StarGlider Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the StarGlider Observatory clear sky chart from 2005-11-06 to 2024-01-17.

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% 10% 4 7% 6% 3 5% 5% 5% 7% 31%	74640
	Night	17% 10% 4 7% 6% 3 5% 4 5% 6% 33%	75188
Transparency	Day	51% 7% 12% 18% 10% 1	74359
	Night	50% 7% 12% 18% 11% 2	74989
Seeing	Day	22% 14% 27% 21% 11% 4	13576
	Night	21% 12% 27% 27% 12% 1	23370

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	17% 17% 8% 11% 7% 4 4 4 4 6% 19%	1691
	11th to 20th	Day	18% 19% 7% 13% 8% 3 4 2 4 4 17%	1674
	21st to end	Day	18% 17% 8% 13% 8% 3 4 4 4 5% 16%	1782
February	1st to 10th	Day	21% 15% 7% 10% 9% 4 4 3 3 4 20%	1620
	11th to 20th	Day	17% 14% 7% 11% 8% 3 6% 4 4 6% 21%	1573
	21st to end	Day	19% 15% 5% 10% 6% 3 4 4 4 5% 25%	1478
March	1st to 10th	Day	17% 11% 5% 8% 6% 3 4 4 4 6% 31%	1980
	11th to 20th	Day	18% 12% 4 9% 7% 3 5% 4 4 6% 29%	1980
	21st to end	Day	21% 10% 4 8% 6% 3 5% 5% 5% 7% 27%	2178
April	1st to 10th	Day	16% 10% 5% 7% 6% 4 5% 4 5% 7% 32%	2078
	11th to 20th	Day	17% 9% 4 8% 6% 3 5% 5% 5% 6% 32%	2161
	21st to end	Day	23% 9% 3 6% 4 3 3 4 5% 6% 33%	2340
May	1st to 10th	Day	19% 9% 3 6% 6% 3 6% 5% 6% 8% 29%	2340
	11th to 20th	Day	21% 7% 3 7% 7% 4 6% 4 6% 7% 30%	2301
	21st to end	Day	12% 5% 2 5% 5% 3 6% 7% 7% 10% 38%	2686
June	1st to 10th	Day	14% 7% 3 6% 6% 3 6% 5% 6% 10% 35%	2520
	11th to 20th	Day	14% 7% 3 5% 6% 4 6% 5% 7% 9% 35%	2520
	21st to end	Day	14% 6% 3 5% 7% 3 5% 5% 6% 10% 38%	2520
July	1st to 10th	Day	12% 6% 3 6% 5% 4 5% 6% 6% 8% 40%	2463
	11th to 20th	Day	12% 7% 2 6% 5% 4 5% 6% 7% 10% 36%	2340
	21st to end	Day	12% 6% 3 7% 7% 4 7% 6% 6% 9% 35%	2547
August	1st to 10th	Day	9% 6% 3 6% 7% 5% 7% 7% 6% 10% 34%	2327
	11th to 20th	Day	13% 6% 3 7% 6% 4 6% 6% 7% 10% 33%	2303
	21st to end	Day	13% 5% 2 5% 5% 4 7% 6% 6% 9% 38%	2453
September	1st to 10th	Day	16% 7% 2 5% 5% 3 4 4 6% 7% 42%	2160
	11th to 20th	Day	12% 5% 3 6% 4 3 4 5% 6% 8% 44%	1993
	21st to end	Day	19% 7% 3 6% 5% 3 5% 4 4 6% 37%	1976
October	1st to 10th	Day	16% 7% 3 5% 5% 3 5% 6% 6% 8% 39%	1800
	11th to 20th	Day	19% 9% 3 6% 6% 4 5% 5% 5% 5% 34%	1800
	21st to end	Day	24% 10% 3 6% 5% 3 4 4 5% 7% 30%	1980
November	1st to 10th	Day	11% 9% 3 6% 5% 3 5% 5% 4 6% 42%	1781
	11th to 20th	Day	18% 14% 4 5% 6% 3 4 3 4 7% 32%	1573
	21st to end	Day	18% 14% 6% 7% 6% 4 5% 5% 5% 5% 26%	1520
December	1st to 10th	Day	22% 17% 8% 10% 7% 3 4 3 4 5% 17%	1520
	11th to 20th	Day	20% 17% 7% 10% 5% 3 3 3 4 5% 21%	1910
	21st to end	Day	26% 22% 7% 9% 6% 2 3 4 4 4 13%	2772
January	1st to 10th	Night	19% 16% 7% 10% 6% 4 5% 5% 4 4 20%	2472
	11th to 20th	Night	21% 15% 6% 10% 8% 3 5% 4 4 4 20%	2430
	21st to end	Night	18% 15% 7% 10% 7% 4 4 4 4 4 22%	2574
February	1st to 10th	Night	22% 13% 5% 10% 7% 3 5% 4 4 5% 23%	2340
	11th to 20th	Night	20% 12% 5% 10% 7% 3 5% 4 5% 5% 24%	2273
	21st to end	Night	21% 12% 5% 10% 6% 3 5% 4 4 5% 25%	1773
March	1st to 10th	Night	19% 9% 5% 6% 6% 3 4 4 4 6% 33%	1980
	11th to 20th	Night	20% 10% 4 7% 5% 3 3 3 4 6% 35%	1980
	21st to end	Night	19% 10% 4 7% 6% 3 5% 4 4 4 35%	2178
April	1st to 10th	Night	18% 10% 4 6% 6% 3 4 4 4 6% 35%	1882
	11th to 20th	Night	16% 8% 3 6% 4 4 5% 5% 4 7% 38%	1763
	21st to end	Night	17% 6% 4 7% 5% 4 5% 3 6% 8% 37%	1620
May	1st to 10th	Night	18% 8% 3 6% 6% 4 6% 5% 4 7% 33%	1620
	11th to 20th	Night	17% 8% 3 6% 5% 2 4 5% 4 9% 36%	1592
	21st to end	Night	10% 6% 2 4 4 3 4 5% 7% 10% 45%	1585
June	1st to 10th	Night	10% 6% 3 5% 6% 2 4 6% 6% 8% 44%	1440
	11th to 20th	Night	12% 6% 2 5% 6% 4 4 5% 7% 8% 40%	1440
	21st to end	Night	10% 4 1 5% 5% 3 6% 5% 7% 9% 45%	1440
July	1st to 10th	Night	8% 5% 2 7% 5% 4 6% 5% 5% 9% 43%	1492
	11th to 20th	Night	9% 4 3 5% 4 4 5% 4 6% 9% 46%	1620
	21st to end	Night	10% 5% 2 6% 6% 4 6% 6% 7% 9% 39%	1771
August	1st to 10th	Night	7% 6% 2 6% 5% 3 5% 6% 8% 9% 45%	1611
	11th to 20th	Night	11% 6% 2 6% 5% 3 5% 4 5% 9% 44%	1598
	21st to end	Night	11% 5% 2 5% 5% 3 5% 6% 6% 9% 44%	1884
September	1st to 10th	Night	13% 5% 1 4 5% 4 4 5% 5% 7% 48%	1800
	11th to 20th	Night	10% 4 2 4 4 2 5% 4 5% 9% 51%	1965
	21st to end	Night	15% 7% 2 6% 5% 3 5% 4 4 6% 42%	1984
October	1st to 10th	Night	14% 6% 3 6% 4 3 4 5% 5% 6% 44%	2160
	11th to 20th	Night	19% 5% 3 6% 6% 4 4 4 5% 6% 38%	2160
	21st to end	Night	24% 8% 2 5% 5% 3 3 4 4 6% 35%	2376
November	1st to 10th	Night	14% 7% 3 5% 5% 3 5% 4 5% 6% 44%	2292
	11th to 20th	Night	19% 11% 3 6% 5% 3 5% 4 5% 7% 32%	2604
	21st to end	Night	21% 13% 4 6% 5% 3 4 4 5% 6% 30%	2660
December	1st to 10th	Night	23% 14% 7% 9% 6% 3 4 4 4 5% 20%	2660
	11th to 20th	Night	20% 17% 6% 9% 6% 3 4 4 4 5% 24%	3285
	21st to end	Night	26% 18% 6% 8% 7% 4 4 4 4 5% 14%	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	69% 6% 10% 11% 5%	1692
	11th to 20th	Day	71% 4 12% 11% 2	1674
	21st to end	Day	71% 5% 10% 10% 3 1	1782
February	1st to 10th	Day	70% 4 8% 12% 5% 1	1620
	11th to 20th	Day	66% 6% 11% 12% 5%	1582
	21st to end	Day	62% 5% 11% 14% 7% 1	1478
March	1st to 10th	Day	53% 6% 14% 20% 7%	1980
	11th to 20th	Day	57% 5% 11% 16% 9% 2	1980
	21st to end	Day	57% 6% 9% 16% 10% 2	2178
April	1st to 10th	Day	51% 6% 13% 17% 11% 2	2078
	11th to 20th	Day	51% 7% 11% 15% 14% 2	2161
	21st to end	Day	51% 6% 11% 20% 9% 2	2340
May	1st to 10th	Day	51% 9% 13% 17% 9% 1	2340
	11th to 20th	Day	53% 8% 12% 14% 10% 2	2305
	21st to end	Day	39% 11% 17% 22% 9% 2	2686
June	1st to 10th	Day	45% 9% 14% 21% 11% 1	2520
	11th to 20th	Day	44% 9% 14% 20% 11% 1	2520
	21st to end	Day	41% 9% 12% 26% 11% 1	2520
July	1st to 10th	Day	41% 10% 13% 20% 13% 3	2396
	11th to 20th	Day	40% 9% 15% 23% 14% 1	2271
	21st to end	Day	42% 10% 14% 20% 13% 1	2405
August	1st to 10th	Day	40% 11% 17% 23% 8%	2314
	11th to 20th	Day	43% 10% 16% 22% 9% 1	2303
	21st to end	Day	39% 10% 14% 22% 12% 3	2453
September	1st to 10th	Day	41% 8% 11% 20% 17% 4	2160
	11th to 20th	Day	36% 8% 15% 26% 14% 1	1993
	21st to end	Day	47% 6% 12% 21% 14%	1976
October	1st to 10th	Day	41% 8% 16% 19% 15% 1	1800
	11th to 20th	Day	50% 7% 12% 22% 9%	1800
	21st to end	Day	53% 6% 14% 16% 9% 1	1980
November	1st to 10th	Day	43% 6% 12% 21% 16% 2	1781
	11th to 20th	Day	54% 5% 12% 18% 9% 2	1573
	21st to end	Day	60% 5% 12% 16% 6% 1	1520
December	1st to 10th	Day	70% 5% 10% 12% 3	1520
	11th to 20th	Day	66% 5% 11% 14% 3	1906
	21st to end	Day	74% 7% 9% 8% 2	2772
January	1st to 10th	Night	67% 6% 9% 14% 5%	2472
	11th to 20th	Night	68% 6% 11% 12% 3	2430
	21st to end	Night	65% 6% 9% 14% 4 1	2574
February	1st to 10th	Night	64% 6% 10% 15% 5%	2340
	11th to 20th	Night	62% 5% 11% 14% 7%	2275
	21st to end	Night	62% 6% 8% 15% 9% 1	1773
March	1st to 10th	Night	52% 6% 11% 20% 11% 1	1980
	11th to 20th	Night	50% 6% 13% 21% 9% 2	1980
	21st to end	Night	52% 5% 9% 17% 14% 3	2178
April	1st to 10th	Night	51% 6% 10% 18% 14% 2	1882
	11th to 20th	Night	45% 8% 12% 17% 16% 3	1763
	21st to end	Night	45% 6% 12% 21% 12% 3	1620
May	1st to 10th	Night	49% 6% 11% 19% 12% 3	1620
	11th to 20th	Night	46% 9% 11% 17% 15% 2	1593
	21st to end	Night	33% 9% 17% 26% 13% 2	1586
June	1st to 10th	Night	35% 9% 15% 24% 15% 2	1440
	11th to 20th	Night	38% 10% 14% 22% 15% 1	1440
	21st to end	Night	35% 10% 13% 24% 16% 2	1440
July	1st to 10th	Night	36% 10% 12% 20% 17% 4	1450
	11th to 20th	Night	31% 9% 17% 23% 19% 2	1566
	21st to end	Night	37% 11% 15% 21% 13% 3	1672
August	1st to 10th	Night	32% 11% 16% 25% 16% 1	1600
	11th to 20th	Night	36% 9% 15% 24% 14% 2	1598
	21st to end	Night	33% 12% 13% 24% 15% 4	1884
September	1st to 10th	Night	33% 7% 14% 22% 18% 5%	1800
	11th to 20th	Night	30% 7% 15% 26% 21% 1	1965
	21st to end	Night	42% 7% 12% 21% 17% 1	1984
October	1st to 10th	Night	39% 8% 14% 22% 14% 2	2160
	11th to 20th	Night	46% 6% 11% 23% 13% 1	2160
	21st to end	Night	50% 6% 11% 19% 11% 2	2376
November	1st to 10th	Night	40% 7% 11% 22% 16% 3	2292
	11th to 20th	Night	52% 5% 11% 19% 10% 2	2604
	21st to end	Night	56% 6% 12% 20% 6% 1	2660
December	1st to 10th	Night	66% 6% 12% 12% 4	2660
	11th to 20th	Night	64% 5% 12% 15% 3	3288
	21st to end	Night	71% 8% 11% 9% 2	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	29% 26% 29% 11% 4 1	263
	11th to 20th	Day	31% 29% 29% 9% 3	258
	21st to end	Day	26% 32% 30% 8% 4	264
February	1st to 10th	Day	34% 23% 38% 5%	240
	11th to 20th	Day	34% 30% 28% 7%	240
	21st to end	Day	40% 22% 28% 9% 1	228
March	1st to 10th	Day	28% 22% 30% 16% 4	420
	11th to 20th	Day	31% 19% 32% 12% 5% 1	420
	21st to end	Day	30% 22% 27% 16% 5% 1	462
April	1st to 10th	Day	27% 22% 25% 16% 8% 1	420
	11th to 20th	Day	23% 19% 27% 17% 9% 4	412
	21st to end	Day	30% 18% 28% 17% 5% 2	420
May	1st to 10th	Day	30% 17% 25% 17% 7% 3	423
	11th to 20th	Day	23% 11% 33% 20% 9% 4	447
	21st to end	Day	14% 9% 26% 27% 18% 7%	485
June	1st to 10th	Day	14% 10% 30% 29% 14% 4	450
	11th to 20th	Day	18% 12% 25% 26% 15% 4	450
	21st to end	Day	14% 10% 32% 28% 14% 3	450
July	1st to 10th	Day	13% 7% 19% 28% 23% 11%	445
	11th to 20th	Day	14% 5% 25% 29% 18% 9%	444
	21st to end	Day	13% 5% 26% 30% 19% 7%	488
August	1st to 10th	Day	10% 7% 22% 30% 21% 10%	446
	11th to 20th	Day	16% 4 22% 30% 16% 11%	439
	21st to end	Day	12% 4 25% 27% 21% 11%	495
September	1st to 10th	Day	16% 8% 22% 27% 19% 8%	450
	11th to 20th	Day	12% 5% 20% 32% 21% 11%	450
	21st to end	Day	23% 11% 23% 23% 14% 6%	450
October	1st to 10th	Day	17% 10% 25% 25% 18% 5%	450
	11th to 20th	Day	22% 16% 29% 23% 8% 2	450
	21st to end	Day	31% 15% 29% 18% 7%	495
November	1st to 10th	Day	19% 18% 31% 19% 11% 3	379
	11th to 20th	Day	29% 20% 29% 14% 5% 2	205
	21st to end	Day	23% 23% 42% 9% 3	180
December	1st to 10th	Day	31% 27% 26% 11% 4 2	180
	11th to 20th	Day	32% 21% 33% 11% 3	180
	21st to end	Day	35% 18% 31% 13% 3	198
January	1st to 10th	Night	27% 21% 34% 15% 3	760
	11th to 20th	Night	31% 16% 32% 18% 3	748
	21st to end	Night	26% 19% 34% 19% 3	792
February	1st to 10th	Night	29% 16% 29% 23% 2	720
	11th to 20th	Night	30% 19% 35% 15% 1	703
	21st to end	Night	28% 16% 36% 17% 3	592
March	1st to 10th	Night	24% 18% 33% 21% 3	720
	11th to 20th	Night	26% 18% 28% 21% 6% 1	720
	21st to end	Night	24% 14% 33% 23% 5% 1	792
April	1st to 10th	Night	25% 16% 30% 24% 5%	720
	11th to 20th	Night	21% 13% 32% 23% 10% 1	699
	21st to end	Night	19% 10% 36% 29% 6%	540
May	1st to 10th	Night	21% 9% 28% 32% 9% 1	540
	11th to 20th	Night	21% 8% 27% 33% 11%	531
	21st to end	Night	12% 3 26% 34% 23% 3	425
June	1st to 10th	Night	11% 5% 25% 41% 18% 1	360
	11th to 20th	Night	12% 6% 24% 38% 20% 1	360
	21st to end	Night	11% 3 28% 36% 20% 1	360
July	1st to 10th	Night	9% 2 20% 42% 23% 5%	400
	11th to 20th	Night	9% 5% 18% 35% 28% 5%	522
	21st to end	Night	11% 3 14% 42% 28% 2	591
August	1st to 10th	Night	10% 1 15% 43% 28% 2	537
	11th to 20th	Night	12% 4 16% 34% 30% 4	534
	21st to end	Night	12% 3 20% 33% 27% 5%	696
September	1st to 10th	Night	15% 3 19% 32% 26% 4	720
	11th to 20th	Night	12% 4 16% 36% 29% 4	720
	21st to end	Night	17% 9% 25% 25% 20% 5%	720
October	1st to 10th	Night	17% 8% 23% 29% 22% 1	720
	11th to 20th	Night	22% 10% 31% 27% 9% 1	720
	21st to end	Night	29% 12% 26% 26% 7%	792
November	1st to 10th	Night	18% 12% 26% 31% 12%	740
	11th to 20th	Night	25% 15% 28% 24% 6% 1	760
	21st to end	Night	26% 16% 30% 21% 6%	760
December	1st to 10th	Night	31% 16% 28% 19% 4	760
	11th to 20th	Night	27% 19% 30% 21% 3	760
	21st to end	Night	28% 17% 32% 21% 2	836

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