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 2024-10-25.

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% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	101449
	Night	18% 11% 4 8% 5% 4 4 3 5% 6% 32%	97364
Transparency	Day	27% 48% 7% 13% 5%	101210
	Night	29% 48% 7% 11% 5%	97223
Seeing	Day	20% 9% 28% 26% 13% 4	14534
	Night	22% 8% 28% 30% 11% 1	23494

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	16% 16% 5% 11% 6% 4 5% 4 4 6% 22%	2517
	11th to 20th	Day	15% 13% 6% 10% 5% 5% 6% 4 6% 6% 24%	2520
	21st to end	Day	14% 16% 6% 10% 5% 5% 5% 4 6% 6% 22%	2772
February	1st to 10th	Day	15% 16% 4 11% 6% 4 5% 4 5% 5% 25%	1695
	11th to 20th	Day	14% 12% 4 9% 7% 5% 5% 5% 6% 6% 27%	1743
	21st to end	Day	18% 14% 4 9% 6% 5% 4 3 5% 4 27%	1570
March	1st to 10th	Day	19% 12% 5% 9% 6% 5% 4 3 4 5% 27%	3116
	11th to 20th	Day	11% 16% 5% 13% 6% 5% 4 4 5% 6% 27%	3320
	21st to end	Day	15% 7% 3 8% 6% 4 5% 3 5% 5% 39%	4132
April	1st to 10th	Day	21% 11% 3 7% 5% 4 5% 3 5% 6% 32%	2280
	11th to 20th	Day	20% 8% 3 7% 5% 4 4 4 5% 5% 35%	2254
	21st to end	Day	22% 9% 3 8% 5% 4 4 3 5% 5% 32%	2408
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% 3 7% 4 4 4 4 6% 6% 28%	1825
November	1st to 10th	Day	16% 10% 3 8% 4 4 5% 4 6% 8% 33%	1671
	11th to 20th	Day	17% 10% 3 9% 5% 5% 5% 4 6% 7% 29%	1615
	21st to end	Day	21% 10% 3 9% 7% 6% 6% 4 5% 6% 23%	1520
December	1st to 10th	Day	24% 16% 4 9% 5% 5% 5% 4 6% 6% 18%	1520
	11th to 20th	Day	21% 13% 4 9% 6% 4 4 4 5% 5% 24%	1906
	21st to end	Day	28% 16% 4 10% 5% 4 3 3 3 4 20%	2772
January	1st to 10th	Night	17% 17% 6% 11% 6% 5% 5% 4 5% 6% 21%	4429
	11th to 20th	Night	12% 14% 5% 13% 8% 7% 5% 4 6% 6% 20%	4440
	21st to end	Night	16% 18% 5% 11% 6% 5% 4 3 4 5% 24%	4884
February	1st to 10th	Night	14% 15% 5% 11% 8% 5% 5% 4 5% 6% 21%	2481
	11th to 20th	Night	14% 11% 5% 10% 6% 5% 6% 5% 5% 6% 28%	2323
	21st to end	Night	14% 15% 4 10% 5% 5% 3 3 4 5% 31%	1884
March	1st to 10th	Night	20% 10% 3 8% 5% 4 5% 3 4 5% 32%	3724
	11th to 20th	Night	19% 17% 4 9% 5% 4 4 3 4 4 27%	3624
	21st to end	Night	13% 11% 4 8% 4 5% 5% 3 5% 5% 39%	3498
April	1st to 10th	Night	24% 9% 2 7% 5% 5% 4 3 6% 6% 30%	1900
	11th to 20th	Night	22% 9% 3 6% 5% 4 4 3 5% 6% 34%	1875
	21st to end	Night	21% 9% 3 8% 5% 3 4 3 5% 6% 34%	1771
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%	2637
November	1st to 10th	Night	19% 9% 3 7% 5% 4 5% 3 5% 7% 33%	2407
	11th to 20th	Night	18% 11% 4 7% 5% 5% 5% 4 5% 5% 32%	2563
	21st to end	Night	22% 12% 3 9% 6% 5% 4 3 5% 5% 26%	2660
December	1st to 10th	Night	22% 15% 6% 10% 6% 4 5% 3 5% 5% 20%	2660
	11th to 20th	Night	23% 15% 4 9% 5% 4 4 3 4 4 24%	3288
	21st to end	Night	30% 16% 4 9% 4 3 3 2 3 4 21%	4884

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	40% 37% 12% 7% 4 1	2518
	11th to 20th	Day	36% 38% 11% 11% 4	2520
	21st to end	Day	41% 40% 11% 8% 1	2772
February	1st to 10th	Day	22% 64% 3 8% 2 1	1695
	11th to 20th	Day	20% 63% 3 8% 5%	1752
	21st to end	Day	21% 64% 3 5% 6% 1	1570
March	1st to 10th	Day	38% 40% 5% 12% 5% 1	3119
	11th to 20th	Day	38% 40% 5% 8% 8% 2	3320
	21st to end	Day	27% 40% 4 14% 13% 1	4132
April	1st to 10th	Day	19% 65% 3 7% 6% 1	2280
	11th to 20th	Day	18% 65% 2 9% 5% 1	2254
	21st to end	Day	19% 64% 4 7% 6%	2408
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	19% 59% 4 12% 5%	1935
	11th to 20th	Day	19% 59% 7% 11% 3	1881
	21st to end	Day	24% 60% 6% 8% 2	1825
November	1st to 10th	Day	23% 59% 6% 8% 3	1671
	11th to 20th	Day	20% 60% 5% 11% 4 1	1615
	21st to end	Day	25% 60% 5% 6% 3	1520
December	1st to 10th	Day	28% 59% 4 7% 2	1520
	11th to 20th	Day	34% 48% 7% 8% 3	1906
	21st to end	Day	46% 36% 6% 8% 4	2772
January	1st to 10th	Night	41% 36% 13% 7% 3	4429
	11th to 20th	Night	38% 38% 10% 11% 3	4440
	21st to end	Night	42% 39% 10% 7% 2	4884
February	1st to 10th	Night	23% 64% 3 7% 2	2481
	11th to 20th	Night	19% 64% 3 9% 5%	2325
	21st to end	Night	18% 64% 3 7% 8%	1884
March	1st to 10th	Night	32% 40% 6% 13% 7% 3	3734
	11th to 20th	Night	39% 40% 4 8% 8% 1	3625
	21st to end	Night	30% 40% 2 16% 11% 2	3498
April	1st to 10th	Night	21% 64% 2 7% 6%	1900
	11th to 20th	Night	21% 65% 1 7% 5% 1	1875
	21st to end	Night	19% 65% 3 6% 6% 1	1771
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	18% 60% 5% 12% 5% 1	2242
	11th to 20th	Night	19% 59% 5% 13% 5%	2299
	21st to end	Night	25% 60% 6% 6% 3	2637
November	1st to 10th	Night	24% 58% 6% 8% 4 1	2407
	11th to 20th	Night	23% 60% 5% 9% 4	2563
	21st to end	Night	25% 60% 5% 7% 3	2660
December	1st to 10th	Night	26% 60% 4 8% 2	2660
	11th to 20th	Night	35% 48% 4 8% 4	3288
	21st to end	Night	45% 36% 5% 11% 3	4884

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	23% 12% 28% 25% 11% 1	180
	11th to 20th	Day	28% 12% 32% 21% 7% 1	180
	21st to end	Day	20% 11% 35% 24% 9% 1	198
February	1st to 10th	Day	29% 16% 35% 16% 5%	263
	11th to 20th	Day	22% 10% 42% 23% 3	350
	21st to end	Day	28% 9% 30% 29% 4	382
March	1st to 10th	Day	23% 10% 35% 24% 8%	460
	11th to 20th	Day	19% 13% 35% 23% 7% 2	460
	21st to end	Day	24% 10% 31% 19% 13% 2	506
April	1st to 10th	Day	27% 11% 26% 23% 10% 2	460
	11th to 20th	Day	26% 13% 27% 20% 11% 3	452
	21st to end	Day	27% 11% 28% 22% 10% 2	460
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% 12% 25% 25% 9%	411
November	1st to 10th	Day	26% 13% 23% 28% 9% 1	365
	11th to 20th	Day	23% 15% 33% 20% 8% 1	275
	21st to end	Day	25% 14% 33% 21% 7% 1	180
December	1st to 10th	Day	32% 8% 27% 24% 8% 1	180
	11th to 20th	Day	24% 13% 39% 17% 6% 1	180
	21st to end	Day	32% 13% 29% 18% 6% 2	198
January	1st to 10th	Night	25% 12% 30% 28% 5%	760
	11th to 20th	Night	27% 9% 33% 26% 5% 1	760
	21st to end	Night	19% 10% 34% 28% 8% 1	836
February	1st to 10th	Night	21% 10% 36% 26% 6% 1	760
	11th to 20th	Night	20% 11% 36% 29% 5%	743
	21st to end	Night	22% 9% 34% 29% 5%	628
March	1st to 10th	Night	24% 11% 31% 29% 6%	760
	11th to 20th	Night	25% 10% 30% 27% 7% 1	760
	21st to end	Night	22% 9% 29% 31% 9%	836
April	1st to 10th	Night	27% 10% 26% 31% 6%	760
	11th to 20th	Night	25% 11% 30% 28% 7% 1	751
	21st to end	Night	26% 9% 27% 28% 9%	631
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	27% 9% 29% 28% 6%	812
November	1st to 10th	Night	25% 11% 28% 27% 9% 1	740
	11th to 20th	Night	23% 13% 33% 20% 9% 1	760
	21st to end	Night	29% 12% 29% 21% 8% 1	760
December	1st to 10th	Night	31% 11% 30% 23% 5%	760
	11th to 20th	Night	29% 13% 29% 23% 5% 1	760
	21st to end	Night	35% 11% 27% 22% 5%	836

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