 3 3 3 4 6% 53%	1990
	21st to end	Day	13% 7% 3 6% 4 3 4 3 4 6% 47%	1890
December	1st to 10th	Day	16% 9% 3 6% 4 4 3 3 5% 5% 41%	3020
	11th to 20th	Day	11% 9% 3 7% 5% 4 4 3 4 5% 43%	3446
	21st to end	Day	16% 7% 2 5% 3 4 4 3 4 5% 47%	4565
January	1st to 10th	Night	21% 9% 3 6% 3 3 3 3 4 5% 41%	7634
	11th to 20th	Night	16% 7% 3 5% 3 3 3 2 4 5% 49%	7680
	21st to end	Night	26% 10% 3 7% 3 3 3 2 3 4 36%	8183
February	1st to 10th	Night	15% 8% 3 5% 4 4 4 3 5% 6% 43%	2520
	11th to 20th	Night	20% 10% 3 6% 3 3 3 2 4 5% 41%	2458
	21st to end	Night	19% 9% 3 7% 4 4 3 3 5% 6% 38%	2076
March	1st to 10th	Night	17% 12% 3 6% 4 4 4 3 6% 7% 36%	5240
	11th to 20th	Night	12% 7% 2 5% 4 4 4 3 5% 7% 47%	5090
	21st to end	Night	17% 7% 2 6% 4 4 4 4 5% 5% 42%	5357
April	1st to 10th	Night	13% 7% 2 5% 5% 3 4 4 6% 7% 43%	2100
	11th to 20th	Night	11% 7% 2 6% 5% 4 4 4 5% 7% 46%	2075
	21st to end	Night	8% 6% 2 4 4 4 4 4 5% 8% 52%	2100
May	1st to 10th	Night	21% 7% 3 7% 4 4 4 3 5% 6% 38%	5832
	11th to 20th	Night	13% 6% 2 6% 4 4 4 3 6% 9% 42%	5829
	21st to end	Night	11% 8% 3 7% 6% 5% 6% 5% 7% 10% 32%	6415
June	1st to 10th	Night	7% 7% 2 7% 5% 5% 5% 5% 7% 11% 39%	1875
	11th to 20th	Night	4 5% 2 5% 5% 5% 4 6% 7% 10% 46%	1781
	21st to end	Night	5% 5% 1 6% 5% 4 6% 6% 10% 10% 41%	1890
July	1st to 10th	Night	4 5% 3 7% 6% 6% 7% 5% 9% 12% 37%	4168
	11th to 20th	Night	5% 4 2 6% 5% 6% 7% 6% 11% 14% 34%	4167
	21st to end	Night	6% 6% 3 8% 6% 6% 7% 5% 9% 11% 31%	4801
August	1st to 10th	Night	2 2 1 3 2 3 4 4 6% 11% 61%	3244
	11th to 20th	Night	4 3 1 5% 5% 5% 6% 5% 8% 13% 44%	3200
	21st to end	Night	7% 6% 3 8% 5% 5% 5% 5% 6% 8% 43%	3586
September	1st to 10th	Night	7% 5% 2 6% 4 5% 6% 6% 9% 10% 40%	2000
	11th to 20th	Night	9% 6% 2 6% 4 4 4 3 7% 8% 47%	2015
	21st to end	Night	9% 5% 2 5% 3 3 4 3 5% 7% 54%	2200
October	1st to 10th	Night	7% 4 2 5% 3 4 4 3 5% 6% 58%	2342
	11th to 20th	Night	8% 4 2 5% 4 3 3 3 4 5% 59%	2400
	21st to end	Night	12% 6% 2 5% 3 2 3 2 4 4 57%	2640
November	1st to 10th	Night	12% 5% 1 4 3 3 3 3 4 5% 56%	2460
	11th to 20th	Night	12% 6% 2 4 3 3 3 2 4 6% 56%	2625
	21st to end	Night	13% 7% 2 4 4 3 4 3 5% 6% 48%	2730
December	1st to 10th	Night	20% 7% 2 4 3 4 3 3 3 5% 45%	4380
	11th to 20th	Night	14% 10% 3 6% 4 3 4 3 4 5% 44%	4952
	21st to end	Night	19% 10% 3 5% 3 3 2 2 3 4 46%	6633

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	47% 3 5% 19% 22% 3	5290
	11th to 20th	Day	35% 2 5% 18% 31% 9%	5310
	21st to end	Day	52% 2 4 11% 17% 14%	6015
February	1st to 10th	Day	46% 2 6% 16% 19% 11%	2100
	11th to 20th	Day	47% 3 6% 16% 20% 8%	2058
	21st to end	Day	47% 4 6% 15% 17% 11%	1730
March	1st to 10th	Day	52% 3 6% 14% 17% 8%	4862
	11th to 20th	Day	41% 3 6% 14% 19% 17%	5090
	21st to end	Day	47% 3 8% 16% 18% 8%	5814
April	1st to 10th	Day	42% 4 9% 19% 17% 8%	2520
	11th to 20th	Day	43% 3 7% 18% 22% 7%	2494
	21st to end	Day	36% 5% 12% 23% 21% 3	2520
May	1st to 10th	Day	50% 4 11% 19% 12% 3	7056
	11th to 20th	Day	40% 5% 16% 20% 17% 2	7035
	21st to end	Day	44% 6% 15% 22% 10% 2	7749
June	1st to 10th	Day	44% 6% 14% 23% 12% 1	2703
	11th to 20th	Day	41% 6% 16% 22% 13% 2	2801
	21st to end	Day	43% 7% 18% 21% 9% 2	2730
July	1st to 10th	Day	42% 10% 21% 19% 7%	5966
	11th to 20th	Day	40% 11% 29% 18% 2 1	5961
	21st to end	Day	45% 9% 17% 24% 5%	6100
August	1st to 10th	Day	24% 4 13% 30% 27% 1	3893
	11th to 20th	Day	32% 7% 20% 29% 11% 1	3885
	21st to end	Day	43% 6% 15% 18% 16% 3	4311
September	1st to 10th	Day	44% 5% 14% 18% 14% 5%	2400
	11th to 20th	Day	42% 6% 10% 18% 18% 7%	2365
	21st to end	Day	33% 3 10% 24% 20% 10%	2200
October	1st to 10th	Day	24% 3 6% 14% 23% 31%	3338
	11th to 20th	Day	18% 2 5% 16% 28% 31%	3240
	21st to end	Day	23% 2 3 14% 37% 19%	3538
November	1st to 10th	Day	33% 3 7% 18% 27% 12%	2058
	11th to 20th	Day	33% 3 7% 15% 29% 13%	1990
	21st to end	Day	39% 2 8% 18% 23% 10%	1890
December	1st to 10th	Day	44% 2 7% 16% 24% 6%	3020
	11th to 20th	Day	43% 4 8% 14% 21% 10%	3443
	21st to end	Day	41% 2 5% 22% 27% 3	4565
January	1st to 10th	Night	47% 3 6% 17% 22% 6%	7635
	11th to 20th	Night	40% 2 4 15% 26% 13%	7680
	21st to end	Night	55% 2 4 12% 18% 9%	8183
February	1st to 10th	Night	43% 3 5% 18% 21% 10%	2520
	11th to 20th	Night	47% 3 5% 15% 20% 9%	2459
	21st to end	Night	49% 3 7% 15% 15% 12%	2076
March	1st to 10th	Night	48% 3 8% 12% 22% 8%	5240
	11th to 20th	Night	38% 3 8% 16% 19% 16%	5090
	21st to end	Night	44% 3 9% 18% 18% 7%	5357
April	1st to 10th	Night	40% 4 10% 18% 18% 10%	2100
	11th to 20th	Night	37% 3 10% 21% 20% 10%	2075
	21st to end	Night	31% 4 11% 23% 25% 6%	2100
May	1st to 10th	Night	48% 3 13% 18% 15% 3	5862
	11th to 20th	Night	39% 4 16% 22% 16% 2	5830
	21st to end	Night	46% 6% 16% 21% 9% 1	6427
June	1st to 10th	Night	37% 6% 19% 24% 13% 1	1875
	11th to 20th	Night	32% 7% 19% 27% 13% 3	1781
	21st to end	Night	31% 9% 24% 22% 12% 1	1890
July	1st to 10th	Night	37% 11% 27% 18% 6% 1	4125
	11th to 20th	Night	36% 14% 33% 14% 2 1	4116
	21st to end	Night	44% 12% 18% 21% 4	4664
August	1st to 10th	Night	17% 6% 20% 32% 24% 1	3232
	11th to 20th	Night	28% 10% 22% 30% 9% 1	3230
	21st to end	Night	40% 7% 15% 18% 17% 4	3586
September	1st to 10th	Night	34% 8% 19% 19% 14% 6%	2000
	11th to 20th	Night	34% 6% 16% 18% 18% 8%	2015
	21st to end	Night	30% 4 11% 24% 21% 10%	2200
October	1st to 10th	Night	23% 3 7% 13% 24% 31%	3836
	11th to 20th	Night	18% 2 3 20% 24% 32%	3899
	21st to end	Night	21% 2 5% 13% 38% 21%	4252
November	1st to 10th	Night	30% 3 8% 18% 25% 15%	2460
	11th to 20th	Night	31% 3 5% 17% 29% 15%	2625
	21st to end	Night	38% 3 7% 19% 21% 12%	2730
December	1st to 10th	Night	44% 1 5% 18% 26% 5%	4380
	11th to 20th	Night	43% 3 7% 16% 23% 8%	4954
	21st to end	Night	44% 2 5% 19% 24% 6%	6633

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	25% 23% 34% 15% 3 1	330
	11th to 20th	Day	24% 18% 36% 16% 4 1	330
	21st to end	Day	29% 22% 35% 11% 2	363
February	1st to 10th	Day	25% 18% 39% 14% 3	330
	11th to 20th	Day	34% 16% 33% 13% 3	330
	21st to end	Day	28% 26% 35% 10% 1	273
March	1st to 10th	Day	29% 15% 36% 16% 3	330
	11th to 20th	Day	27% 18% 39% 13% 4	330
	21st to end	Day	23% 14% 32% 24% 6%	463
April	1st to 10th	Day	19% 19% 31% 23% 8% 1	540
	11th to 20th	Day	23% 13% 32% 21% 10% 1	532
	21st to end	Day	13% 11% 32% 29% 14% 1	540
May	1st to 10th	Day	18% 12% 27% 26% 14% 3	543
	11th to 20th	Day	14% 6% 28% 28% 17% 7%	567
	21st to end	Day	8% 5% 23% 32% 23% 9%	617
June	1st to 10th	Day	11% 5% 13% 35% 24% 11%	705
	11th to 20th	Day	9% 4 14% 34% 31% 8%	780
	21st to end	Day	9% 3 13% 27% 31% 19%	780
July	1st to 10th	Day	6% 2 6% 29% 43% 15%	771
	11th to 20th	Day	7% 2 5% 24% 40% 21%	768
	21st to end	Day	7% 3 7% 26% 40% 18%	720
August	1st to 10th	Day	6% 2 5% 26% 36% 26%	526
	11th to 20th	Day	8% 1 4 20% 43% 24%	519
	21st to end	Day	14% 2 7% 23% 33% 20%	583
September	1st to 10th	Day	12% 2 7% 28% 34% 17%	530
	11th to 20th	Day	12% 4 11% 22% 35% 15%	530
	21st to end	Day	12% 4 11% 33% 27% 12%	530
October	1st to 10th	Day	7% 7% 23% 36% 18% 8%	382
	11th to 20th	Day	10% 5% 26% 32% 22% 6%	330
	21st to end	Day	20% 9% 23% 30% 14% 5%	363
November	1st to 10th	Day	18% 9% 26% 32% 13% 3	330
	11th to 20th	Day	18% 10% 32% 29% 11% 1	330
	21st to end	Day	15% 19% 35% 19% 8% 3	330
December	1st to 10th	Day	25% 13% 36% 21% 5% 1	330
	11th to 20th	Day	20% 17% 39% 19% 5% 1	330
	21st to end	Day	25% 15% 33% 18% 8% 1	363
January	1st to 10th	Night	25% 14% 33% 22% 5%	1050
	11th to 20th	Night	23% 15% 32% 23% 7%	1050
	21st to end	Night	25% 16% 31% 24% 4	1079
February	1st to 10th	Night	21% 15% 35% 25% 4	840
	11th to 20th	Night	29% 15% 29% 22% 4	823
	21st to end	Night	23% 20% 35% 19% 3	692
March	1st to 10th	Night	20% 15% 34% 27% 5%	730
	11th to 20th	Night	18% 14% 33% 29% 6% 1	630
	21st to end	Night	17% 11% 37% 28% 7%	693
April	1st to 10th	Night	20% 12% 33% 27% 9% 1	630
	11th to 20th	Night	16% 12% 34% 28% 10% 1	623
	21st to end	Night	10% 9% 33% 34% 14% 1	630
May	1st to 10th	Night	13% 5% 30% 35% 16% 2	630
	11th to 20th	Night	12% 4 25% 35% 20% 3	621
	21st to end	Night	6% 4 20% 39% 26% 5%	683
June	1st to 10th	Night	10% 4 11% 40% 29% 7%	630
	11th to 20th	Night	5% 2 10% 37% 39% 7%	630
	21st to end	Night	6% 2 8% 31% 39% 14%	630
July	1st to 10th	Night	5% 2 2 29% 48% 13%	615
	11th to 20th	Night	6% 2 4 25% 45% 19%	612
	21st to end	Night	5% 2 5% 28% 46% 14%	687
August	1st to 10th	Night	3 1 4 24% 49% 19%	597
	11th to 20th	Night	7% 1 4 23% 48% 18%	594
	21st to end	Night	10% 2 6% 24% 39% 18%	660
September	1st to 10th	Night	8% 2 7% 31% 42% 11%	600
	11th to 20th	Night	9% 3 9% 28% 42% 9%	615
	21st to end	Night	10% 2 12% 35% 36% 6%	800
October	1st to 10th	Night	8% 2 18% 34% 33% 5%	800
	11th to 20th	Night	10% 4 24% 36% 23% 3	800
	21st to end	Night	16% 6% 21% 34% 22% 2	880
November	1st to 10th	Night	13% 7% 21% 36% 20% 2	820
	11th to 20th	Night	14% 8% 28% 34% 14% 1	840
	21st to end	Night	16% 10% 30% 28% 14% 2	840
December	1st to 10th	Night	23% 10% 29% 29% 9%	840
	11th to 20th	Night	21% 10% 34% 27% 9%	840
	21st to end	Night	27% 12% 28% 27% 6%	1155

Page generated at 2026-07-28 02:25:23. Version ac07

Copyright © 2026 Attilla Danko.