Watch Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Watch Observatory clear sky chart from 2005-11-06 to 2025-04-26.

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	16% 10% 3 8% 5% 5% 5% 4 6% 7% 32%	113573
	Night	17% 12% 4 9% 5% 4 4 3 5% 6% 30%	115034
Transparency	Day	30% 42% 8% 14% 6% 1	117065
	Night	34% 40% 7% 12% 7% 1	119693
Seeing	Day	20% 10% 28% 25% 13% 4	15072
	Night	22% 9% 28% 30% 10% 1	24223

By Month

Percentage Cloud Cover NE (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	15% 20% 6% 12% 6% 4 4 3 4 5% 20%	3597
	11th to 20th	Day	13% 15% 6% 11% 5% 5% 6% 4 5% 6% 24%	3570
	21st to end	Day	11% 15% 6% 11% 7% 5% 5% 4 6% 6% 25%	3960
February	1st to 10th	Day	14% 16% 4 11% 6% 5% 5% 4 5% 5% 25%	1785
	11th to 20th	Day	14% 12% 4 9% 7% 6% 5% 4 6% 6% 27%	1838
	21st to end	Day	18% 14% 4 9% 6% 5% 4 4 5% 4 26%	1650
March	1st to 10th	Day	17% 12% 5% 10% 7% 6% 5% 4 4 5% 25%	4476
	11th to 20th	Day	10% 13% 4 11% 6% 5% 4 4 6% 7% 31%	4750
	21st to end	Day	18% 8% 4 9% 5% 4 4 3 5% 5% 35%	5922
April	1st to 10th	Day	20% 11% 3 7% 4 4 5% 3 5% 6% 32%	2400
	11th to 20th	Day	20% 9% 3 7% 5% 4 4 3 5% 5% 35%	2374
	21st to end	Day	22% 9% 3 8% 5% 4 4 4 5% 5% 31%	2470
May	1st to 10th	Day	20% 9% 3 8% 4 5% 5% 4 5% 7% 29%	4411
	11th to 20th	Day	17% 9% 3 8% 5% 4 4 4 6% 7% 33%	4398
	21st to end	Day	15% 6% 2 7% 5% 4 5% 4 7% 8% 37%	4837
June	1st to 10th	Day	21% 8% 3 7% 4 5% 5% 4 6% 7% 30%	2660
	11th to 20th	Day	14% 6% 2 6% 5% 5% 5% 5% 7% 7% 38%	2660
	21st to end	Day	19% 8% 3 6% 5% 5% 5% 4 7% 8% 31%	2660
July	1st to 10th	Day	11% 6% 3 8% 5% 5% 6% 5% 6% 9% 37%	4440
	11th to 20th	Day	13% 6% 2 6% 5% 5% 6% 4 7% 9% 38%	4440
	21st to end	Day	11% 5% 2 5% 4 4 5% 4 7% 9% 44%	4814
August	1st to 10th	Day	14% 7% 3 8% 5% 5% 5% 4 7% 9% 33%	4423
	11th to 20th	Day	13% 6% 3 7% 5% 5% 4 4 6% 9% 39%	3966
	21st to end	Day	16% 5% 2 6% 4 4 5% 5% 7% 9% 40%	4180
September	1st to 10th	Day	12% 6% 2 6% 4 5% 5% 4 6% 8% 43%	2280
	11th to 20th	Day	13% 5% 2 6% 3 4 4 4 5% 7% 49%	2218
	21st to end	Day	16% 6% 2 5% 4 4 5% 3 5% 7% 41%	2090
October	1st to 10th	Day	20% 7% 3 6% 5% 4 4 4 5% 6% 36%	1935
	11th to 20th	Day	18% 9% 3 8% 6% 5% 6% 4 4 5% 31%	1881
	21st to end	Day	24% 11% 4 7% 3 3 4 4 6% 6% 29%	1881
November	1st to 10th	Day	16% 10% 3 8% 4 5% 5% 4 6% 8% 33%	1761
	11th to 20th	Day	17% 10% 3 9% 5% 5% 5% 4 6% 7% 30%	1700
	21st to end	Day	21% 11% 3 9% 7% 6% 6% 4 5% 6% 23%	1600
December	1st to 10th	Day	20% 20% 5% 10% 5% 4 5% 3 6% 6% 16%	2600
	11th to 20th	Day	18% 13% 4 10% 6% 5% 5% 4 5% 4 26%	2986
	21st to end	Day	23% 14% 4 10% 5% 4 5% 4 5% 6% 20%	3960
January	1st to 10th	Night	14% 21% 7% 13% 5% 4 4 3 4 5% 19%	6349
	11th to 20th	Night	10% 16% 6% 16% 8% 7% 5% 4 5% 5% 17%	6315
	21st to end	Night	13% 17% 5% 12% 7% 5% 4 3 4 6% 24%	6996
February	1st to 10th	Night	14% 15% 5% 11% 8% 5% 5% 3 5% 6% 22%	2611
	11th to 20th	Night	14% 11% 5% 10% 6% 5% 5% 5% 5% 6% 27%	2448
	21st to end	Night	14% 16% 4 10% 5% 5% 3 3 4 5% 31%	1980
March	1st to 10th	Night	21% 13% 4 9% 5% 4 5% 3 4 5% 29%	5364
	11th to 20th	Night	15% 13% 3 8% 5% 5% 5% 4 6% 6% 30%	5194
	21st to end	Night	16% 10% 4 8% 5% 5% 5% 3 5% 5% 32%	5008
April	1st to 10th	Night	23% 9% 2 7% 5% 5% 4 3 5% 6% 30%	2000
	11th to 20th	Night	22% 9% 4 6% 5% 4 4 3 5% 5% 33%	1975
	21st to end	Night	21% 9% 3 8% 5% 3 3 3 5% 6% 34%	1826
May	1st to 10th	Night	21% 11% 3 8% 4 4 5% 4 5% 6% 30%	2559
	11th to 20th	Night	21% 7% 2 7% 4 4 4 3 4 6% 37%	2495
	21st to end	Night	15% 7% 3 7% 4 5% 4 3 5% 9% 37%	2738
June	1st to 10th	Night	19% 7% 3 7% 5% 5% 6% 5% 6% 8% 30%	1520
	11th to 20th	Night	16% 6% 2 6% 4 3 5% 4 6% 7% 40%	1520
	21st to end	Night	18% 8% 2 7% 5% 5% 4 4 6% 6% 35%	1520
July	1st to 10th	Night	12% 8% 3 8% 6% 5% 5% 5% 6% 7% 35%	2520
	11th to 20th	Night	13% 6% 2 6% 5% 4 5% 4 6% 9% 41%	2520
	21st to end	Night	11% 7% 3 6% 4 4 4 4 6% 7% 43%	2739
August	1st to 10th	Night	15% 7% 3 8% 4 4 4 4 6% 7% 39%	2506
	11th to 20th	Night	15% 7% 3 7% 4 4 4 4 6% 6% 39%	2826
	21st to end	Night	14% 6% 3 7% 5% 4 4 4 6% 7% 40%	3476
September	1st to 10th	Night	11% 6% 3 7% 5% 4 5% 3 6% 7% 43%	1900
	11th to 20th	Night	15% 6% 2 5% 3 3 3 3 5% 6% 50%	1962
	21st to end	Night	16% 6% 2 5% 4 4 5% 3 5% 8% 42%	2090
October	1st to 10th	Night	18% 7% 3 7% 5% 4 5% 3 5% 6% 37%	2242
	11th to 20th	Night	19% 8% 3 8% 5% 4 4 4 5% 6% 34%	2299
	21st to end	Night	22% 8% 3 9% 5% 5% 4 3 5% 5% 30%	2717
November	1st to 10th	Night	19% 9% 3 7% 5% 4 5% 3 5% 7% 32%	2537
	11th to 20th	Night	18% 10% 3 8% 5% 5% 5% 4 5% 5% 32%	2698
	21st to end	Night	21% 12% 4 9% 6% 5% 4 3 5% 6% 25%	2800
December	1st to 10th	Night	18% 17% 6% 12% 6% 5% 6% 3 5% 5% 18%	4580
	11th to 20th	Night	23% 14% 4 10% 5% 4 4 3 5% 4 23%	5208
	21st to end	Night	25% 16% 4 9% 4 3 3 4 5% 6% 21%	6996

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	49% 26% 10% 12% 3	3598
	11th to 20th	Day	44% 28% 9% 13% 5% 1	3600
	21st to end	Day	44% 28% 9% 10% 7% 1	3960
February	1st to 10th	Day	23% 61% 3 8% 3 1	1785
	11th to 20th	Day	23% 60% 4 8% 5%	1847
	21st to end	Day	23% 61% 3 6% 6% 1	1650
March	1st to 10th	Day	45% 29% 5% 10% 8% 2	4479
	11th to 20th	Day	37% 29% 13% 11% 9% 1	4750
	21st to end	Day	39% 29% 4 13% 14% 1	5922
April	1st to 10th	Day	20% 62% 3 8% 7% 1	2400
	11th to 20th	Day	20% 62% 2 9% 6% 1	2374
	21st to end	Day	21% 62% 4 7% 6%	2470
May	1st to 10th	Day	35% 41% 6% 15% 4	4425
	11th to 20th	Day	33% 39% 12% 13% 4	4402
	21st to end	Day	27% 40% 8% 18% 7%	4837
June	1st to 10th	Day	18% 64% 3 9% 5% 1	2660
	11th to 20th	Day	16% 64% 5% 8% 6% 1	2660
	21st to end	Day	18% 63% 4 9% 5% 1	2660
July	1st to 10th	Day	29% 37% 9% 18% 7%	4370
	11th to 20th	Day	26% 36% 11% 23% 4	4365
	21st to end	Day	22% 37% 9% 26% 6%	4661
August	1st to 10th	Day	30% 38% 12% 15% 6%	4409
	11th to 20th	Day	30% 37% 11% 19% 2	4008
	21st to end	Day	25% 38% 12% 15% 10%	4180
September	1st to 10th	Day	14% 61% 6% 13% 6%	2280
	11th to 20th	Day	13% 59% 6% 14% 7% 1	2218
	21st to end	Day	14% 60% 6% 13% 7% 1	2090
October	1st to 10th	Day	28% 37% 9% 21% 5%	3208
	11th to 20th	Day	23% 37% 14% 22% 5%	3109
	21st to end	Day	26% 37% 16% 16% 5%	3081
November	1st to 10th	Day	25% 57% 6% 9% 3	1761
	11th to 20th	Day	20% 57% 5% 12% 6% 1	1700
	21st to end	Day	27% 58% 6% 6% 3	1600
December	1st to 10th	Day	46% 36% 6% 11% 1	2600
	11th to 20th	Day	43% 33% 6% 11% 7%	2986
	21st to end	Day	47% 27% 8% 12% 5%	3960
January	1st to 10th	Night	52% 25% 10% 10% 3	6349
	11th to 20th	Night	49% 27% 9% 11% 3	6360
	21st to end	Night	46% 28% 8% 8% 9%	6996
February	1st to 10th	Night	25% 61% 3 8% 3	2611
	11th to 20th	Night	21% 61% 3 9% 5%	2450
	21st to end	Night	20% 61% 3 8% 8%	1980
March	1st to 10th	Night	42% 29% 5% 11% 10% 4	5374
	11th to 20th	Night	37% 29% 11% 12% 9% 2	5195
	21st to end	Night	41% 29% 3 17% 9% 1	5008
April	1st to 10th	Night	22% 61% 2 8% 6%	2000
	11th to 20th	Night	23% 62% 2 7% 6% 1	1975
	21st to end	Night	20% 63% 3 7% 6% 1	1826
May	1st to 10th	Night	34% 40% 9% 13% 3	2565
	11th to 20th	Night	31% 40% 10% 13% 6%	2496
	21st to end	Night	30% 39% 7% 12% 11%	2739
June	1st to 10th	Night	18% 65% 3 8% 6% 1	1520
	11th to 20th	Night	14% 65% 4 9% 8% 1	1520
	21st to end	Night	19% 63% 4 8% 6% 1	1520
July	1st to 10th	Night	33% 37% 10% 12% 9%	2480
	11th to 20th	Night	26% 37% 12% 21% 4	2472
	21st to end	Night	24% 36% 12% 18% 10%	2651
August	1st to 10th	Night	30% 38% 9% 18% 5%	2496
	11th to 20th	Night	33% 38% 12% 14% 3	2850
	21st to end	Night	28% 38% 9% 13% 10% 1	3476
September	1st to 10th	Night	13% 60% 6% 13% 8%	1900
	11th to 20th	Night	14% 59% 6% 15% 5% 1	1962
	21st to end	Night	15% 60% 5% 13% 8%	2090
October	1st to 10th	Night	27% 37% 10% 20% 5%	3736
	11th to 20th	Night	25% 37% 9% 23% 6%	3810
	21st to end	Night	29% 37% 13% 13% 9%	4467
November	1st to 10th	Night	25% 55% 6% 9% 4 1	2537
	11th to 20th	Night	24% 57% 4 10% 5%	2698
	21st to end	Night	26% 58% 6% 7% 3	2800
December	1st to 10th	Night	45% 36% 8% 8% 2	4580
	11th to 20th	Night	45% 32% 5% 9% 9%	5208
	21st to end	Night	48% 26% 6% 12% 8%	6996

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% 14% 29% 23% 10% 1	200
	11th to 20th	Day	27% 13% 33% 21% 7% 1	200
	21st to end	Day	20% 14% 35% 22% 8% 1	220
February	1st to 10th	Day	28% 17% 35% 15% 4	293
	11th to 20th	Day	23% 11% 41% 22% 3	385
	21st to end	Day	27% 11% 30% 29% 4	414
March	1st to 10th	Day	24% 11% 35% 23% 7%	500
	11th to 20th	Day	19% 13% 37% 23% 6% 2	500
	21st to end	Day	25% 11% 31% 19% 12% 2	550
April	1st to 10th	Day	28% 13% 26% 22% 9% 2	500
	11th to 20th	Day	25% 14% 27% 20% 11% 3	492
	21st to end	Day	26% 12% 29% 21% 10% 2	480
May	1st to 10th	Day	21% 9% 29% 27% 10% 3	463
	11th to 20th	Day	18% 6% 30% 25% 16% 4	487
	21st to end	Day	17% 9% 29% 26% 13% 5%	529
June	1st to 10th	Day	21% 9% 23% 30% 13% 4	490
	11th to 20th	Day	16% 8% 25% 31% 17% 2	490
	21st to end	Day	22% 7% 25% 25% 15% 5%	490
July	1st to 10th	Day	10% 8% 25% 31% 20% 7%	485
	11th to 20th	Day	13% 6% 26% 31% 18% 6%	484
	21st to end	Day	13% 6% 21% 36% 19% 5%	532
August	1st to 10th	Day	15% 5% 20% 31% 21% 7%	486
	11th to 20th	Day	12% 4 20% 33% 22% 10%	479
	21st to end	Day	14% 6% 25% 25% 20% 9%	539
September	1st to 10th	Day	15% 7% 24% 26% 19% 9%	490
	11th to 20th	Day	13% 6% 21% 25% 21% 13%	490
	21st to end	Day	18% 8% 25% 24% 19% 6%	490
October	1st to 10th	Day	21% 9% 25% 25% 14% 6%	490
	11th to 20th	Day	20% 13% 30% 23% 13% 2	480
	21st to end	Day	29% 13% 24% 24% 9%	429
November	1st to 10th	Day	27% 13% 23% 27% 9% 1	395
	11th to 20th	Day	22% 15% 35% 20% 8% 1	300
	21st to end	Day	27% 16% 32% 19% 7% 1	200
December	1st to 10th	Day	31% 9% 29% 24% 7% 2	200
	11th to 20th	Day	25% 14% 39% 17% 5% 1	200
	21st to end	Day	30% 12% 30% 20% 5% 3	220
January	1st to 10th	Night	25% 12% 31% 27% 5%	800
	11th to 20th	Night	27% 9% 33% 25% 6% 1	800
	21st to end	Night	19% 10% 35% 28% 8% 1	880
February	1st to 10th	Night	20% 11% 36% 26% 6% 1	800
	11th to 20th	Night	21% 11% 36% 27% 5%	783
	21st to end	Night	22% 9% 34% 30% 5%	660
March	1st to 10th	Night	24% 12% 31% 28% 6%	800
	11th to 20th	Night	25% 10% 31% 28% 6% 1	800
	21st to end	Night	22% 10% 28% 30% 9%	880
April	1st to 10th	Night	27% 10% 27% 29% 6%	800
	11th to 20th	Night	25% 12% 30% 27% 6% 1	791
	21st to end	Night	27% 9% 27% 27% 9%	652
May	1st to 10th	Night	25% 6% 29% 29% 9% 2	412
	11th to 20th	Night	21% 7% 26% 34% 10% 1	374
	21st to end	Night	15% 5% 32% 38% 10%	411
June	1st to 10th	Night	25% 4 26% 34% 11% 1	380
	11th to 20th	Night	17% 5% 25% 36% 16% 1	380
	21st to end	Night	19% 6% 26% 32% 17% 1	380
July	1st to 10th	Night	12% 4 24% 40% 19% 1	370
	11th to 20th	Night	11% 3 24% 44% 17% 1	368
	21st to end	Night	13% 4 21% 42% 17% 1	416
August	1st to 10th	Night	16% 3 15% 44% 21% 1	378
	11th to 20th	Night	14% 3 20% 36% 24% 3	576
	21st to end	Night	15% 3 22% 35% 22% 2	836
September	1st to 10th	Night	14% 4 23% 39% 19% 1	760
	11th to 20th	Night	18% 5% 17% 35% 23% 3	760
	21st to end	Night	19% 6% 23% 31% 19% 2	760
October	1st to 10th	Night	21% 7% 26% 31% 14% 1	760
	11th to 20th	Night	23% 9% 28% 29% 11% 1	760
	21st to end	Night	26% 10% 30% 28% 6%	836
November	1st to 10th	Night	25% 12% 28% 27% 8% 1	780
	11th to 20th	Night	23% 13% 33% 21% 9% 1	800
	21st to end	Night	29% 13% 29% 21% 7% 1	800
December	1st to 10th	Night	30% 10% 31% 24% 5%	800
	11th to 20th	Night	29% 14% 29% 23% 5% 1	800
	21st to end	Night	34% 10% 27% 23% 5%	880

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