SkyWatcher Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the SkyWatcher Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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% 7% 2 6% 4 4 4 4 5% 6% 48%	124302
	Night	9% 8% 3 7% 5% 4 4 4 6% 6% 45%	123600
Transparency	Day	34% 7% 12% 23% 16% 8%	124046
	Night	37% 8% 14% 22% 14% 5%	123443
Seeing	Day	11% 14% 26% 27% 17% 5%	15408
	Night	12% 8% 21% 37% 20% 2	24422

By Month

Percentage Cloud Cover SW (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% 16% 7% 15% 8% 7% 6% 4 6% 6% 11%	3639
	11th to 20th	Day	14% 12% 3 6% 4 5% 5% 4 5% 6% 34%	3640
	21st to end	Day	12% 11% 3 9% 6% 5% 6% 5% 7% 6% 32%	4170
February	1st to 10th	Day	13% 14% 3 9% 7% 5% 5% 4 5% 7% 28%	1800
	11th to 20th	Day	10% 11% 3 9% 6% 5% 7% 4 6% 7% 31%	1755
	21st to end	Day	9% 13% 4 8% 7% 6% 6% 5% 7% 7% 29%	1650
March	1st to 10th	Day	7% 10% 3 9% 5% 6% 6% 5% 8% 9% 32%	4518
	11th to 20th	Day	8% 14% 4 8% 5% 5% 5% 4 5% 6% 36%	4995
	21st to end	Day	8% 10% 4 11% 7% 7% 7% 5% 7% 8% 25%	5764
April	1st to 10th	Day	10% 11% 3 7% 6% 5% 6% 5% 7% 8% 32%	2400
	11th to 20th	Day	8% 9% 2 6% 6% 5% 5% 4 6% 7% 41%	2369
	21st to end	Day	7% 7% 2 6% 5% 5% 5% 4 7% 8% 45%	2521
May	1st to 10th	Day	9% 6% 2 5% 3 3 4 3 5% 6% 55%	6320
	11th to 20th	Day	6% 5% 2 6% 4 4 4 4 5% 7% 53%	6277
	21st to end	Day	7% 5% 2 5% 4 4 5% 3 6% 8% 52%	6902
June	1st to 10th	Day	9% 7% 3 6% 5% 4 4 4 5% 7% 48%	2800
	11th to 20th	Day	6% 4 2 5% 5% 4 5% 3 5% 7% 53%	2800
	21st to end	Day	3 2 1 3 3 3 3 3 5% 6% 68%	2800
July	1st to 10th	Day	1 1 2 1 1 2 2 2 4 85%	4560
	11th to 20th	Day	2 1 1 1 2 1 1 2 3 84%	4560
	21st to end	Day	1 1 1 2 1 2 1 2 3 4 82%	4791
August	1st to 10th	Day	3 2 1 3 2 2 2 2 4 5% 75%	4294
	11th to 20th	Day	2 1 1 3 2 3 3 3 5% 6% 72%	4010
	21st to end	Day	3 3 1 3 2 2 3 2 3 6% 71%	4180
September	1st to 10th	Day	2 1 1 2 2 2 3 2 3 6% 76%	2280
	11th to 20th	Day	4 4 2 4 4 4 5% 4 5% 6% 59%	2280
	21st to end	Day	8% 5% 1 4 4 3 4 3 4 6% 58%	2038
October	1st to 10th	Day	6% 4 1 4 3 3 4 4 6% 6% 60%	3180
	11th to 20th	Day	6% 4 2 7% 4 6% 6% 5% 7% 9% 45%	3161
	21st to end	Day	9% 8% 3 9% 4 5% 6% 4 7% 8% 36%	3237
November	1st to 10th	Day	12% 12% 3 7% 6% 6% 6% 4 7% 8% 29%	1759
	11th to 20th	Day	11% 13% 5% 11% 8% 7% 6% 5% 7% 6% 21%	1600
	21st to end	Day	10% 14% 4 9% 5% 6% 5% 4 7% 7% 29%	1600
December	1st to 10th	Day	9% 9% 3 8% 5% 5% 5% 4 8% 7% 37%	2620
	11th to 20th	Day	12% 17% 6% 12% 7% 6% 6% 5% 7% 6% 15%	3028
	21st to end	Day	14% 15% 5% 13% 8% 7% 6% 4 5% 5% 17%	4004
January	1st to 10th	Night	16% 16% 6% 15% 8% 7% 6% 4 6% 5% 12%	6306
	11th to 20th	Night	15% 12% 3 5% 4 3 3 4 6% 6% 37%	6320
	21st to end	Night	13% 12% 4 8% 5% 5% 4 3 4 6% 35%	6778
February	1st to 10th	Night	13% 13% 4 8% 5% 5% 5% 4 6% 7% 30%	2600
	11th to 20th	Night	10% 11% 3 9% 5% 4 5% 4 5% 8% 36%	2532
	21st to end	Night	10% 14% 3 7% 6% 5% 5% 4 6% 8% 31%	1980
March	1st to 10th	Night	6% 8% 3 8% 6% 6% 6% 5% 8% 9% 36%	5376
	11th to 20th	Night	7% 10% 3 7% 5% 5% 5% 5% 6% 7% 40%	5004
	21st to end	Night	8% 10% 3 9% 7% 7% 7% 6% 8% 9% 25%	5210
April	1st to 10th	Night	11% 8% 3 6% 5% 4 4 5% 7% 8% 39%	2000
	11th to 20th	Night	7% 7% 3 5% 6% 4 6% 3 6% 7% 46%	1979
	21st to end	Night	8% 6% 2 5% 4 4 5% 4 7% 8% 48%	1874
May	1st to 10th	Night	8% 5% 2 4 3 3 3 2 4 5% 61%	3640
	11th to 20th	Night	5% 5% 2 6% 4 2 4 3 6% 6% 58%	3616
	21st to end	Night	7% 5% 1 5% 3 4 4 3 5% 8% 56%	3972
June	1st to 10th	Night	10% 5% 2 5% 4 4 4 3 5% 7% 51%	1600
	11th to 20th	Night	6% 5% 2 4 3 4 3 4 5% 7% 59%	1600
	21st to end	Night	4 3 1 4 2 3 4 3 4 4 70%	1600
July	1st to 10th	Night	1 1 1 1 1 1 1 2 3 87%	2620
	11th to 20th	Night	1 1 1 2 1 1 2 2 3 4 83%	2620
	21st to end	Night	1 1 1 1 1 1 1 1 2 3 86%	2761
August	1st to 10th	Night	2 2 1 3 3 2 3 3 4 5% 74%	2623
	11th to 20th	Night	1 2 1 2 2 3 3 3 4 4 76%	2828
	21st to end	Night	2 2 1 3 2 2 2 2 4 5% 76%	3498
September	1st to 10th	Night	2 2 1 2 1 2 2 3 4 5% 77%	1900
	11th to 20th	Night	5% 3 1 3 3 4 4 3 5% 6% 63%	1900
	21st to end	Night	6% 4 1 4 3 3 3 3 5% 6% 61%	2138
October	1st to 10th	Night	5% 2 2 3 3 3 3 2 4 6% 67%	3800
	11th to 20th	Night	6% 4 2 6% 4 5% 5% 4 7% 8% 49%	3777
	21st to end	Night	10% 7% 2 7% 4 6% 6% 5% 7% 7% 39%	4325
November	1st to 10th	Night	12% 9% 2 8% 6% 5% 5% 4 6% 8% 35%	2535
	11th to 20th	Night	12% 12% 4 10% 7% 5% 5% 5% 6% 6% 28%	2800
	21st to end	Night	10% 9% 4 9% 5% 4 5% 5% 5% 8% 36%	2800
December	1st to 10th	Night	7% 8% 3 7% 5% 5% 5% 5% 8% 7% 40%	4560
	11th to 20th	Night	17% 16% 5% 12% 8% 7% 6% 4 5% 5% 14%	5176
	21st to end	Night	16% 12% 5% 12% 7% 6% 6% 5% 5% 6% 20%	6952

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% 12% 10% 8% 1	3639
	11th to 20th	Day	47% 11% 12% 23% 6% 1	3640
	21st to end	Day	47% 10% 10% 21% 10% 2	4170
February	1st to 10th	Day	55% 8% 16% 16% 4 1	1800
	11th to 20th	Day	48% 11% 15% 18% 8%	1764
	21st to end	Day	49% 11% 16% 14% 9%	1650
March	1st to 10th	Day	42% 7% 13% 24% 13% 1	4518
	11th to 20th	Day	46% 7% 11% 27% 9%	4995
	21st to end	Day	50% 11% 16% 18% 5%	5764
April	1st to 10th	Day	47% 10% 17% 17% 9%	2400
	11th to 20th	Day	41% 9% 16% 18% 14% 3	2372
	21st to end	Day	35% 8% 16% 27% 11% 2	2521
May	1st to 10th	Day	29% 6% 10% 27% 25% 3	6320
	11th to 20th	Day	29% 7% 13% 27% 22% 3	6280
	21st to end	Day	28% 8% 20% 31% 12% 1	6904
June	1st to 10th	Day	35% 7% 13% 28% 15% 3	2800
	11th to 20th	Day	29% 7% 13% 32% 16% 3	2800
	21st to end	Day	17% 5% 12% 34% 23% 8%	2800
July	1st to 10th	Day	7% 2 5% 16% 25% 44%	4491
	11th to 20th	Day	8% 2 5% 10% 25% 50%	4485
	21st to end	Day	8% 2 7% 19% 36% 27%	4640
August	1st to 10th	Day	13% 3 5% 17% 35% 26%	4278
	11th to 20th	Day	14% 4 10% 35% 28% 9%	4049
	21st to end	Day	17% 3 11% 28% 30% 11%	4179
September	1st to 10th	Day	12% 3 9% 30% 30% 16%	2280
	11th to 20th	Day	24% 7% 12% 25% 22% 9%	2280
	21st to end	Day	27% 6% 14% 27% 20% 6%	2038
October	1st to 10th	Day	24% 6% 8% 30% 25% 7%	3180
	11th to 20th	Day	32% 8% 18% 25% 16% 1	3161
	21st to end	Day	42% 8% 11% 27% 11% 1	3237
November	1st to 10th	Day	50% 10% 14% 18% 7% 1	1759
	11th to 20th	Day	59% 12% 12% 12% 4	1600
	21st to end	Day	53% 8% 14% 16% 7% 1	1600
December	1st to 10th	Day	42% 11% 13% 25% 9%	2620
	11th to 20th	Day	63% 10% 15% 11% 1	3028
	21st to end	Day	63% 10% 13% 12% 2	4004
January	1st to 10th	Night	69% 10% 12% 7% 2	6306
	11th to 20th	Night	45% 11% 15% 23% 6% 1	6320
	21st to end	Night	47% 9% 11% 20% 11% 1	6778
February	1st to 10th	Night	52% 10% 17% 16% 5%	2600
	11th to 20th	Night	46% 10% 17% 20% 8% 1	2534
	21st to end	Night	48% 10% 15% 17% 9% 1	1980
March	1st to 10th	Night	39% 7% 14% 26% 14% 1	5376
	11th to 20th	Night	39% 9% 14% 27% 11%	5004
	21st to end	Night	48% 13% 20% 16% 3	5210
April	1st to 10th	Night	40% 10% 18% 24% 7% 1	2000
	11th to 20th	Night	36% 9% 19% 22% 11% 3	1979
	21st to end	Night	34% 10% 16% 26% 12% 3	1874
May	1st to 10th	Night	25% 5% 10% 28% 32% 1	3637
	11th to 20th	Night	24% 5% 17% 34% 18% 1	3616
	21st to end	Night	24% 8% 28% 30% 9% 1	3972
June	1st to 10th	Night	34% 8% 13% 28% 14% 3	1600
	11th to 20th	Night	25% 7% 14% 38% 14% 3	1600
	21st to end	Night	19% 5% 12% 34% 22% 8%	1600
July	1st to 10th	Night	6% 2 7% 17% 28% 41%	2580
	11th to 20th	Night	9% 2 5% 13% 23% 47%	2572
	21st to end	Night	7% 1 6% 20% 38% 29%	2674
August	1st to 10th	Night	13% 3 6% 16% 37% 25%	2615
	11th to 20th	Night	12% 3 11% 39% 25% 9%	2855
	21st to end	Night	12% 3 13% 29% 31% 11%	3498
September	1st to 10th	Night	10% 4 12% 29% 29% 17%	1900
	11th to 20th	Night	22% 6% 12% 27% 21% 12%	1900
	21st to end	Night	23% 5% 12% 29% 23% 8%	2138
October	1st to 10th	Night	20% 4 11% 34% 28% 3	3800
	11th to 20th	Night	31% 7% 20% 24% 16% 2	3777
	21st to end	Night	40% 8% 14% 26% 13% 1	4325
November	1st to 10th	Night	46% 10% 13% 22% 8%	2535
	11th to 20th	Night	53% 11% 14% 18% 5%	2800
	21st to end	Night	45% 10% 17% 21% 7%	2800
December	1st to 10th	Night	37% 11% 15% 30% 6%	4560
	11th to 20th	Night	67% 9% 12% 9% 3	5176
	21st to end	Night	58% 12% 14% 12% 4	6952

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	22% 17% 28% 21% 10% 3	200
	11th to 20th	Day	20% 10% 24% 30% 14% 3	200
	21st to end	Day	19% 9% 25% 28% 15% 4	295
February	1st to 10th	Day	21% 15% 25% 27% 11% 1	400
	11th to 20th	Day	17% 16% 30% 23% 13% 1	395
	21st to end	Day	18% 18% 27% 25% 10% 2	414
March	1st to 10th	Day	17% 18% 30% 25% 8% 2	500
	11th to 20th	Day	14% 22% 26% 25% 12% 2	500
	21st to end	Day	19% 17% 27% 26% 9% 1	550
April	1st to 10th	Day	21% 18% 27% 22% 8% 4	500
	11th to 20th	Day	15% 18% 34% 20% 9% 5%	492
	21st to end	Day	10% 19% 30% 24% 14% 3	500
May	1st to 10th	Day	12% 17% 27% 25% 16% 4	503
	11th to 20th	Day	16% 16% 24% 27% 13% 4	528
	21st to end	Day	12% 13% 24% 28% 18% 6%	573
June	1st to 10th	Day	11% 15% 25% 26% 16% 7%	530
	11th to 20th	Day	9% 19% 28% 25% 15% 3	530
	21st to end	Day	3 13% 25% 32% 22% 5%	530
July	1st to 10th	Day	2 9% 28% 33% 22% 7%	526
	11th to 20th	Day	1 11% 25% 32% 24% 7%	524
	21st to end	Day	2 6% 27% 33% 25% 8%	532
August	1st to 10th	Day	3 9% 23% 28% 25% 12%	487
	11th to 20th	Day	2 7% 18% 31% 28% 14%	477
	21st to end	Day	4 11% 23% 28% 28% 6%	539
September	1st to 10th	Day	2 8% 24% 32% 25% 9%	490
	11th to 20th	Day	5% 9% 24% 32% 25% 4	490
	21st to end	Day	10% 12% 22% 29% 22% 6%	490
October	1st to 10th	Day	10% 10% 26% 30% 20% 4	490
	11th to 20th	Day	12% 12% 23% 26% 22% 5%	490
	21st to end	Day	15% 19% 25% 24% 13% 5%	416
November	1st to 10th	Day	18% 16% 28% 24% 13% 1	297
	11th to 20th	Day	17% 17% 29% 18% 17% 4	200
	21st to end	Day	18% 14% 21% 30% 17% 2	200
December	1st to 10th	Day	18% 8% 22% 28% 16% 9%	200
	11th to 20th	Day	19% 12% 33% 28% 9%	200
	21st to end	Day	18% 15% 30% 24% 13%	220
January	1st to 10th	Night	19% 12% 25% 29% 14% 2	800
	11th to 20th	Night	18% 10% 21% 30% 19% 2	800
	21st to end	Night	19% 7% 21% 35% 16% 2	880
February	1st to 10th	Night	18% 11% 24% 33% 13% 1	800
	11th to 20th	Night	16% 10% 27% 33% 14% 1	784
	21st to end	Night	18% 12% 26% 33% 10%	660
March	1st to 10th	Night	13% 12% 26% 35% 13% 1	800
	11th to 20th	Night	12% 12% 21% 38% 16% 1	800
	21st to end	Night	12% 12% 26% 36% 14% 1	880
April	1st to 10th	Night	16% 10% 22% 34% 16% 2	800
	11th to 20th	Night	11% 12% 27% 30% 17% 3	791
	21st to end	Night	10% 9% 26% 36% 17% 1	674
May	1st to 10th	Night	7% 7% 24% 39% 22% 2	400
	11th to 20th	Night	10% 6% 16% 39% 27% 2	394
	21st to end	Night	6% 6% 17% 39% 28% 3	433
June	1st to 10th	Night	11% 6% 17% 43% 23% 2	400
	11th to 20th	Night	6% 7% 24% 45% 17% 1	400
	21st to end	Night	7% 2 17% 46% 28% 1	400
July	1st to 10th	Night	2 1 10% 54% 32% 1	390
	11th to 20th	Night	1 1 12% 57% 27% 3	388
	21st to end	Night	1 1 9% 54% 32% 2	416
August	1st to 10th	Night	2 1 9% 48% 36% 5%	433
	11th to 20th	Night	2 1 7% 38% 43% 8%	567
	21st to end	Night	3 3 16% 46% 28% 5%	836
September	1st to 10th	Night	2 3 15% 44% 32% 4	760
	11th to 20th	Night	5% 2 14% 40% 34% 5%	760
	21st to end	Night	7% 4 15% 39% 30% 4	760
October	1st to 10th	Night	9% 3 19% 43% 23% 2	760
	11th to 20th	Night	8% 8% 19% 38% 24% 4	760
	21st to end	Night	14% 8% 23% 34% 17% 3	836
November	1st to 10th	Night	13% 10% 22% 39% 16% 1	780
	11th to 20th	Night	16% 14% 24% 31% 12% 2	800
	21st to end	Night	12% 10% 22% 37% 17% 3	800
December	1st to 10th	Night	16% 9% 23% 33% 17% 3	800
	11th to 20th	Night	19% 11% 24% 32% 13% 1	800
	21st to end	Night	20% 12% 27% 30% 10% 1	880

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