Watch Observatory 2
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Watch Observatory 2 clear sky chart from 2005-11-06 to 2025-02-25.

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 5% 7% 32%	108668
	Night	17% 12% 4 9% 5% 5% 4 3 5% 6% 30%	109980
Transparency	Day	30% 43% 7% 13% 5%	112146
	Night	34% 41% 7% 12% 6%	114653
Seeing	Day	24% 16% 30% 19% 8% 2	14819
	Night	24% 14% 29% 24% 9% 1	23994

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% 19% 6% 13% 6% 5% 4 3 4 5% 20%	3597
	11th to 20th	Day	13% 16% 6% 12% 6% 5% 5% 4 5% 4 23%	3570
	21st to end	Day	12% 17% 6% 12% 6% 5% 5% 4 6% 6% 22%	3978
February	1st to 10th	Day	15% 15% 4 12% 7% 4 4 4 5% 5% 24%	1800
	11th to 20th	Day	14% 14% 4 10% 8% 6% 5% 3 4 6% 25%	1823
	21st to end	Day	20% 13% 5% 10% 6% 5% 4 3 4 5% 26%	1610
March	1st to 10th	Day	20% 13% 4 11% 6% 4 4 3 3 6% 26%	3116
	11th to 20th	Day	13% 17% 5% 11% 6% 5% 5% 3 4 5% 27%	3351
	21st to end	Day	15% 8% 3 7% 6% 5% 5% 4 4 6% 39%	4120
April	1st to 10th	Day	21% 12% 3 8% 5% 4 4 3 4 5% 33%	2280
	11th to 20th	Day	21% 9% 2 8% 5% 4 4 3 5% 5% 35%	2254
	21st to end	Day	23% 8% 3 8% 5% 4 4 3 4 5% 32%	2393
May	1st to 10th	Day	21% 9% 4 8% 4 6% 5% 4 6% 6% 29%	4426
	11th to 20th	Day	18% 9% 3 8% 5% 4 5% 4 5% 7% 33%	4398
	21st to end	Day	15% 7% 3 7% 5% 4 5% 4 6% 8% 36%	4837
June	1st to 10th	Day	22% 7% 3 7% 5% 4 5% 4 5% 8% 29%	2660
	11th to 20th	Day	14% 6% 3 7% 5% 5% 5% 4 7% 8% 37%	2660
	21st to end	Day	19% 7% 3 7% 4 5% 5% 5% 6% 7% 31%	2660
July	1st to 10th	Day	11% 7% 3 7% 6% 6% 6% 4 6% 9% 35%	4440
	11th to 20th	Day	13% 6% 2 6% 5% 5% 5% 5% 8% 8% 37%	4440
	21st to end	Day	11% 6% 2 5% 4 4 4 4 7% 9% 43%	4814
August	1st to 10th	Day	14% 7% 3 7% 5% 5% 5% 4 6% 10% 33%	4423
	11th to 20th	Day	13% 6% 3 7% 5% 4 5% 4 6% 8% 39%	3955
	21st to end	Day	15% 5% 1 6% 4 5% 5% 4 7% 9% 38%	4180
September	1st to 10th	Day	12% 6% 3 5% 4 4 4 5% 6% 8% 43%	2280
	11th to 20th	Day	12% 4 2 6% 3 4 4 3 4 7% 49%	2229
	21st to end	Day	15% 7% 3 6% 4 4 4 3 5% 7% 41%	2090
October	1st to 10th	Day	19% 9% 3 6% 5% 5% 4 3 5% 6% 36%	1916
	11th to 20th	Day	20% 9% 3 9% 5% 6% 4 3 5% 5% 31%	1895
	21st to end	Day	23% 11% 4 8% 4 4 5% 3 5% 5% 28%	1881
November	1st to 10th	Day	16% 9% 3 7% 5% 5% 5% 4 5% 8% 33%	1761
	11th to 20th	Day	16% 10% 3 8% 6% 7% 5% 4 5% 7% 29%	1685
	21st to end	Day	20% 13% 4 9% 7% 6% 5% 3 7% 5% 22%	1600
December	1st to 10th	Day	20% 19% 6% 10% 6% 5% 4 3 6% 6% 15%	2600
	11th to 20th	Day	19% 15% 5% 10% 5% 5% 5% 4 4 5% 24%	2986
	21st to end	Day	24% 15% 4 10% 5% 4 5% 3 4 5% 21%	3960
January	1st to 10th	Night	14% 22% 8% 13% 5% 5% 4 3 4 4 19%	6349
	11th to 20th	Night	10% 16% 7% 17% 8% 6% 6% 4 4 5% 17%	6315
	21st to end	Night	14% 17% 5% 12% 6% 5% 5% 3 5% 5% 23%	6973
February	1st to 10th	Night	15% 13% 5% 11% 7% 6% 6% 4 5% 6% 22%	2600
	11th to 20th	Night	13% 13% 4 11% 6% 6% 6% 4 5% 5% 27%	2461
	21st to end	Night	15% 14% 4 10% 6% 4 4 4 4 5% 30%	1939
March	1st to 10th	Night	20% 10% 3 8% 6% 4 4 3 5% 5% 32%	3724
	11th to 20th	Night	18% 17% 5% 9% 6% 5% 4 3 4 5% 26%	3593
	21st to end	Night	13% 11% 3 8% 5% 4 4 3 4 5% 38%	3512
April	1st to 10th	Night	23% 9% 3 7% 5% 5% 4 3 4 6% 31%	1900
	11th to 20th	Night	22% 8% 3 7% 5% 3 4 3 5% 5% 34%	1875
	21st to end	Night	22% 8% 3 7% 5% 4 4 3 5% 6% 33%	1786
May	1st to 10th	Night	21% 10% 3 6% 5% 4 5% 4 5% 7% 29%	2528
	11th to 20th	Night	21% 8% 3 6% 4 3 3 3 5% 6% 37%	2495
	21st to end	Night	14% 8% 3 6% 5% 4 5% 3 6% 8% 38%	2738
June	1st to 10th	Night	20% 6% 3 6% 6% 4 5% 4 7% 9% 30%	1520
	11th to 20th	Night	16% 7% 3 5% 4 4 4 3 6% 7% 41%	1520
	21st to end	Night	17% 7% 3 8% 5% 4 5% 5% 5% 8% 34%	1520
July	1st to 10th	Night	12% 8% 3 9% 5% 5% 5% 5% 6% 7% 35%	2520
	11th to 20th	Night	13% 6% 2 6% 5% 4 4 4 6% 8% 41%	2520
	21st to end	Night	12% 6% 2 6% 4 4 5% 4 6% 7% 43%	2739
August	1st to 10th	Night	14% 8% 3 7% 4 4 4 3 6% 8% 39%	2506
	11th to 20th	Night	15% 7% 3 6% 5% 4 5% 4 5% 7% 40%	2837
	21st to end	Night	13% 7% 2 7% 5% 4 4 4 6% 8% 40%	3476
September	1st to 10th	Night	10% 6% 2 8% 5% 5% 4 3 6% 7% 43%	1900
	11th to 20th	Night	14% 6% 2 5% 4 4 3 2 5% 7% 49%	1949
	21st to end	Night	16% 5% 2 5% 4 5% 4 3 5% 7% 43%	2090
October	1st to 10th	Night	19% 7% 3 6% 5% 4 5% 3 5% 6% 38%	2261
	11th to 20th	Night	19% 8% 4 7% 5% 5% 5% 4 4 6% 33%	2283
	21st to end	Night	22% 10% 3 8% 5% 5% 5% 3 5% 5% 30%	2717
November	1st to 10th	Night	19% 10% 3 8% 5% 4 4 3 5% 6% 33%	2537
	11th to 20th	Night	18% 10% 3 8% 5% 5% 5% 4 5% 5% 32%	2713
	21st to end	Night	20% 12% 4 10% 6% 5% 4 4 5% 6% 25%	2800
December	1st to 10th	Night	17% 17% 6% 12% 7% 5% 5% 4 5% 5% 18%	4580
	11th to 20th	Night	23% 16% 4 10% 5% 4 4 3 4 4 24%	5208
	21st to end	Night	26% 17% 4 9% 4 4 3 3 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	52% 26% 8% 11% 3 1	3598
	11th to 20th	Day	48% 26% 7% 11% 6% 1	3600
	21st to end	Day	48% 28% 8% 8% 6% 1	3978
February	1st to 10th	Day	25% 61% 3 8% 3 1	1800
	11th to 20th	Day	24% 61% 3 8% 5%	1832
	21st to end	Day	23% 63% 3 5% 5% 1	1610
March	1st to 10th	Day	39% 39% 4 12% 5%	3119
	11th to 20th	Day	40% 39% 4 8% 7% 1	3351
	21st to end	Day	29% 39% 3 14% 13% 1	4120
April	1st to 10th	Day	19% 65% 3 7% 5% 1	2280
	11th to 20th	Day	19% 65% 2 8% 5% 1	2254
	21st to end	Day	20% 65% 3 6% 5%	2393
May	1st to 10th	Day	37% 39% 4 16% 4	4440
	11th to 20th	Day	35% 39% 11% 11% 4	4402
	21st to end	Day	29% 39% 7% 18% 7%	4837
June	1st to 10th	Day	19% 65% 3 8% 5%	2660
	11th to 20th	Day	16% 65% 5% 8% 5% 1	2660
	21st to end	Day	18% 64% 4 9% 5% 1	2660
July	1st to 10th	Day	30% 38% 9% 16% 7%	4370
	11th to 20th	Day	26% 38% 10% 22% 4	4365
	21st to end	Day	22% 38% 9% 26% 5%	4661
August	1st to 10th	Day	31% 38% 11% 15% 5%	4409
	11th to 20th	Day	30% 39% 10% 19% 2	3996
	21st to end	Day	26% 38% 12% 16% 7%	4180
September	1st to 10th	Day	14% 61% 7% 12% 6%	2280
	11th to 20th	Day	13% 60% 6% 14% 7% 1	2229
	21st to end	Day	14% 60% 6% 13% 6% 1	2090
October	1st to 10th	Day	28% 37% 9% 22% 4	3176
	11th to 20th	Day	25% 37% 12% 21% 4	3123
	21st to end	Day	27% 37% 15% 16% 4	3081
November	1st to 10th	Day	25% 57% 6% 9% 3 1	1761
	11th to 20th	Day	20% 59% 5% 11% 5% 1	1685
	21st to end	Day	27% 59% 6% 5% 2	1600
December	1st to 10th	Day	45% 38% 8% 8% 1	2600
	11th to 20th	Day	45% 33% 6% 10% 5%	2986
	21st to end	Day	48% 27% 6% 14% 5%	3960
January	1st to 10th	Night	53% 26% 10% 9% 3	6349
	11th to 20th	Night	53% 26% 7% 10% 3	6360
	21st to end	Night	49% 28% 7% 7% 9% 1	6973
February	1st to 10th	Night	25% 62% 4 7% 2	2600
	11th to 20th	Night	23% 61% 3 8% 4	2463
	21st to end	Night	20% 63% 3 7% 6%	1939
March	1st to 10th	Night	32% 40% 5% 15% 7% 2	3734
	11th to 20th	Night	41% 39% 5% 8% 6% 1	3594
	21st to end	Night	32% 39% 1 15% 12% 1	3512
April	1st to 10th	Night	20% 65% 2 7% 6%	1900
	11th to 20th	Night	20% 65% 2 7% 5%	1875
	21st to end	Night	20% 65% 3 6% 6% 1	1786
May	1st to 10th	Night	35% 40% 8% 14% 3	2534
	11th to 20th	Night	34% 38% 9% 12% 6%	2496
	21st to end	Night	30% 39% 6% 14% 10% 1	2739
June	1st to 10th	Night	18% 65% 3 8% 5% 1	1520
	11th to 20th	Night	15% 65% 4 8% 8%	1520
	21st to end	Night	18% 64% 4 8% 6% 1	1520
July	1st to 10th	Night	33% 38% 9% 11% 9%	2480
	11th to 20th	Night	26% 37% 11% 23% 3	2472
	21st to end	Night	25% 37% 11% 16% 10%	2651
August	1st to 10th	Night	29% 38% 10% 18% 5%	2496
	11th to 20th	Night	31% 38% 11% 17% 2	2862
	21st to end	Night	27% 39% 10% 15% 9% 1	3476
September	1st to 10th	Night	14% 60% 6% 12% 7%	1900
	11th to 20th	Night	13% 60% 6% 15% 5% 1	1949
	21st to end	Night	15% 60% 5% 13% 7%	2090
October	1st to 10th	Night	25% 38% 11% 21% 5%	3768
	11th to 20th	Night	25% 37% 10% 22% 6%	3794
	21st to end	Night	29% 38% 13% 14% 7%	4467
November	1st to 10th	Night	25% 56% 7% 7% 4 1	2537
	11th to 20th	Night	24% 57% 5% 9% 5%	2713
	21st to end	Night	26% 58% 6% 8% 2	2800
December	1st to 10th	Night	45% 37% 9% 7% 2	4580
	11th to 20th	Night	47% 32% 3 10% 8%	5208
	21st to end	Night	50% 26% 4 11% 9%	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	32% 18% 32% 16% 3	200
	11th to 20th	Day	31% 26% 29% 10% 4 1	200
	21st to end	Day	19% 22% 39% 17% 3	223
February	1st to 10th	Day	33% 30% 26% 9% 2	300
	11th to 20th	Day	29% 28% 36% 7%	370
	21st to end	Day	30% 20% 34% 15% 2	398
March	1st to 10th	Day	28% 22% 31% 15% 4	460
	11th to 20th	Day	29% 22% 33% 13% 2 1	460
	21st to end	Day	32% 22% 24% 14% 6% 1	506
April	1st to 10th	Day	31% 19% 31% 14% 5%	460
	11th to 20th	Day	34% 19% 26% 13% 6% 1	452
	21st to end	Day	32% 19% 29% 13% 7% 1	460
May	1st to 10th	Day	25% 17% 34% 16% 6% 1	463
	11th to 20th	Day	21% 16% 33% 20% 9% 2	487
	21st to end	Day	20% 17% 31% 22% 7% 1	529
June	1st to 10th	Day	28% 11% 31% 23% 7% 2	490
	11th to 20th	Day	18% 15% 32% 25% 8% 2	490
	21st to end	Day	25% 11% 33% 18% 10% 3	490
July	1st to 10th	Day	16% 8% 33% 26% 13% 2	485
	11th to 20th	Day	14% 11% 33% 27% 13% 2	484
	21st to end	Day	17% 8% 33% 29% 11% 2	532
August	1st to 10th	Day	17% 8% 28% 31% 14% 3	486
	11th to 20th	Day	16% 6% 27% 31% 13% 6%	479
	21st to end	Day	17% 9% 32% 23% 14% 6%	539
September	1st to 10th	Day	18% 10% 30% 21% 15% 5%	490
	11th to 20th	Day	15% 8% 26% 22% 20% 11%	490
	21st to end	Day	19% 14% 31% 19% 14% 4	490
October	1st to 10th	Day	23% 15% 27% 20% 12% 3	490
	11th to 20th	Day	25% 20% 27% 20% 6% 1	487
	21st to end	Day	33% 14% 30% 18% 5%	429
November	1st to 10th	Day	30% 18% 27% 18% 6% 1	395
	11th to 20th	Day	26% 23% 30% 16% 5%	285
	21st to end	Day	31% 21% 28% 16% 5% 1	200
December	1st to 10th	Day	30% 18% 33% 16% 4	200
	11th to 20th	Day	31% 26% 30% 11% 1 1	200
	21st to end	Day	32% 22% 28% 15% 2	220
January	1st to 10th	Night	27% 19% 35% 14% 4	800
	11th to 20th	Night	28% 20% 26% 20% 5% 1	800
	21st to end	Night	21% 18% 35% 21% 5%	880
February	1st to 10th	Night	23% 20% 33% 21% 3	800
	11th to 20th	Night	23% 22% 34% 17% 3	783
	21st to end	Night	26% 18% 31% 22% 3	647
March	1st to 10th	Night	24% 19% 30% 23% 4	760
	11th to 20th	Night	27% 18% 32% 19% 3 1	760
	21st to end	Night	25% 17% 27% 25% 6% 1	836
April	1st to 10th	Night	31% 15% 29% 21% 4	760
	11th to 20th	Night	26% 19% 29% 21% 5%	751
	21st to end	Night	29% 14% 29% 20% 8%	646
May	1st to 10th	Night	26% 11% 30% 24% 7% 2	394
	11th to 20th	Night	24% 10% 29% 26% 9% 1	374
	21st to end	Night	17% 10% 38% 27% 9%	411
June	1st to 10th	Night	22% 8% 28% 32% 10% 1	380
	11th to 20th	Night	21% 8% 27% 31% 12% 1	380
	21st to end	Night	18% 8% 30% 27% 16% 1	380
July	1st to 10th	Night	13% 7% 30% 34% 15% 1	370
	11th to 20th	Night	14% 5% 29% 36% 15%	368
	21st to end	Night	14% 4 31% 37% 14% 1	416
August	1st to 10th	Night	14% 6% 22% 37% 19% 1	378
	11th to 20th	Night	15% 2 26% 34% 22% 2	588
	21st to end	Night	17% 6% 27% 31% 17% 2	836
September	1st to 10th	Night	14% 6% 31% 34% 14% 1	760
	11th to 20th	Night	19% 6% 21% 30% 21% 3	760
	21st to end	Night	19% 10% 28% 24% 18% 2	760
October	1st to 10th	Night	23% 10% 27% 28% 11% 1	760
	11th to 20th	Night	25% 12% 29% 24% 10%	760
	21st to end	Night	28% 16% 28% 24% 4	836
November	1st to 10th	Night	28% 17% 24% 23% 7% 1	780
	11th to 20th	Night	24% 19% 32% 17% 9% 1	800
	21st to end	Night	29% 19% 26% 20% 7%	800
December	1st to 10th	Night	31% 18% 29% 19% 4	800
	11th to 20th	Night	32% 21% 26% 16% 5% 1	800
	21st to end	Night	37% 17% 26% 15% 4	880

Page generated at 2025-02-22 06:51:38. Version ac07

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