SkyWatch Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the SkyWatch Observatory clear sky chart from 2005-11-06 to 2025-02-21.

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	15% 12% 4 10% 5% 5% 4 4 5% 6% 29%	108674
	Night	15% 13% 5% 10% 5% 4 4 3 5% 6% 28%	109913
Transparency	Day	53% 6% 11% 19% 10% 1	112336
	Night	55% 5% 11% 17% 10% 1	114390
Seeing	Day	22% 17% 29% 19% 9% 3	14854
	Night	22% 15% 27% 24% 10% 1	24694

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	14% 22% 9% 25% 5% 3 3 2 3 3 12%	4056
	11th to 20th	Day	11% 23% 8% 20% 8% 5% 3 2 3 3 11%	4060
	21st to end	Day	20% 19% 9% 17% 6% 4 3 2 3 3 12%	4466
February	1st to 10th	Day	16% 21% 6% 14% 8% 4 3 2 3 4 18%	1800
	11th to 20th	Day	15% 17% 7% 15% 8% 5% 4 3 3 5% 18%	1763
	21st to end	Day	17% 18% 4 13% 4 4 4 4 5% 5% 22%	1570
March	1st to 10th	Day	21% 12% 7% 9% 5% 4 4 3 5% 6% 25%	3395
	11th to 20th	Day	12% 16% 6% 10% 5% 4 3 3 4 5% 31%	3480
	21st to end	Day	21% 13% 5% 9% 4 4 3 3 4 5% 29%	3815
April	1st to 10th	Day	15% 16% 4 7% 4 4 4 3 6% 7% 31%	2157
	11th to 20th	Day	19% 13% 3 7% 4 4 4 3 4 5% 33%	2376
	21st to end	Day	20% 12% 3 7% 5% 3 4 2 4 5% 34%	2470
May	1st to 10th	Day	19% 9% 4 8% 5% 5% 5% 4 5% 6% 31%	4082
	11th to 20th	Day	19% 9% 3 7% 4 4 4 3 6% 7% 33%	4081
	21st to end	Day	11% 6% 2 7% 5% 5% 5% 5% 6% 8% 41%	4714
June	1st to 10th	Day	14% 8% 2 6% 5% 5% 5% 5% 6% 8% 35%	2660
	11th to 20th	Day	12% 7% 3 7% 5% 5% 5% 4 7% 8% 38%	2660
	21st to end	Day	13% 5% 2 6% 4 4 6% 4 7% 9% 39%	2660
July	1st to 10th	Day	8% 5% 2 7% 5% 5% 5% 5% 7% 10% 40%	4291
	11th to 20th	Day	9% 5% 3 6% 4 4 5% 5% 7% 10% 43%	4120
	21st to end	Day	10% 6% 2 7% 5% 5% 6% 4 7% 8% 40%	4466
August	1st to 10th	Day	12% 7% 3 8% 5% 5% 5% 4 6% 9% 35%	4104
	11th to 20th	Day	11% 6% 3 9% 5% 5% 6% 5% 8% 9% 33%	4044
	21st to end	Day	9% 6% 3 7% 5% 5% 5% 5% 6% 9% 40%	4415
September	1st to 10th	Day	15% 6% 3 6% 4 4 4 4 6% 7% 40%	2229
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 4 6% 8% 43%	2090
	21st to end	Day	19% 9% 3 7% 4 3 4 3 6% 7% 35%	2090
October	1st to 10th	Day	15% 7% 3 8% 5% 4 4 3 7% 7% 37%	1933
	11th to 20th	Day	22% 9% 2 9% 5% 5% 4 4 5% 6% 31%	1900
	21st to end	Day	24% 11% 3 8% 5% 5% 4 3 4 6% 26%	2090
November	1st to 10th	Day	13% 12% 3 8% 5% 5% 4 4 6% 6% 34%	1783
	11th to 20th	Day	16% 18% 4 10% 5% 4 5% 3 4 7% 23%	1690
	21st to end	Day	17% 22% 6% 10% 6% 5% 4 3 5% 5% 16%	1600
December	1st to 10th	Day	16% 20% 12% 14% 6% 5% 5% 3 4 4 12%	2600
	11th to 20th	Day	21% 24% 7% 11% 5% 5% 4 3 3 5% 12%	2990
	21st to end	Day	23% 25% 7% 15% 6% 4 3 2 3 3 8%	3974
January	1st to 10th	Night	13% 20% 11% 24% 6% 4 3 2 3 4 11%	5889
	11th to 20th	Night	11% 20% 11% 20% 8% 5% 5% 3 4 3 10%	5900
	21st to end	Night	21% 19% 8% 13% 6% 4 5% 2 3 3 15%	6490
February	1st to 10th	Night	18% 17% 5% 14% 7% 4 5% 3 4 5% 17%	2600
	11th to 20th	Night	16% 16% 6% 14% 7% 6% 5% 3 4 4 19%	2520
	21st to end	Night	19% 13% 6% 12% 6% 5% 3 3 4 4 23%	1885
March	1st to 10th	Night	20% 16% 4 8% 5% 3 3 3 4 4 31%	3457
	11th to 20th	Night	15% 12% 6% 10% 5% 5% 4 4 4 5% 32%	3480
	21st to end	Night	17% 12% 5% 8% 4 4 4 3 4 4 33%	3817
April	1st to 10th	Night	17% 15% 3 8% 5% 5% 3 3 4 5% 31%	2022
	11th to 20th	Night	16% 11% 3 6% 4 3 6% 3 6% 7% 34%	1768
	21st to end	Night	15% 10% 3 8% 6% 4 5% 3 5% 6% 36%	1710
May	1st to 10th	Night	23% 10% 3 6% 4 5% 5% 3 5% 7% 30%	2813
	11th to 20th	Night	14% 10% 3 7% 5% 3 4 4 5% 8% 37%	2812
	21st to end	Night	13% 4 1 5% 4 3 4 4 6% 10% 46%	2840
June	1st to 10th	Night	10% 7% 2 8% 5% 6% 4 5% 5% 7% 41%	1520
	11th to 20th	Night	8% 6% 3 7% 5% 5% 5% 5% 6% 7% 43%	1520
	21st to end	Night	10% 6% 2 6% 5% 5% 4 4 6% 10% 42%	1520
July	1st to 10th	Night	8% 6% 2 7% 5% 4 4 3 5% 9% 47%	2664
	11th to 20th	Night	8% 5% 2 5% 4 4 4 4 6% 9% 48%	2840
	21st to end	Night	6% 5% 2 7% 5% 5% 5% 4 6% 9% 45%	3087
August	1st to 10th	Night	10% 8% 3 8% 6% 5% 4 5% 6% 8% 39%	2825
	11th to 20th	Night	13% 8% 3 8% 5% 4 5% 3 4 6% 40%	2785
	21st to end	Night	8% 5% 2 6% 5% 5% 6% 4 7% 10% 43%	3208
September	1st to 10th	Night	13% 5% 2 5% 3 4 4 3 6% 8% 45%	1947
	11th to 20th	Night	11% 5% 2 5% 3 4 4 3 4 7% 52%	2090
	21st to end	Night	17% 7% 2 7% 5% 5% 5% 4 5% 5% 37%	2090
October	1st to 10th	Night	15% 7% 3 5% 4 4 4 3 6% 8% 42%	2245
	11th to 20th	Night	19% 7% 3 8% 4 4 5% 4 6% 7% 32%	2280
	21st to end	Night	24% 10% 3 7% 4 5% 4 3 5% 6% 29%	2508
November	1st to 10th	Night	14% 11% 2 8% 5% 5% 4 4 5% 7% 35%	2512
	11th to 20th	Night	16% 16% 5% 10% 5% 5% 5% 3 5% 6% 24%	2706
	21st to end	Night	20% 19% 4 9% 5% 5% 4 3 5% 6% 20%	2800
December	1st to 10th	Night	14% 19% 9% 13% 7% 6% 5% 3 4 5% 15%	4580
	11th to 20th	Night	17% 25% 8% 12% 5% 4 4 2 3 4 15%	5205
	21st to end	Night	21% 23% 7% 13% 5% 4 4 3 4 5% 11%	6978

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	79% 3 7% 7% 3 1	4057
	11th to 20th	Day	78% 3 8% 8% 3	4060
	21st to end	Day	79% 3 7% 8% 3	4466
February	1st to 10th	Day	71% 4 8% 12% 5%	1800
	11th to 20th	Day	70% 5% 8% 11% 6%	1772
	21st to end	Day	63% 6% 10% 12% 6% 2	1570
March	1st to 10th	Day	58% 7% 9% 22% 4	3395
	11th to 20th	Day	56% 6% 14% 17% 6% 1	3480
	21st to end	Day	58% 4 9% 17% 11% 1	3815
April	1st to 10th	Day	52% 5% 12% 16% 13% 1	2157
	11th to 20th	Day	53% 5% 10% 17% 13% 2	2376
	21st to end	Day	52% 5% 10% 20% 10% 3	2470
May	1st to 10th	Day	53% 7% 8% 23% 9% 1	4120
	11th to 20th	Day	48% 8% 12% 21% 10% 1	4085
	21st to end	Day	38% 9% 12% 21% 15% 4	4728
June	1st to 10th	Day	44% 7% 11% 25% 12% 1	2660
	11th to 20th	Day	41% 8% 13% 21% 14% 2	2660
	21st to end	Day	40% 7% 12% 25% 15% 1	2660
July	1st to 10th	Day	37% 8% 12% 24% 15% 3	4225
	11th to 20th	Day	34% 8% 12% 27% 18% 1	4051
	21st to end	Day	39% 7% 14% 21% 18% 1	4324
August	1st to 10th	Day	42% 9% 14% 23% 11%	4091
	11th to 20th	Day	43% 9% 11% 29% 7% 1	4083
	21st to end	Day	37% 7% 14% 28% 13% 1	4415
September	1st to 10th	Day	41% 8% 13% 21% 14% 3	2229
	11th to 20th	Day	38% 8% 12% 24% 17% 1	2090
	21st to end	Day	49% 6% 10% 22% 13% 1	2090
October	1st to 10th	Day	35% 5% 15% 25% 19% 1	3206
	11th to 20th	Day	47% 5% 12% 18% 14% 3	3140
	21st to end	Day	45% 5% 14% 24% 11% 1	3428
November	1st to 10th	Day	50% 6% 13% 19% 12% 1	1783
	11th to 20th	Day	61% 5% 11% 13% 8% 2	1690
	21st to end	Day	69% 5% 9% 12% 5%	1600
December	1st to 10th	Day	75% 5% 10% 8% 2	2600
	11th to 20th	Day	75% 6% 8% 9% 2	2986
	21st to end	Day	82% 4 7% 6% 1	3974
January	1st to 10th	Night	79% 3 6% 8% 4	5889
	11th to 20th	Night	78% 5% 7% 8% 2	5900
	21st to end	Night	75% 3 9% 8% 5%	6490
February	1st to 10th	Night	69% 5% 10% 11% 4	2600
	11th to 20th	Night	68% 5% 8% 14% 5%	2522
	21st to end	Night	65% 5% 8% 13% 7% 1	1885
March	1st to 10th	Night	55% 7% 11% 19% 8%	3457
	11th to 20th	Night	54% 5% 15% 16% 8% 1	3480
	21st to end	Night	53% 4 11% 19% 12% 1	3817
April	1st to 10th	Night	55% 5% 10% 17% 12% 2	2022
	11th to 20th	Night	49% 6% 11% 19% 12% 2	1768
	21st to end	Night	48% 6% 11% 20% 12% 3	1710
May	1st to 10th	Night	52% 7% 10% 18% 10% 3	2840
	11th to 20th	Night	45% 7% 13% 22% 12% 2	2813
	21st to end	Night	32% 8% 14% 26% 17% 3	2847
June	1st to 10th	Night	40% 7% 10% 25% 15% 2	1520
	11th to 20th	Night	38% 7% 13% 21% 18% 3	1520
	21st to end	Night	37% 8% 13% 24% 18% 1	1520
July	1st to 10th	Night	35% 5% 13% 24% 19% 4	2620
	11th to 20th	Night	31% 7% 13% 28% 18% 3	2786
	21st to end	Night	35% 8% 13% 19% 23% 2	2988
August	1st to 10th	Night	41% 7% 11% 26% 13% 1	2814
	11th to 20th	Night	43% 6% 12% 28% 10% 1	2812
	21st to end	Night	35% 7% 13% 29% 15% 2	3208
September	1st to 10th	Night	36% 8% 12% 26% 16% 2	1947
	11th to 20th	Night	32% 7% 14% 24% 21% 2	2090
	21st to end	Night	47% 6% 10% 21% 15% 1	2090
October	1st to 10th	Night	34% 6% 10% 27% 22% 1	3752
	11th to 20th	Night	46% 6% 10% 20% 16% 2	3779
	21st to end	Night	42% 4 14% 24% 14% 1	4120
November	1st to 10th	Night	48% 6% 13% 20% 11% 2	2512
	11th to 20th	Night	61% 5% 11% 15% 6% 3	2706
	21st to end	Night	65% 5% 11% 14% 4	2800
December	1st to 10th	Night	71% 6% 13% 8% 3	4580
	11th to 20th	Night	73% 4 10% 11% 1	5208
	21st to end	Night	75% 6% 10% 9% 1	6978

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	30% 32% 27% 8% 3	300
	11th to 20th	Day	34% 30% 25% 7% 2	300
	21st to end	Day	27% 34% 30% 7% 2	330
February	1st to 10th	Day	33% 28% 32% 7% 1	300
	11th to 20th	Day	34% 33% 26% 7%	300
	21st to end	Day	34% 32% 28% 5%	225
March	1st to 10th	Day	28% 25% 30% 13% 3	431
	11th to 20th	Day	26% 25% 32% 14% 2 1	460
	21st to end	Day	33% 23% 26% 14% 3	506
April	1st to 10th	Day	26% 26% 27% 15% 5% 1	460
	11th to 20th	Day	30% 21% 25% 15% 7% 2	452
	21st to end	Day	27% 20% 32% 16% 4 1	460
May	1st to 10th	Day	26% 20% 29% 19% 4 2	463
	11th to 20th	Day	21% 13% 32% 23% 8% 3	487
	21st to end	Day	14% 10% 32% 26% 14% 4	529
June	1st to 10th	Day	16% 12% 36% 24% 10% 2	490
	11th to 20th	Day	17% 12% 29% 27% 13% 2	490
	21st to end	Day	14% 11% 35% 27% 10% 3	490
July	1st to 10th	Day	9% 8% 24% 31% 20% 7%	485
	11th to 20th	Day	13% 10% 28% 27% 16% 6%	484
	21st to end	Day	10% 7% 32% 29% 16% 6%	532
August	1st to 10th	Day	11% 8% 26% 28% 20% 6%	486
	11th to 20th	Day	14% 8% 27% 28% 15% 8%	479
	21st to end	Day	13% 7% 26% 28% 20% 6%	539
September	1st to 10th	Day	16% 10% 24% 25% 20% 6%	490
	11th to 20th	Day	10% 6% 23% 27% 23% 10%	490
	21st to end	Day	23% 12% 28% 20% 11% 5%	490
October	1st to 10th	Day	15% 13% 30% 22% 17% 3	490
	11th to 20th	Day	26% 20% 32% 15% 6% 1	490
	21st to end	Day	34% 18% 28% 17% 2	539
November	1st to 10th	Day	23% 25% 28% 17% 7% 1	321
	11th to 20th	Day	32% 25% 28% 11% 4	245
	21st to end	Day	29% 27% 36% 9%	200
December	1st to 10th	Day	26% 33% 29% 9% 3 1	200
	11th to 20th	Day	28% 30% 30% 12% 1	200
	21st to end	Day	33% 29% 29% 9% 2	221
January	1st to 10th	Night	27% 27% 32% 11% 3	800
	11th to 20th	Night	34% 22% 29% 14% 2	800
	21st to end	Night	27% 23% 31% 16% 3	880
February	1st to 10th	Night	29% 22% 29% 18% 2	800
	11th to 20th	Night	28% 23% 32% 16% 2	783
	21st to end	Night	30% 23% 32% 14% 1	629
March	1st to 10th	Night	24% 21% 32% 20% 3	760
	11th to 20th	Night	25% 21% 29% 20% 4 1	760
	21st to end	Night	24% 18% 30% 23% 5%	836
April	1st to 10th	Night	26% 15% 32% 23% 4	760
	11th to 20th	Night	24% 14% 32% 22% 6% 1	644
	21st to end	Night	18% 14% 33% 28% 6% 1	570
May	1st to 10th	Night	21% 12% 27% 29% 11%	570
	11th to 20th	Night	18% 9% 28% 34% 10% 2	561
	21st to end	Night	12% 6% 27% 35% 20% 1	481
June	1st to 10th	Night	13% 4 32% 36% 14% 1	380
	11th to 20th	Night	11% 6% 27% 39% 16% 1	380
	21st to end	Night	10% 9% 25% 40% 16%	380
July	1st to 10th	Night	6% 5% 20% 41% 23% 4	445
	11th to 20th	Night	9% 5% 21% 37% 24% 4	552
	21st to end	Night	10% 3 21% 40% 24% 2	624
August	1st to 10th	Night	10% 4 17% 39% 26% 4	567
	11th to 20th	Night	13% 3 15% 37% 29% 4	564
	21st to end	Night	11% 7% 20% 32% 27% 3	672
September	1st to 10th	Night	15% 7% 17% 33% 24% 4	760
	11th to 20th	Night	13% 4 17% 36% 26% 4	760
	21st to end	Night	20% 10% 23% 28% 16% 4	760
October	1st to 10th	Night	19% 10% 26% 25% 18% 2	760
	11th to 20th	Night	24% 16% 27% 25% 7% 1	760
	21st to end	Night	31% 17% 28% 20% 4	836
November	1st to 10th	Night	23% 16% 30% 23% 8% 1	780
	11th to 20th	Night	28% 21% 29% 16% 5% 1	800
	21st to end	Night	33% 22% 27% 14% 4	800
December	1st to 10th	Night	32% 23% 29% 13% 4	800
	11th to 20th	Night	31% 25% 29% 14% 2	800
	21st to end	Night	30% 24% 32% 13% 2	880

Page generated at 2025-02-18 03:26:28. Version ac07

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