SkyShed Observatories
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the SkyShed Observatories clear sky chart from 2005-11-06 to 2024-01-15.

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	15% 10% 5% 8% 6% 3 5% 5% 5% 7% 30%	74833
	Night	16% 12% 5% 9% 6% 3 5% 4 5% 6% 29%	74901
Transparency	Day	52% 7% 12% 18% 9% 1	74544
	Night	56% 6% 11% 17% 9% 1	74726
Seeing	Day	19% 12% 28% 24% 13% 5%	13939
	Night	20% 11% 29% 28% 11% 1	23332

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% 17% 9% 14% 10% 4 5% 4 4 6% 13%	1423
	11th to 20th	Day	15% 18% 11% 17% 8% 5% 5% 3 3 5% 11%	1617
	21st to end	Day	13% 18% 9% 15% 11% 5% 5% 4 3 4 12%	1782
February	1st to 10th	Day	16% 17% 9% 14% 13% 5% 5% 3 3 3 12%	1620
	11th to 20th	Day	13% 12% 9% 13% 12% 5% 4 5% 4 5% 17%	1573
	21st to end	Day	16% 16% 8% 10% 8% 3 5% 4 5% 5% 21%	1474
March	1st to 10th	Day	15% 12% 6% 10% 8% 5% 5% 4 5% 6% 26%	1918
	11th to 20th	Day	12% 13% 7% 11% 6% 3 6% 4 4 6% 29%	1980
	21st to end	Day	17% 12% 6% 8% 7% 3 3 4 5% 6% 29%	2178
April	1st to 10th	Day	17% 16% 6% 6% 5% 3 4 5% 5% 7% 27%	2047
	11th to 20th	Day	18% 12% 3 5% 4 3 4 4 5% 8% 35%	2309
	21st to end	Day	20% 10% 4 6% 5% 3 4 4 5% 7% 32%	2340
May	1st to 10th	Day	17% 10% 3 6% 6% 3 4 5% 6% 8% 34%	2340
	11th to 20th	Day	18% 8% 3 6% 4 3 4 4 5% 7% 38%	2319
	21st to end	Day	13% 5% 2 4 5% 4 6% 5% 5% 9% 41%	2725
June	1st to 10th	Day	13% 6% 3 6% 6% 3 6% 4 6% 8% 39%	2558
	11th to 20th	Day	13% 5% 3 6% 5% 3 5% 5% 6% 9% 42%	2700
	21st to end	Day	15% 6% 2 5% 4 3 5% 5% 6% 8% 40%	2700
July	1st to 10th	Day	9% 4 2 5% 4 2 5% 5% 6% 9% 49%	2700
	11th to 20th	Day	11% 5% 3 5% 5% 3 5% 6% 7% 9% 40%	2476
	21st to end	Day	10% 5% 3 5% 6% 4 4 6% 6% 9% 42%	2547
August	1st to 10th	Day	9% 5% 2 6% 5% 3 6% 6% 9% 10% 38%	2327
	11th to 20th	Day	11% 4 3 5% 6% 5% 6% 6% 7% 10% 37%	2303
	21st to end	Day	13% 6% 3 5% 5% 3 5% 5% 6% 9% 41%	2552
September	1st to 10th	Day	12% 6% 3 6% 6% 3 5% 6% 6% 9% 38%	2111
	11th to 20th	Day	12% 5% 2 5% 4 4 5% 6% 7% 7% 41%	1980
	21st to end	Day	18% 7% 3 6% 5% 4 5% 5% 6% 7% 34%	1980
October	1st to 10th	Day	18% 7% 3 7% 6% 3 5% 5% 5% 6% 34%	1838
	11th to 20th	Day	22% 12% 5% 7% 7% 4 4 4 5% 6% 22%	1800
	21st to end	Day	27% 13% 4 7% 6% 4 5% 5% 5% 7% 17%	1931
November	1st to 10th	Day	16% 16% 4 7% 6% 3 4 4 5% 8% 27%	1672
	11th to 20th	Day	15% 17% 5% 10% 7% 4 6% 5% 5% 6% 21%	1543
	21st to end	Day	18% 19% 9% 9% 8% 4 5% 5% 4 5% 13%	1520
December	1st to 10th	Day	14% 23% 11% 14% 9% 4 4 3 4 4 10%	1520
	11th to 20th	Day	19% 17% 9% 15% 9% 4 5% 4 5% 3 11%	1906
	21st to end	Day	27% 20% 9% 10% 6% 3 4 4 4 5% 8%	2524
January	1st to 10th	Night	17% 17% 7% 14% 12% 5% 5% 4 3 4 12%	2739
	11th to 20th	Night	18% 20% 8% 14% 10% 4 5% 4 3 4 12%	2426
	21st to end	Night	13% 16% 10% 14% 11% 4 5% 5% 4 4 14%	2574
February	1st to 10th	Night	16% 18% 7% 12% 12% 4 5% 4 4 4 13%	2340
	11th to 20th	Night	14% 14% 7% 13% 9% 4 5% 5% 5% 5% 17%	2273
	21st to end	Night	18% 12% 7% 13% 8% 4 5% 5% 4 4 21%	1780
March	1st to 10th	Night	15% 10% 6% 10% 7% 4 6% 4 5% 6% 29%	2029
	11th to 20th	Night	13% 12% 5% 8% 6% 4 5% 4 5% 7% 31%	1979
	21st to end	Night	17% 11% 5% 8% 6% 3 4 4 4 5% 32%	2178
April	1st to 10th	Night	20% 11% 5% 5% 6% 3 4 5% 5% 6% 31%	1913
	11th to 20th	Night	17% 8% 4 6% 5% 3 5% 4 4 8% 36%	1615
	21st to end	Night	16% 11% 4 6% 6% 3 4 4 5% 7% 34%	1620
May	1st to 10th	Night	17% 8% 3 5% 5% 3 4 5% 5% 8% 36%	1620
	11th to 20th	Night	15% 7% 3 6% 5% 3 4 4 6% 8% 39%	1571
	21st to end	Night	10% 4 2 4 5% 3 4 5% 7% 7% 49%	1550
June	1st to 10th	Night	13% 5% 3 5% 5% 4 4 5% 5% 7% 43%	1397
	11th to 20th	Night	10% 6% 2 6% 5% 3 5% 5% 7% 8% 42%	1260
	21st to end	Night	12% 5% 2 5% 5% 3 5% 4 4 9% 46%	1260
July	1st to 10th	Night	6% 4 2 4 4 2 5% 5% 6% 8% 55%	1260
	11th to 20th	Night	9% 4 2 4 4 2 4 5% 5% 11% 49%	1478
	21st to end	Night	9% 5% 2 5% 6% 3 4 6% 6% 8% 46%	1771
August	1st to 10th	Night	8% 4 1 6% 6% 2 4 6% 6% 9% 47%	1611
	11th to 20th	Night	8% 5% 2 5% 5% 3 4 6% 6% 9% 48%	1598
	21st to end	Night	11% 5% 2 4 5% 4 4 4 7% 8% 46%	1785
September	1st to 10th	Night	12% 4 2 5% 5% 4 5% 6% 6% 6% 45%	1845
	11th to 20th	Night	12% 5% 2 4 4 2 3 5% 5% 8% 50%	1980
	21st to end	Night	16% 7% 2 5% 6% 3 4 4 6% 7% 40%	1980
October	1st to 10th	Night	15% 8% 4 7% 6% 2 4 5% 5% 6% 37%	2120
	11th to 20th	Night	23% 11% 4 7% 5% 3 4 4 5% 6% 28%	2160
	21st to end	Night	26% 10% 4 8% 7% 4 5% 5% 4 5% 22%	2422
November	1st to 10th	Night	15% 13% 4 8% 5% 3 4 4 6% 7% 31%	2406
	11th to 20th	Night	17% 16% 6% 10% 6% 3 6% 5% 4 7% 20%	2636
	21st to end	Night	20% 19% 6% 10% 7% 4 5% 3 4 4 17%	2660
December	1st to 10th	Night	18% 23% 12% 12% 6% 3 5% 3 4 4 11%	2660
	11th to 20th	Night	14% 21% 9% 14% 7% 3 4 4 4 5% 14%	3288
	21st to end	Night	25% 23% 9% 11% 6% 3 3 3 4 4 8%	5117

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	74% 6% 9% 9% 2	1424
	11th to 20th	Day	78% 6% 7% 5% 4 1	1617
	21st to end	Day	75% 5% 8% 8% 4 1	1782
February	1st to 10th	Day	78% 5% 7% 9% 2	1620
	11th to 20th	Day	69% 6% 8% 12% 4	1582
	21st to end	Day	65% 6% 11% 13% 6% 1	1474
March	1st to 10th	Day	59% 6% 14% 14% 7% 1	1921
	11th to 20th	Day	57% 6% 9% 17% 9% 2	1980
	21st to end	Day	56% 5% 10% 18% 10% 2	2178
April	1st to 10th	Day	56% 6% 12% 17% 9%	2047
	11th to 20th	Day	47% 6% 12% 20% 12% 1	2309
	21st to end	Day	52% 6% 11% 18% 11% 2	2340
May	1st to 10th	Day	48% 6% 11% 23% 12% 1	2340
	11th to 20th	Day	46% 7% 11% 23% 11% 2	2323
	21st to end	Day	40% 8% 14% 22% 14% 2	2725
June	1st to 10th	Day	41% 8% 13% 23% 14% 1	2558
	11th to 20th	Day	39% 7% 11% 25% 17% 1	2700
	21st to end	Day	40% 8% 12% 25% 14%	2700
July	1st to 10th	Day	31% 8% 12% 26% 20% 2	2626
	11th to 20th	Day	37% 7% 15% 27% 14% 1	2399
	21st to end	Day	37% 8% 14% 26% 14% 2	2405
August	1st to 10th	Day	37% 11% 16% 26% 10% 1	2314
	11th to 20th	Day	39% 10% 17% 24% 10% 1	2303
	21st to end	Day	39% 7% 15% 25% 13% 1	2552
September	1st to 10th	Day	41% 8% 14% 23% 12% 1	2111
	11th to 20th	Day	37% 8% 14% 29% 12% 1	1980
	21st to end	Day	46% 8% 14% 23% 8% 1	1980
October	1st to 10th	Day	49% 8% 12% 19% 12% 1	1838
	11th to 20th	Day	61% 8% 11% 13% 7% 1	1800
	21st to end	Day	65% 7% 13% 10% 5%	1931
November	1st to 10th	Day	55% 8% 11% 18% 7% 1	1672
	11th to 20th	Day	62% 6% 14% 10% 6% 1	1543
	21st to end	Day	72% 7% 10% 9% 2	1520
December	1st to 10th	Day	77% 6% 9% 6% 1	1520
	11th to 20th	Day	75% 7% 9% 7% 1	1906
	21st to end	Day	79% 7% 9% 5%	2524
January	1st to 10th	Night	76% 5% 9% 8% 3	2739
	11th to 20th	Night	77% 6% 8% 6% 3	2426
	21st to end	Night	73% 6% 9% 8% 3 1	2574
February	1st to 10th	Night	74% 6% 8% 10% 2	2340
	11th to 20th	Night	68% 5% 11% 12% 4	2275
	21st to end	Night	65% 6% 10% 13% 6%	1780
March	1st to 10th	Night	55% 6% 11% 19% 8% 1	2039
	11th to 20th	Night	53% 5% 12% 19% 9% 2	1980
	21st to end	Night	55% 5% 8% 18% 12% 3	2178
April	1st to 10th	Night	53% 6% 9% 19% 12% 1	1913
	11th to 20th	Night	47% 5% 11% 18% 17% 2	1615
	21st to end	Night	50% 6% 9% 18% 15% 3	1620
May	1st to 10th	Night	45% 6% 10% 21% 15% 2	1620
	11th to 20th	Night	43% 5% 11% 22% 15% 4	1572
	21st to end	Night	31% 8% 15% 26% 17% 4	1551
June	1st to 10th	Night	39% 6% 11% 26% 16% 2	1397
	11th to 20th	Night	36% 8% 13% 24% 17% 3	1260
	21st to end	Night	36% 6% 14% 25% 18% 2	1260
July	1st to 10th	Night	25% 8% 12% 27% 24% 4	1225
	11th to 20th	Night	31% 7% 14% 28% 18% 2	1433
	21st to end	Night	34% 9% 15% 24% 15% 3	1672
August	1st to 10th	Night	32% 9% 14% 31% 14% 1	1600
	11th to 20th	Night	31% 8% 17% 29% 14% 2	1598
	21st to end	Night	35% 9% 13% 27% 15% 2	1785
September	1st to 10th	Night	37% 9% 13% 23% 16% 2	1845
	11th to 20th	Night	32% 6% 14% 29% 18% 1	1980
	21st to end	Night	43% 7% 13% 24% 12%	1980
October	1st to 10th	Night	46% 7% 11% 20% 15% 1	2120
	11th to 20th	Night	56% 7% 12% 19% 6% 1	2160
	21st to end	Night	62% 7% 11% 13% 8%	2422
November	1st to 10th	Night	51% 7% 12% 20% 9%	2406
	11th to 20th	Night	64% 7% 10% 14% 4 1	2636
	21st to end	Night	69% 7% 13% 9% 2	2660
December	1st to 10th	Night	78% 6% 8% 6% 2	2660
	11th to 20th	Night	74% 6% 11% 8% 2	3288
	21st to end	Night	82% 5% 9% 4 1	5117

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	20% 22% 32% 16% 7% 2	225
	11th to 20th	Day	23% 17% 43% 11% 4 1	252
	21st to end	Day	25% 13% 38% 20% 4 1	264
February	1st to 10th	Day	26% 21% 39% 13% 1	240
	11th to 20th	Day	22% 21% 43% 12% 1	240
	21st to end	Day	26% 29% 30% 14% 1	198
March	1st to 10th	Day	23% 15% 37% 19% 6% 1	361
	11th to 20th	Day	20% 14% 34% 24% 8% 1	420
	21st to end	Day	25% 14% 32% 23% 6% 1	462
April	1st to 10th	Day	22% 17% 32% 22% 6% 1	420
	11th to 20th	Day	26% 15% 25% 20% 10% 3	412
	21st to end	Day	24% 13% 33% 20% 8% 2	420
May	1st to 10th	Day	22% 11% 26% 28% 9% 3	423
	11th to 20th	Day	21% 7% 29% 23% 15% 5%	447
	21st to end	Day	15% 7% 24% 29% 18% 8%	485
June	1st to 10th	Day	12% 8% 31% 30% 14% 5%	488
	11th to 20th	Day	14% 9% 30% 29% 14% 3	630
	21st to end	Day	15% 7% 28% 33% 14% 3	630
July	1st to 10th	Day	9% 5% 24% 30% 24% 9%	621
	11th to 20th	Day	15% 4 24% 29% 21% 7%	538
	21st to end	Day	8% 4 24% 38% 18% 7%	488
August	1st to 10th	Day	12% 3 22% 27% 24% 13%	446
	11th to 20th	Day	13% 3 16% 32% 22% 13%	439
	21st to end	Day	16% 5% 22% 24% 22% 11%	495
September	1st to 10th	Day	14% 5% 23% 29% 18% 12%	450
	11th to 20th	Day	10% 6% 20% 26% 26% 12%	450
	21st to end	Day	16% 12% 26% 22% 14% 9%	450
October	1st to 10th	Day	20% 10% 27% 20% 17% 5%	450
	11th to 20th	Day	24% 17% 31% 21% 6% 1	450
	21st to end	Day	29% 17% 32% 17% 4	446
November	1st to 10th	Day	24% 19% 29% 20% 8% 1	270
	11th to 20th	Day	26% 18% 34% 15% 6% 1	191
	21st to end	Day	24% 21% 38% 14% 2	180
December	1st to 10th	Day	26% 26% 33% 12% 4	180
	11th to 20th	Day	28% 28% 27% 12% 4	180
	21st to end	Day	33% 23% 30% 13% 1	198
January	1st to 10th	Night	23% 18% 36% 17% 5% 1	950
	11th to 20th	Night	29% 16% 30% 21% 4	770
	21st to end	Night	19% 13% 39% 23% 6%	792
February	1st to 10th	Night	25% 16% 36% 21% 2	720
	11th to 20th	Night	22% 15% 32% 24% 6%	703
	21st to end	Night	24% 17% 34% 21% 3	592
March	1st to 10th	Night	20% 13% 35% 24% 7%	720
	11th to 20th	Night	19% 15% 35% 23% 6% 1	720
	21st to end	Night	23% 11% 31% 26% 9% 1	792
April	1st to 10th	Night	24% 14% 30% 27% 5%	720
	11th to 20th	Night	20% 13% 31% 24% 11% 1	551
	21st to end	Night	21% 13% 28% 30% 8%	540
May	1st to 10th	Night	20% 7% 30% 30% 11% 1	540
	11th to 20th	Night	19% 4 30% 34% 12% 1	510
	21st to end	Night	11% 4 28% 39% 17% 1	389
June	1st to 10th	Night	16% 4 28% 37% 15%	360
	11th to 20th	Night	11% 6% 27% 42% 14%	360
	21st to end	Night	11% 4 24% 47% 13% 1	360
July	1st to 10th	Night	6% 2 18% 50% 22% 1	350
	11th to 20th	Night	11% 2 21% 40% 24% 2	484
	21st to end	Night	9% 3 18% 48% 22% 1	591
August	1st to 10th	Night	9% 2 14% 42% 30% 3	537
	11th to 20th	Night	10% 2 15% 40% 29% 4	534
	21st to end	Night	13% 3 20% 36% 26% 2	597
September	1st to 10th	Night	14% 5% 21% 34% 20% 5%	720
	11th to 20th	Night	12% 5% 19% 36% 24% 4	720
	21st to end	Night	18% 6% 23% 32% 17% 4	720
October	1st to 10th	Night	18% 8% 28% 29% 15% 2	720
	11th to 20th	Night	25% 15% 26% 25% 8% 1	720
	21st to end	Night	31% 11% 31% 24% 3	792
November	1st to 10th	Night	24% 12% 26% 29% 8% 1	740
	11th to 20th	Night	24% 16% 36% 17% 5% 1	760
	21st to end	Night	29% 13% 31% 23% 4	760
December	1st to 10th	Night	29% 19% 30% 18% 4	760
	11th to 20th	Night	24% 21% 36% 16% 3	760
	21st to end	Night	30% 15% 36% 17% 2	978

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