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 2024-02-25.

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	17% 9% 4 7% 6% 3 5% 5% 5% 7% 33%	78233
	Night	18% 10% 4 7% 6% 3 5% 4 5% 6% 31%	81063
Transparency	Day	50% 5% 13% 21% 10% 1	77938
	Night	54% 5% 11% 20% 10% 1	80892
Seeing	Day	21% 9% 27% 26% 13% 4	13564
	Night	23% 8% 28% 30% 11% 1	22739

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	17% 15% 6% 10% 8% 3 5% 4 4 6% 22%	2517
	11th to 20th	Day	15% 13% 6% 8% 7% 4 6% 6% 5% 7% 24%	2520
	21st to end	Day	14% 16% 6% 9% 7% 4 5% 5% 6% 6% 22%	2772
February	1st to 10th	Day	15% 15% 5% 10% 7% 3 5% 5% 5% 5% 25%	1695
	11th to 20th	Day	14% 12% 4 9% 7% 4 5% 5% 6% 6% 27%	1743
	21st to end	Day	19% 13% 5% 9% 7% 4 4 4 5% 4 27%	1520
March	1st to 10th	Day	15% 12% 6% 7% 7% 4 6% 4 4 5% 30%	1797
	11th to 20th	Day	16% 11% 4 7% 7% 3 5% 5% 5% 6% 31%	1890
	21st to end	Day	20% 8% 3 6% 6% 2 4 4 5% 5% 36%	2356
April	1st to 10th	Day	21% 9% 4 6% 6% 3 5% 4 4 6% 32%	2160
	11th to 20th	Day	20% 7% 3 6% 5% 3 5% 5% 5% 5% 36%	2134
	21st to end	Day	22% 8% 4 6% 6% 3 4 4 5% 5% 32%	2281
May	1st to 10th	Day	19% 7% 4 6% 5% 3 5% 5% 5% 6% 35%	2505
	11th to 20th	Day	17% 7% 2 6% 5% 3 4 4 6% 7% 38%	2478
	21st to end	Day	16% 6% 3 6% 5% 3 5% 5% 7% 7% 37%	2725
June	1st to 10th	Day	21% 7% 3 6% 5% 3 5% 5% 6% 7% 31%	2520
	11th to 20th	Day	15% 6% 3 5% 6% 3 5% 6% 6% 7% 39%	2520
	21st to end	Day	19% 7% 3 6% 5% 4 6% 5% 6% 8% 31%	2520
July	1st to 10th	Day	12% 5% 3 6% 6% 3 5% 6% 6% 8% 42%	2520
	11th to 20th	Day	12% 5% 2 5% 5% 4 6% 6% 7% 9% 38%	2520
	21st to end	Day	12% 4 3 5% 6% 3 5% 6% 7% 10% 38%	2744
August	1st to 10th	Day	12% 7% 2 5% 6% 4 5% 6% 7% 8% 38%	2506
	11th to 20th	Day	10% 5% 3 5% 6% 4 5% 6% 6% 9% 41%	2270
	21st to end	Day	13% 5% 2 5% 5% 3 4 6% 6% 8% 41%	2376
September	1st to 10th	Day	13% 5% 2 5% 5% 3 5% 5% 6% 8% 42%	2160
	11th to 20th	Day	14% 4 2 5% 4 3 4 4 5% 7% 48%	2102
	21st to end	Day	16% 6% 2 5% 5% 3 5% 4 5% 7% 41%	1980
October	1st to 10th	Day	21% 6% 3 5% 6% 3 4 5% 5% 6% 36%	1834
	11th to 20th	Day	18% 9% 3 8% 8% 4 5% 5% 4 6% 29%	1782
	21st to end	Day	25% 11% 4 6% 5% 3 4 5% 5% 6% 27%	1782
November	1st to 10th	Day	17% 9% 3 7% 5% 3 5% 5% 6% 8% 33%	1671
	11th to 20th	Day	18% 9% 4 8% 6% 4 5% 5% 6% 7% 29%	1615
	21st to end	Day	21% 9% 3 8% 7% 5% 6% 5% 5% 7% 23%	1520
December	1st to 10th	Day	24% 15% 4 8% 6% 3 5% 5% 5% 6% 18%	1520
	11th to 20th	Day	21% 12% 5% 8% 7% 3 4 5% 5% 5% 25%	1906
	21st to end	Day	28% 16% 5% 9% 6% 2 4 3 3 4 20%	2772
January	1st to 10th	Night	17% 16% 6% 10% 7% 4 5% 5% 5% 6% 21%	4429
	11th to 20th	Night	13% 13% 6% 11% 10% 5% 6% 5% 6% 6% 20%	4440
	21st to end	Night	17% 17% 6% 9% 8% 4 4 3 3 5% 24%	4884
February	1st to 10th	Night	14% 14% 6% 10% 8% 4 6% 5% 5% 6% 21%	2481
	11th to 20th	Night	15% 10% 5% 8% 7% 4 6% 6% 5% 6% 28%	2323
	21st to end	Night	15% 13% 6% 9% 6% 3 4 4 4 5% 32%	1831
March	1st to 10th	Night	17% 12% 5% 9% 7% 3 5% 4 5% 5% 28%	2150
	11th to 20th	Night	18% 13% 4 6% 6% 3 5% 4 5% 5% 33%	2068
	21st to end	Night	18% 11% 4 6% 4 4 4 3 4 5% 38%	1998
April	1st to 10th	Night	24% 8% 3 6% 6% 3 5% 3 5% 6% 31%	1800
	11th to 20th	Night	22% 7% 4 5% 5% 2 4 4 5% 6% 35%	1775
	21st to end	Night	21% 9% 3 7% 6% 3 3 4 4 6% 33%	1678
May	1st to 10th	Night	21% 8% 3 7% 5% 3 4 4 5% 5% 35%	1471
	11th to 20th	Night	18% 7% 3 6% 6% 2 5% 3 5% 6% 39%	1415
	21st to end	Night	16% 5% 3 5% 4 4 4 5% 4 9% 41%	1550
June	1st to 10th	Night	20% 7% 3 6% 5% 3 6% 6% 5% 9% 30%	1440
	11th to 20th	Night	17% 6% 3 5% 4 2 5% 5% 6% 7% 40%	1440
	21st to end	Night	16% 7% 3 6% 6% 3 5% 5% 6% 8% 36%	1440
July	1st to 10th	Night	10% 4 2 5% 5% 3 4 7% 5% 8% 44%	1440
	11th to 20th	Night	11% 5% 3 5% 6% 3 5% 6% 6% 10% 40%	1440
	21st to end	Night	11% 7% 3 5% 4 2 5% 6% 6% 8% 42%	1574
August	1st to 10th	Night	12% 5% 2 5% 4 3 4 5% 7% 8% 44%	1432
	11th to 20th	Night	12% 5% 3 5% 6% 3 4 5% 5% 6% 45%	1608
	21st to end	Night	13% 5% 2 6% 5% 3 4 5% 5% 8% 43%	1980
September	1st to 10th	Night	12% 6% 3 6% 6% 3 5% 5% 6% 7% 43%	1800
	11th to 20th	Night	16% 6% 2 4 4 2 2 3 5% 7% 49%	1858
	21st to end	Night	16% 5% 2 5% 4 3 5% 4 5% 8% 42%	1980
October	1st to 10th	Night	18% 7% 3 6% 6% 3 4 5% 5% 6% 36%	2124
	11th to 20th	Night	20% 8% 3 7% 6% 3 5% 5% 5% 6% 33%	2178
	21st to end	Night	23% 8% 3 8% 7% 4 4 4 5% 5% 29%	2574
November	1st to 10th	Night	19% 9% 4 6% 6% 4 4 4 4 7% 33%	2407
	11th to 20th	Night	18% 10% 4 7% 5% 3 5% 5% 5% 6% 32%	2563
	21st to end	Night	22% 12% 4 8% 7% 4 5% 4 5% 6% 26%	2660
December	1st to 10th	Night	22% 14% 6% 9% 7% 3 6% 4 5% 5% 20%	2660
	11th to 20th	Night	23% 14% 5% 8% 5% 3 4 4 4 4 25%	3288
	21st to end	Night	30% 16% 4 8% 5% 2 3 3 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	63% 3 15% 13% 5% 1	2518
	11th to 20th	Day	58% 5% 13% 16% 8% 1	2520
	21st to end	Day	61% 5% 16% 13% 5%	2772
February	1st to 10th	Day	60% 4 10% 19% 5% 2	1695
	11th to 20th	Day	56% 4 13% 17% 9% 1	1752
	21st to end	Day	61% 3 9% 16% 9% 2	1520
March	1st to 10th	Day	57% 3 10% 18% 10% 2	1800
	11th to 20th	Day	53% 3 13% 16% 13% 3	1890
	21st to end	Day	50% 3 10% 19% 14% 4	2356
April	1st to 10th	Day	53% 5% 7% 21% 12% 2	2160
	11th to 20th	Day	48% 3 8% 23% 16% 2	2134
	21st to end	Day	54% 3 9% 18% 13% 4	2281
May	1st to 10th	Day	49% 5% 8% 21% 15% 2	2505
	11th to 20th	Day	45% 4 10% 22% 16% 3	2482
	21st to end	Day	45% 5% 13% 21% 14% 2	2725
June	1st to 10th	Day	51% 5% 11% 20% 12% 1	2520
	11th to 20th	Day	43% 5% 11% 23% 14% 3	2520
	21st to end	Day	50% 5% 14% 19% 10% 1	2520
July	1st to 10th	Day	37% 6% 15% 28% 14% 1	2450
	11th to 20th	Day	39% 5% 17% 28% 10%	2445
	21st to end	Day	40% 6% 17% 27% 9%	2591
August	1st to 10th	Day	41% 6% 18% 27% 9%	2492
	11th to 20th	Day	37% 6% 19% 27% 10% 1	2270
	21st to end	Day	37% 6% 15% 28% 14% 1	2376
September	1st to 10th	Day	38% 6% 15% 28% 13%	2160
	11th to 20th	Day	36% 4 12% 32% 15% 1	2102
	21st to end	Day	42% 5% 14% 28% 10% 1	1980
October	1st to 10th	Day	49% 5% 11% 23% 11%	1834
	11th to 20th	Day	55% 4 14% 19% 7%	1782
	21st to end	Day	56% 5% 13% 18% 8% 1	1782
November	1st to 10th	Day	48% 5% 16% 22% 8% 1	1671
	11th to 20th	Day	55% 5% 12% 19% 10%	1615
	21st to end	Day	60% 6% 13% 16% 5% 1	1520
December	1st to 10th	Day	68% 5% 11% 13% 4	1520
	11th to 20th	Day	61% 6% 12% 16% 5%	1906
	21st to end	Day	70% 3 10% 12% 5%	2772
January	1st to 10th	Night	64% 3 15% 13% 4	4429
	11th to 20th	Night	61% 5% 13% 16% 4 1	4440
	21st to end	Night	64% 4 13% 12% 6% 1	4884
February	1st to 10th	Night	62% 6% 9% 17% 6% 1	2481
	11th to 20th	Night	53% 5% 12% 21% 8% 1	2325
	21st to end	Night	56% 3 9% 18% 14% 1	1831
March	1st to 10th	Night	58% 4 9% 17% 11% 2	2160
	11th to 20th	Night	54% 4 10% 17% 13% 3	2069
	21st to end	Night	50% 3 6% 21% 15% 4	1998
April	1st to 10th	Night	54% 4 8% 19% 14% 2	1800
	11th to 20th	Night	51% 4 7% 21% 15% 2	1775
	21st to end	Night	52% 4 7% 19% 13% 4	1678
May	1st to 10th	Night	50% 4 6% 18% 16% 5%	1471
	11th to 20th	Night	47% 4 7% 19% 19% 4	1416
	21st to end	Night	43% 4 13% 21% 17% 2	1551
June	1st to 10th	Night	51% 5% 10% 21% 11% 1	1440
	11th to 20th	Night	43% 5% 12% 23% 16% 2	1440
	21st to end	Night	46% 6% 14% 20% 14%	1440
July	1st to 10th	Night	36% 6% 13% 27% 18% 1	1400
	11th to 20th	Night	37% 6% 14% 30% 12%	1392
	21st to end	Night	37% 6% 17% 27% 12%	1486
August	1st to 10th	Night	36% 6% 19% 27% 11%	1422
	11th to 20th	Night	39% 5% 17% 25% 13%	1608
	21st to end	Night	38% 5% 16% 28% 12% 1	1980
September	1st to 10th	Night	40% 6% 11% 27% 16% 1	1800
	11th to 20th	Night	37% 3 13% 34% 12% 1	1858
	21st to end	Night	41% 6% 11% 28% 14%	1980
October	1st to 10th	Night	47% 6% 9% 24% 12% 1	2124
	11th to 20th	Night	52% 4 11% 24% 9%	2178
	21st to end	Night	57% 4 13% 17% 8% 1	2574
November	1st to 10th	Night	52% 4 12% 22% 9% 1	2407
	11th to 20th	Night	53% 5% 13% 18% 10% 1	2563
	21st to end	Night	61% 5% 11% 16% 6%	2660
December	1st to 10th	Night	66% 5% 11% 15% 4	2660
	11th to 20th	Night	63% 5% 8% 17% 6%	3288
	21st to end	Night	68% 4 9% 15% 4	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% 3	362
March	1st to 10th	Day	23% 10% 35% 25% 8%	420
	11th to 20th	Day	20% 12% 35% 24% 7% 2	420
	21st to end	Day	25% 9% 29% 20% 14% 2	462
April	1st to 10th	Day	27% 12% 26% 24% 11% 2	420
	11th to 20th	Day	25% 12% 26% 22% 12% 3	412
	21st to end	Day	27% 11% 27% 23% 10% 2	420
May	1st to 10th	Day	21% 9% 30% 26% 11% 3	423
	11th to 20th	Day	19% 6% 32% 25% 14% 4	447
	21st to end	Day	18% 8% 30% 26% 13% 5%	485
June	1st to 10th	Day	22% 8% 24% 29% 13% 4	450
	11th to 20th	Day	17% 7% 24% 31% 18% 3	450
	21st to end	Day	21% 6% 25% 27% 16% 5%	450
July	1st to 10th	Day	11% 7% 23% 31% 21% 7%	445
	11th to 20th	Day	13% 4 26% 31% 19% 6%	444
	21st to end	Day	13% 6% 21% 35% 19% 5%	488
August	1st to 10th	Day	15% 4 21% 30% 22% 8%	446
	11th to 20th	Day	11% 4 21% 33% 21% 10%	439
	21st to end	Day	14% 7% 25% 24% 21% 9%	495
September	1st to 10th	Day	15% 6% 23% 26% 19% 10%	450
	11th to 20th	Day	14% 7% 23% 25% 20% 12%	450
	21st to end	Day	18% 8% 25% 23% 20% 7%	450
October	1st to 10th	Day	22% 8% 25% 25% 14% 6%	450
	11th to 20th	Day	20% 12% 30% 22% 14% 2	441
	21st to end	Day	30% 12% 24% 24% 9%	396
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%	611
March	1st to 10th	Night	23% 11% 31% 29% 6%	720
	11th to 20th	Night	25% 10% 31% 28% 7% 1	720
	21st to end	Night	23% 7% 28% 32% 10% 1	792
April	1st to 10th	Night	27% 10% 26% 31% 6%	720
	11th to 20th	Night	24% 10% 29% 29% 7% 1	711
	21st to end	Night	27% 9% 27% 27% 10%	598
May	1st to 10th	Night	24% 6% 29% 30% 10% 2	391
	11th to 20th	Night	21% 6% 28% 34% 10% 1	354
	21st to end	Night	16% 5% 31% 38% 11%	389
June	1st to 10th	Night	26% 3 27% 33% 11% 1	360
	11th to 20th	Night	18% 5% 26% 35% 16% 1	360
	21st to end	Night	18% 5% 26% 33% 18% 1	360
July	1st to 10th	Night	12% 4 23% 40% 20% 1	350
	11th to 20th	Night	11% 3 24% 44% 17% 1	348
	21st to end	Night	13% 4 22% 43% 17% 2	394
August	1st to 10th	Night	15% 3 15% 45% 22% 1	358
	11th to 20th	Night	14% 2 20% 36% 24% 3	544
	21st to end	Night	15% 3 22% 34% 23% 3	792
September	1st to 10th	Night	14% 4 22% 40% 19% 1	720
	11th to 20th	Night	19% 5% 18% 35% 21% 2	720
	21st to end	Night	19% 6% 23% 30% 19% 3	720
October	1st to 10th	Night	21% 7% 27% 31% 14% 1	720
	11th to 20th	Night	23% 8% 28% 29% 11% 1	720
	21st to end	Night	27% 9% 30% 28% 6%	792
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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