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 2024-01-31.

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% 11% 5% 8% 6% 3 5% 5% 5% 7% 30%	74746
	Night	16% 12% 5% 8% 6% 3 5% 4 5% 6% 30%	75382
Transparency	Day	53% 7% 11% 18% 10% 1	74466
	Night	55% 6% 11% 17% 10% 1	75181
Seeing	Day	21% 17% 29% 20% 10% 3	13494
	Night	22% 15% 27% 24% 10% 1	23327

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	16% 20% 9% 12% 8% 3 4 4 4 4 16%	1706
	11th to 20th	Day	17% 21% 8% 13% 8% 3 4 4 4 5% 14%	1710
	21st to end	Day	16% 19% 11% 17% 9% 4 4 4 3 4 11%	1872
February	1st to 10th	Day	17% 20% 7% 12% 10% 4 3 3 3 4 17%	1620
	11th to 20th	Day	15% 16% 8% 13% 9% 4 5% 4 3 5% 17%	1582
	21st to end	Day	17% 18% 4 14% 5% 3 4 5% 4 5% 21%	1480
March	1st to 10th	Day	17% 13% 7% 9% 6% 3 4 4 6% 6% 26%	1952
	11th to 20th	Day	16% 14% 6% 9% 6% 3 3 3 4 5% 30%	1980
	21st to end	Day	21% 13% 5% 7% 5% 3 4 3 4 5% 29%	2178
April	1st to 10th	Day	15% 13% 5% 6% 5% 3 4 4 5% 7% 32%	2043
	11th to 20th	Day	19% 12% 3 6% 5% 3 4 4 4 5% 33%	2249
	21st to end	Day	21% 11% 4 6% 6% 3 3 3 4 5% 34%	2340
May	1st to 10th	Day	17% 10% 4 7% 6% 4 5% 6% 5% 6% 31%	2340
	11th to 20th	Day	19% 8% 3 6% 5% 3 5% 5% 5% 7% 34%	2301
	21st to end	Day	11% 6% 3 5% 6% 4 5% 6% 6% 8% 39%	2658
June	1st to 10th	Day	15% 7% 3 4 6% 3 6% 6% 6% 8% 36%	2520
	11th to 20th	Day	13% 7% 3 6% 6% 4 4 5% 7% 8% 38%	2520
	21st to end	Day	13% 5% 2 5% 5% 3 5% 6% 7% 9% 40%	2520
July	1st to 10th	Day	9% 6% 2 6% 6% 4 5% 5% 5% 9% 44%	2441
	11th to 20th	Day	12% 6% 3 6% 4 3 5% 6% 6% 9% 40%	2340
	21st to end	Day	10% 5% 2 5% 6% 4 6% 6% 7% 9% 39%	2547
August	1st to 10th	Day	7% 6% 4 6% 5% 4 6% 6% 7% 9% 40%	2327
	11th to 20th	Day	10% 6% 3 7% 7% 4 6% 6% 8% 9% 34%	2303
	21st to end	Day	11% 6% 3 6% 5% 4 5% 6% 6% 10% 38%	2513
September	1st to 10th	Day	16% 5% 4 6% 5% 3 5% 5% 6% 7% 38%	2112
	11th to 20th	Day	12% 6% 2 5% 6% 4 4 5% 6% 8% 41%	1980
	21st to end	Day	18% 8% 3 5% 5% 3 4 4 6% 7% 37%	1980
October	1st to 10th	Day	16% 7% 3 7% 6% 3 4 4 7% 7% 35%	1832
	11th to 20th	Day	22% 9% 3 8% 6% 3 5% 4 4 6% 30%	1800
	21st to end	Day	25% 11% 3 8% 6% 4 5% 4 4 6% 24%	1980
November	1st to 10th	Day	12% 12% 4 8% 5% 4 5% 5% 5% 7% 34%	1692
	11th to 20th	Day	17% 17% 5% 9% 6% 3 5% 4 4 6% 24%	1606
	21st to end	Day	17% 21% 7% 8% 7% 3 5% 4 5% 6% 17%	1520
December	1st to 10th	Day	16% 22% 11% 11% 7% 4 5% 4 3 4 14%	1520
	11th to 20th	Day	17% 20% 8% 14% 6% 3 4 5% 4 6% 14%	1910
	21st to end	Day	24% 26% 8% 10% 7% 4 3 3 3 4 10%	2772
January	1st to 10th	Night	17% 18% 9% 15% 7% 3 4 4 3 4 15%	2459
	11th to 20th	Night	18% 18% 9% 13% 7% 3 4 3 4 3 16%	2470
	21st to end	Night	16% 19% 9% 11% 10% 4 5% 4 3 4 15%	2711
February	1st to 10th	Night	19% 16% 6% 12% 9% 3 5% 4 4 5% 17%	2340
	11th to 20th	Night	16% 15% 7% 12% 8% 4 5% 5% 4 4 19%	2261
	21st to end	Night	20% 12% 7% 11% 6% 4 4 4 4 5% 23%	1776
March	1st to 10th	Night	15% 12% 6% 9% 7% 3 5% 5% 5% 5% 28%	2007
	11th to 20th	Night	17% 11% 5% 8% 6% 3 4 5% 4 6% 33%	1980
	21st to end	Night	17% 12% 5% 9% 6% 3 4 4 5% 4 30%	2178
April	1st to 10th	Night	17% 13% 5% 7% 6% 4 4 4 4 5% 32%	1916
	11th to 20th	Night	16% 11% 4 6% 5% 2 5% 5% 5% 8% 34%	1674
	21st to end	Night	15% 9% 3 7% 7% 3 5% 4 5% 6% 35%	1620
May	1st to 10th	Night	18% 9% 3 5% 5% 4 6% 4 5% 7% 33%	1620
	11th to 20th	Night	13% 9% 4 7% 6% 3 4 5% 5% 9% 36%	1592
	21st to end	Night	11% 5% 2 5% 5% 3 5% 5% 6% 10% 44%	1616
June	1st to 10th	Night	9% 6% 3 7% 6% 4 5% 6% 6% 7% 42%	1440
	11th to 20th	Night	9% 6% 3 6% 6% 4 4 5% 6% 7% 43%	1440
	21st to end	Night	10% 5% 3 6% 5% 3 5% 5% 6% 10% 43%	1440
July	1st to 10th	Night	6% 4 3 5% 5% 3 5% 4 5% 8% 51%	1514
	11th to 20th	Night	8% 5% 2 4 5% 3 4 5% 7% 10% 46%	1620
	21st to end	Night	8% 4 2 5% 6% 4 6% 5% 7% 9% 44%	1771
August	1st to 10th	Night	7% 4 2 6% 7% 3 4 6% 6% 8% 47%	1611
	11th to 20th	Night	9% 5% 3 6% 5% 4 6% 6% 5% 9% 44%	1598
	21st to end	Night	11% 5% 2 4 5% 3 5% 5% 6% 9% 45%	1824
September	1st to 10th	Night	14% 5% 2 5% 4 3 4 4 6% 8% 45%	1844
	11th to 20th	Night	12% 5% 2 4 4 3 3 4 4 7% 51%	1980
	21st to end	Night	15% 7% 2 6% 6% 3 6% 5% 5% 6% 39%	1980
October	1st to 10th	Night	15% 7% 3 4 5% 3 4 4 6% 8% 41%	2126
	11th to 20th	Night	20% 7% 3 6% 6% 3 5% 5% 5% 8% 32%	2160
	21st to end	Night	26% 10% 3 6% 5% 4 4 4 5% 6% 27%	2376
November	1st to 10th	Night	14% 10% 3 7% 5% 4 5% 5% 5% 7% 35%	2383
	11th to 20th	Night	16% 15% 5% 9% 6% 4 4 4 6% 6% 25%	2570
	21st to end	Night	20% 17% 5% 8% 6% 3 5% 4 5% 6% 21%	2660
December	1st to 10th	Night	20% 20% 9% 9% 7% 3 5% 4 4 4 16%	2660
	11th to 20th	Night	16% 21% 7% 12% 7% 3 4 4 4 5% 18%	3285
	21st to end	Night	23% 23% 8% 10% 7% 3 5% 3 4 5% 10%	4880

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	73% 5% 10% 9% 4	1707
	11th to 20th	Day	74% 4 11% 9% 2	1710
	21st to end	Day	79% 4 7% 8% 3	1872
February	1st to 10th	Day	73% 5% 8% 10% 4	1620
	11th to 20th	Day	71% 5% 8% 10% 6%	1591
	21st to end	Day	65% 6% 10% 11% 6% 2	1480
March	1st to 10th	Day	58% 5% 13% 18% 5%	1952
	11th to 20th	Day	58% 5% 10% 15% 10% 2	1980
	21st to end	Day	59% 4 8% 18% 9% 2	2178
April	1st to 10th	Day	52% 5% 12% 17% 13% 1	2043
	11th to 20th	Day	53% 5% 10% 17% 13% 2	2249
	21st to end	Day	53% 5% 10% 20% 10% 3	2340
May	1st to 10th	Day	53% 8% 8% 20% 10% 2	2340
	11th to 20th	Day	49% 7% 11% 17% 14% 2	2305
	21st to end	Day	40% 9% 13% 23% 12% 2	2658
June	1st to 10th	Day	44% 7% 11% 25% 13% 1	2520
	11th to 20th	Day	42% 9% 13% 21% 13% 2	2520
	21st to end	Day	39% 7% 11% 25% 16% 1	2520
July	1st to 10th	Day	38% 8% 10% 25% 16% 2	2375
	11th to 20th	Day	39% 8% 12% 25% 15% 1	2271
	21st to end	Day	37% 9% 13% 23% 16% 2	2405
August	1st to 10th	Day	36% 10% 14% 26% 14%	2314
	11th to 20th	Day	43% 11% 13% 21% 11% 2	2303
	21st to end	Day	39% 9% 13% 21% 15% 2	2513
September	1st to 10th	Day	42% 8% 13% 20% 13% 3	2112
	11th to 20th	Day	39% 8% 12% 24% 15% 1	1980
	21st to end	Day	47% 6% 11% 23% 13% 1	1980
October	1st to 10th	Day	46% 7% 13% 20% 13% 1	1832
	11th to 20th	Day	55% 6% 12% 18% 9% 1	1800
	21st to end	Day	62% 4 13% 13% 8% 1	1980
November	1st to 10th	Day	50% 6% 13% 18% 12% 1	1692
	11th to 20th	Day	61% 5% 11% 13% 7% 3	1606
	21st to end	Day	68% 5% 10% 12% 5%	1520
December	1st to 10th	Day	75% 5% 8% 10% 3	1520
	11th to 20th	Day	72% 6% 9% 10% 2	1906
	21st to end	Day	80% 4 8% 7% 1	2772
January	1st to 10th	Night	73% 4 9% 10% 4	2459
	11th to 20th	Night	74% 4 10% 10% 2 1	2470
	21st to end	Night	75% 4 9% 9% 3 1	2711
February	1st to 10th	Night	71% 5% 10% 11% 3	2340
	11th to 20th	Night	68% 5% 8% 14% 5%	2263
	21st to end	Night	65% 5% 8% 13% 7% 1	1776
March	1st to 10th	Night	56% 6% 11% 19% 8%	2007
	11th to 20th	Night	54% 5% 10% 19% 11% 2	1980
	21st to end	Night	57% 4 7% 17% 13% 2	2178
April	1st to 10th	Night	54% 6% 10% 17% 11% 2	1916
	11th to 20th	Night	49% 6% 11% 19% 13% 3	1674
	21st to end	Night	48% 6% 11% 20% 12% 3	1620
May	1st to 10th	Night	50% 6% 12% 17% 12% 3	1620
	11th to 20th	Night	47% 5% 11% 19% 15% 4	1593
	21st to end	Night	35% 7% 16% 25% 15% 1	1617
June	1st to 10th	Night	40% 7% 11% 25% 15% 2	1440
	11th to 20th	Night	39% 7% 12% 21% 18% 3	1440
	21st to end	Night	36% 8% 13% 24% 18% 1	1440
July	1st to 10th	Night	33% 6% 11% 24% 20% 4	1470
	11th to 20th	Night	31% 8% 14% 24% 20% 3	1566
	21st to end	Night	35% 9% 13% 23% 18% 3	1672
August	1st to 10th	Night	32% 8% 13% 30% 16% 1	1600
	11th to 20th	Night	37% 7% 15% 27% 13% 1	1598
	21st to end	Night	34% 7% 13% 25% 17% 3	1824
September	1st to 10th	Night	37% 8% 12% 24% 16% 3	1844
	11th to 20th	Night	33% 7% 14% 24% 20% 2	1980
	21st to end	Night	45% 6% 11% 21% 16% 1	1980
October	1st to 10th	Night	41% 7% 13% 20% 17% 1	2126
	11th to 20th	Night	51% 6% 12% 20% 11% 1	2160
	21st to end	Night	58% 6% 11% 16% 9% 1	2376
November	1st to 10th	Night	48% 6% 13% 19% 11% 3	2383
	11th to 20th	Night	60% 5% 11% 15% 6% 3	2570
	21st to end	Night	65% 5% 11% 14% 4	2660
December	1st to 10th	Night	73% 5% 10% 8% 4	2660
	11th to 20th	Night	70% 5% 12% 11% 2	3288
	21st to end	Night	77% 6% 10% 6% 1	4880

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% 29% 28% 9% 3	270
	11th to 20th	Day	33% 31% 25% 8% 3	270
	21st to end	Day	27% 34% 30% 6% 2	294
February	1st to 10th	Day	35% 26% 33% 6%	240
	11th to 20th	Day	32% 35% 28% 5%	240
	21st to end	Day	33% 34% 29% 5%	198
March	1st to 10th	Day	28% 26% 31% 13% 3	392
	11th to 20th	Day	25% 25% 32% 15% 3 1	420
	21st to end	Day	33% 22% 26% 14% 4	462
April	1st to 10th	Day	26% 26% 26% 15% 6% 1	420
	11th to 20th	Day	29% 19% 26% 15% 8% 2	412
	21st to end	Day	28% 18% 33% 16% 4 1	420
May	1st to 10th	Day	26% 20% 28% 18% 5% 2	423
	11th to 20th	Day	22% 13% 31% 23% 8% 2	447
	21st to end	Day	14% 10% 31% 26% 15% 5%	485
June	1st to 10th	Day	16% 10% 37% 25% 9% 2	450
	11th to 20th	Day	18% 12% 28% 26% 12% 2	450
	21st to end	Day	13% 11% 34% 28% 10% 3	450
July	1st to 10th	Day	9% 8% 22% 31% 21% 8%	445
	11th to 20th	Day	14% 9% 27% 28% 16% 6%	444
	21st to end	Day	10% 8% 32% 30% 15% 6%	488
August	1st to 10th	Day	8% 8% 26% 29% 22% 7%	446
	11th to 20th	Day	13% 8% 25% 29% 15% 8%	439
	21st to end	Day	14% 7% 25% 28% 19% 7%	495
September	1st to 10th	Day	16% 9% 24% 26% 19% 6%	450
	11th to 20th	Day	11% 6% 24% 26% 23% 10%	450
	21st to end	Day	20% 13% 29% 21% 12% 6%	450
October	1st to 10th	Day	16% 13% 29% 22% 18% 3	450
	11th to 20th	Day	26% 21% 33% 15% 5% 1	450
	21st to end	Day	36% 17% 29% 16% 2	495
November	1st to 10th	Day	21% 26% 27% 17% 8% 1	290
	11th to 20th	Day	32% 26% 25% 12% 5%	221
	21st to end	Day	26% 28% 37% 9%	180
December	1st to 10th	Day	25% 34% 28% 9% 3 1	180
	11th to 20th	Day	24% 32% 31% 11% 1	180
	21st to end	Day	33% 29% 28% 8% 2	198
January	1st to 10th	Night	28% 26% 32% 12% 3	760
	11th to 20th	Night	34% 21% 29% 14% 2	760
	21st to end	Night	26% 23% 32% 16% 3	835
February	1st to 10th	Night	30% 22% 29% 17% 2	720
	11th to 20th	Night	28% 23% 32% 15% 2	703
	21st to end	Night	31% 23% 32% 14% 1	592
March	1st to 10th	Night	23% 21% 33% 20% 3	720
	11th to 20th	Night	25% 20% 29% 20% 4 1	720
	21st to end	Night	24% 18% 30% 23% 6%	792
April	1st to 10th	Night	25% 15% 31% 23% 5%	720
	11th to 20th	Night	23% 15% 32% 22% 7% 1	610
	21st to end	Night	18% 14% 34% 28% 6% 1	540
May	1st to 10th	Night	21% 12% 27% 29% 11%	540
	11th to 20th	Night	18% 9% 28% 34% 10% 2	531
	21st to end	Night	12% 5% 28% 33% 21% 2	456
June	1st to 10th	Night	12% 4 33% 36% 15% 1	360
	11th to 20th	Night	11% 6% 27% 39% 16% 1	360
	21st to end	Night	10% 9% 24% 41% 16%	360
July	1st to 10th	Night	6% 5% 20% 41% 24% 5%	420
	11th to 20th	Night	9% 5% 21% 37% 24% 4	522
	21st to end	Night	10% 3 22% 40% 24% 2	591
August	1st to 10th	Night	9% 4 17% 40% 26% 4	537
	11th to 20th	Night	11% 4 15% 37% 29% 4	534
	21st to end	Night	12% 6% 20% 31% 27% 3	636
September	1st to 10th	Night	15% 7% 17% 33% 24% 4	720
	11th to 20th	Night	14% 4 18% 36% 24% 5%	720
	21st to end	Night	18% 10% 23% 28% 17% 4	720
October	1st to 10th	Night	19% 10% 26% 25% 18% 3	720
	11th to 20th	Night	25% 17% 27% 24% 6% 1	720
	21st to end	Night	32% 17% 28% 20% 3	792
November	1st to 10th	Night	22% 16% 29% 24% 8% 1	740
	11th to 20th	Night	27% 22% 29% 16% 5% 1	760
	21st to end	Night	32% 22% 28% 15% 4	760
December	1st to 10th	Night	33% 23% 28% 13% 4	760
	11th to 20th	Night	30% 26% 29% 14% 2	760
	21st to end	Night	31% 24% 32% 12% 2	836

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