Starhoo Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Starhoo Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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	18% 8% 2 6% 4 4 4 3 5% 7% 39%	126514
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 39%	123834
Transparency	Day	45% 3 11% 22% 15% 3	130061
	Night	46% 3 11% 22% 16% 3	128374
Seeing	Day	22% 18% 33% 19% 7% 2	16478
	Night	21% 15% 33% 24% 7% 1	25336

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	11% 9% 2 7% 3 3 3 3 5% 6% 47%	3597
	11th to 20th	Day	16% 9% 2 5% 3 3 3 3 4 4 48%	3570
	21st to end	Day	17% 10% 3 6% 3 4 4 3 4 4 41%	3960
February	1st to 10th	Day	19% 8% 3 6% 4 4 5% 4 5% 7% 35%	1911
	11th to 20th	Day	15% 8% 2 6% 4 3 4 5% 5% 6% 42%	1948
	21st to end	Day	18% 10% 3 6% 5% 3 3 3 5% 5% 39%	1650
March	1st to 10th	Day	22% 13% 3 6% 3 3 2 2 4 6% 37%	4476
	11th to 20th	Day	15% 10% 3 6% 4 3 4 3 5% 7% 40%	4814
	21st to end	Day	25% 11% 2 6% 4 4 4 3 4 5% 31%	5708
April	1st to 10th	Day	21% 9% 2 5% 4 4 4 3 5% 6% 37%	2400
	11th to 20th	Day	19% 7% 2 6% 5% 4 4 3 5% 7% 39%	2397
	21st to end	Day	20% 7% 2 6% 4 4 4 3 5% 6% 39%	2600
May	1st to 10th	Day	27% 12% 3 6% 4 3 4 3 5% 6% 27%	5886
	11th to 20th	Day	22% 11% 3 7% 5% 4 3 3 5% 6% 32%	5861
	21st to end	Day	21% 7% 3 6% 4 4 4 4 6% 7% 35%	6902
June	1st to 10th	Day	20% 6% 2 6% 4 4 4 4 6% 8% 35%	2800
	11th to 20th	Day	17% 5% 2 7% 5% 4 5% 4 6% 8% 38%	2800
	21st to end	Day	15% 7% 2 6% 5% 4 5% 4 6% 10% 38%	2800
July	1st to 10th	Day	18% 7% 3 7% 5% 4 5% 4 6% 10% 32%	6360
	11th to 20th	Day	12% 7% 2 6% 5% 5% 6% 4 8% 10% 35%	6360
	21st to end	Day	16% 6% 2 6% 4 4 4 3 6% 7% 41%	6883
August	1st to 10th	Day	17% 7% 3 8% 5% 5% 5% 4 6% 9% 31%	4235
	11th to 20th	Day	12% 7% 3 8% 5% 5% 6% 4 7% 8% 37%	4142
	21st to end	Day	8% 4 2 5% 4 5% 6% 5% 7% 9% 47%	4312
September	1st to 10th	Day	15% 6% 2 5% 4 3 4 3 5% 6% 46%	2375
	11th to 20th	Day	14% 6% 2 5% 3 5% 3 2 4 6% 51%	2200
	21st to end	Day	20% 6% 2 6% 4 4 4 3 5% 6% 41%	2200
October	1st to 10th	Day	22% 6% 1 4 4 3 3 3 4 5% 45%	2200
	11th to 20th	Day	17% 5% 2 5% 4 3 3 3 4 7% 47%	1978
	21st to end	Day	18% 6% 2 5% 3 3 3 3 4 6% 44%	1980
November	1st to 10th	Day	17% 6% 2 6% 3 4 3 3 4 4 48%	1851
	11th to 20th	Day	17% 7% 2 4 3 3 3 3 4 5% 48%	1863
	21st to end	Day	18% 7% 2 5% 4 4 3 3 5% 7% 43%	1800
December	1st to 10th	Day	14% 10% 2 4 3 3 3 3 4 6% 49%	2749
	11th to 20th	Day	19% 7% 2 4 3 3 3 2 3 5% 49%	2986
	21st to end	Day	20% 10% 3 7% 4 3 3 3 4 4 37%	3960
January	1st to 10th	Night	13% 6% 2 5% 3 3 3 3 4 7% 52%	6349
	11th to 20th	Night	15% 10% 4 7% 4 4 3 3 4 4 42%	6315
	21st to end	Night	17% 10% 3 7% 5% 4 4 4 4 5% 36%	6996
February	1st to 10th	Night	17% 9% 3 6% 4 4 4 4 4 5% 40%	2485
	11th to 20th	Night	18% 8% 3 6% 4 4 4 3 5% 5% 39%	2338
	21st to end	Night	18% 8% 2 6% 4 4 4 3 4 5% 41%	1980
March	1st to 10th	Night	25% 9% 2 4 3 4 4 3 4 5% 37%	5364
	11th to 20th	Night	17% 11% 3 7% 4 3 4 3 5% 5% 38%	5145
	21st to end	Night	24% 10% 3 7% 4 4 4 3 3 4 33%	5220
April	1st to 10th	Night	19% 9% 3 6% 4 4 4 3 5% 5% 37%	2000
	11th to 20th	Night	20% 8% 2 6% 4 4 4 3 5% 6% 37%	1950
	21st to end	Night	19% 8% 2 6% 4 5% 4 3 4 5% 39%	1800
May	1st to 10th	Night	28% 11% 3 7% 4 3 4 3 5% 6% 26%	4054
	11th to 20th	Night	20% 10% 3 8% 5% 4 5% 4 5% 7% 27%	4033
	21st to end	Night	21% 9% 3 7% 5% 5% 6% 3 5% 6% 30%	4001
June	1st to 10th	Night	18% 7% 3 6% 5% 4 5% 4 5% 7% 37%	1600
	11th to 20th	Night	17% 8% 2 6% 6% 5% 4 3 5% 8% 37%	1600
	21st to end	Night	17% 9% 2 5% 3 4 4 3 6% 8% 39%	1600
July	1st to 10th	Night	18% 8% 3 6% 4 4 4 4 5% 10% 33%	3600
	11th to 20th	Night	17% 9% 3 6% 4 4 4 4 6% 9% 33%	3600
	21st to end	Night	14% 7% 2 7% 5% 4 4 4 6% 6% 40%	3926
August	1st to 10th	Night	22% 8% 2 6% 4 5% 5% 4 7% 8% 29%	2910
	11th to 20th	Night	9% 7% 3 8% 5% 5% 5% 5% 6% 9% 38%	2901
	21st to end	Night	9% 4 1 5% 3 4 4 3 6% 7% 54%	3586
September	1st to 10th	Night	13% 6% 3 6% 5% 4 4 4 5% 7% 44%	2025
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 4 7% 51%	2200
	21st to end	Night	19% 7% 3 5% 4 3 5% 3 4 6% 40%	2200
October	1st to 10th	Night	20% 7% 2 6% 3 4 3 2 4 5% 44%	2200
	11th to 20th	Night	16% 5% 2 4 4 4 3 3 6% 7% 47%	2418
	21st to end	Night	19% 8% 3 5% 4 3 5% 3 4 5% 40%	2860
November	1st to 10th	Night	15% 5% 2 6% 3 3 4 3 5% 6% 47%	2667
	11th to 20th	Night	17% 8% 2 5% 3 3 4 3 4 5% 46%	2692
	21st to end	Night	17% 5% 2 5% 4 3 4 4 5% 7% 45%	2600
December	1st to 10th	Night	18% 8% 2 5% 4 3 3 3 3 4 47%	4415
	11th to 20th	Night	22% 9% 2 5% 3 3 3 3 4 5% 41%	5208
	21st to end	Night	17% 11% 3 7% 4 3 3 3 4 5% 40%	6996

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	39% 2 6% 27% 22% 4	3598
	11th to 20th	Day	40% 3 10% 22% 17% 8%	3600
	21st to end	Day	48% 2 8% 21% 18% 3	3960
February	1st to 10th	Day	50% 3 12% 18% 13% 3	1911
	11th to 20th	Day	43% 4 11% 22% 16% 3	1957
	21st to end	Day	48% 3 9% 21% 16% 3	1650
March	1st to 10th	Day	52% 2 10% 14% 12% 11%	4479
	11th to 20th	Day	44% 3 12% 28% 11% 2	4814
	21st to end	Day	57% 3 5% 15% 13% 7%	5708
April	1st to 10th	Day	49% 3 8% 19% 17% 4	2400
	11th to 20th	Day	47% 3 8% 21% 18% 3	2397
	21st to end	Day	46% 4 8% 21% 16% 4	2600
May	1st to 10th	Day	60% 3 14% 15% 7% 1	5897
	11th to 20th	Day	55% 3 14% 17% 10% 1	5865
	21st to end	Day	48% 3 12% 22% 14% 1	6902
June	1st to 10th	Day	47% 5% 12% 23% 12% 1	2800
	11th to 20th	Day	43% 4 14% 23% 14% 2	2800
	21st to end	Day	42% 6% 14% 23% 14% 1	2800
July	1st to 10th	Day	48% 5% 13% 20% 12% 2	6290
	11th to 20th	Day	44% 4 17% 24% 10% 1	6285
	21st to end	Day	42% 4 11% 28% 15% 1	6685
August	1st to 10th	Day	50% 4 15% 21% 9% 1	4222
	11th to 20th	Day	43% 4 15% 26% 10% 1	4181
	21st to end	Day	34% 5% 14% 26% 20% 1	4312
September	1st to 10th	Day	39% 4 9% 26% 21% 1	2375
	11th to 20th	Day	36% 3 9% 25% 23% 3	2200
	21st to end	Day	45% 4 8% 20% 21% 2	2200
October	1st to 10th	Day	39% 4 9% 26% 18% 4	3590
	11th to 20th	Day	33% 2 6% 30% 25% 3	3194
	21st to end	Day	31% 3 13% 30% 20% 3	3180
November	1st to 10th	Day	41% 4 12% 22% 19% 2	1851
	11th to 20th	Day	39% 3 9% 24% 21% 3	1863
	21st to end	Day	43% 5% 10% 26% 14% 2	1800
December	1st to 10th	Day	40% 2 13% 27% 16% 1	2749
	11th to 20th	Day	42% 2 14% 17% 19% 6%	2986
	21st to end	Day	51% 3 7% 21% 13% 4	3960
January	1st to 10th	Night	35% 2 8% 35% 18% 3	6349
	11th to 20th	Night	47% 3 9% 22% 15% 5%	6360
	21st to end	Night	52% 2 8% 18% 16% 5%	6996
February	1st to 10th	Night	48% 3 12% 22% 13% 2	2485
	11th to 20th	Night	47% 3 8% 21% 17% 4	2340
	21st to end	Night	46% 3 7% 24% 16% 4	1980
March	1st to 10th	Night	51% 2 9% 16% 15% 7%	5374
	11th to 20th	Night	50% 2 14% 19% 13% 2	5146
	21st to end	Night	57% 2 4 12% 19% 6%	5220
April	1st to 10th	Night	50% 3 8% 19% 16% 4	2000
	11th to 20th	Night	48% 3 8% 20% 18% 3	1950
	21st to end	Night	49% 3 8% 20% 16% 4	1800
May	1st to 10th	Night	61% 3 14% 13% 7% 2	4060
	11th to 20th	Night	57% 3 15% 15% 8% 2	4034
	21st to end	Night	55% 3 11% 19% 10%	4002
June	1st to 10th	Night	47% 5% 12% 22% 13% 1	1600
	11th to 20th	Night	47% 4 11% 24% 14% 1	1600
	21st to end	Night	43% 4 14% 24% 14% 1	1600
July	1st to 10th	Night	49% 4 10% 21% 15%	3560
	11th to 20th	Night	48% 4 16% 22% 10% 1	3552
	21st to end	Night	44% 4 11% 25% 14% 1	3809
August	1st to 10th	Night	53% 5% 13% 19% 10% 1	2899
	11th to 20th	Night	42% 4 16% 28% 9%	2928
	21st to end	Night	30% 3 15% 29% 23% 1	3586
September	1st to 10th	Night	40% 5% 10% 25% 18% 2	2025
	11th to 20th	Night	36% 3 9% 25% 25% 2	2200
	21st to end	Night	46% 4 7% 22% 19% 2	2200
October	1st to 10th	Night	41% 4 11% 27% 16% 3	3590
	11th to 20th	Night	28% 2 8% 27% 32% 3	3941
	21st to end	Night	34% 3 11% 28% 22% 2	4610
November	1st to 10th	Night	39% 4 11% 24% 18% 3	2667
	11th to 20th	Night	41% 3 9% 22% 20% 4	2692
	21st to end	Night	39% 3 13% 28% 14% 2	2600
December	1st to 10th	Night	43% 2 11% 27% 16% 1	4415
	11th to 20th	Night	47% 3 13% 15% 20% 2	5208
	21st to end	Night	47% 3 8% 23% 17% 1	6996

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	27% 28% 29% 15% 2	200
	11th to 20th	Day	26% 29% 35% 9% 2 1	200
	21st to end	Day	26% 27% 35% 9% 2 1	220
February	1st to 10th	Day	29% 23% 34% 12% 1	453
	11th to 20th	Day	30% 30% 31% 7% 2	496
	21st to end	Day	24% 29% 32% 12% 3	414
March	1st to 10th	Day	29% 25% 31% 12% 2	500
	11th to 20th	Day	26% 28% 32% 11% 3 1	500
	21st to end	Day	29% 23% 31% 11% 4 1	550
April	1st to 10th	Day	30% 25% 28% 11% 5% 1	500
	11th to 20th	Day	27% 27% 31% 12% 3	492
	21st to end	Day	27% 20% 34% 15% 3 2	500
May	1st to 10th	Day	31% 24% 31% 11% 3 1	503
	11th to 20th	Day	23% 17% 41% 13% 6% 1	527
	21st to end	Day	20% 15% 35% 22% 7% 1	573
June	1st to 10th	Day	20% 15% 33% 23% 7% 1	530
	11th to 20th	Day	16% 18% 38% 20% 7% 1	530
	21st to end	Day	16% 13% 38% 25% 6% 2	530
July	1st to 10th	Day	16% 10% 32% 30% 10% 2	525
	11th to 20th	Day	14% 8% 32% 31% 13% 2	524
	21st to end	Day	12% 11% 36% 28% 11% 1	576
August	1st to 10th	Day	13% 9% 36% 29% 10% 2	526
	11th to 20th	Day	14% 10% 30% 29% 14% 3	519
	21st to end	Day	13% 9% 35% 26% 12% 5%	583
September	1st to 10th	Day	19% 10% 32% 23% 12% 4	530
	11th to 20th	Day	17% 11% 28% 25% 11% 8%	530
	21st to end	Day	24% 12% 30% 23% 7% 4	530
October	1st to 10th	Day	25% 13% 27% 21% 9% 6%	530
	11th to 20th	Day	20% 17% 33% 19% 10% 2	462
	21st to end	Day	25% 18% 28% 19% 6% 3	462
November	1st to 10th	Day	20% 19% 34% 17% 9% 2	425
	11th to 20th	Day	25% 23% 33% 14% 4	421
	21st to end	Day	23% 24% 30% 15% 8%	400
December	1st to 10th	Day	21% 24% 36% 15% 4	297
	11th to 20th	Day	31% 28% 32% 9% 2 1	200
	21st to end	Day	26% 23% 34% 11% 4 2	220
January	1st to 10th	Night	22% 22% 37% 17% 3	800
	11th to 20th	Night	28% 22% 33% 15% 3	800
	21st to end	Night	22% 20% 36% 19% 3	880
February	1st to 10th	Night	24% 18% 37% 19% 1	800
	11th to 20th	Night	26% 22% 38% 12% 1	783
	21st to end	Night	24% 22% 33% 18% 3	660
March	1st to 10th	Night	26% 19% 34% 18% 3	800
	11th to 20th	Night	21% 20% 38% 17% 4	800
	21st to end	Night	26% 19% 35% 17% 3	880
April	1st to 10th	Night	25% 18% 35% 18% 4	800
	11th to 20th	Night	24% 19% 35% 19% 3	766
	21st to end	Night	22% 18% 37% 20% 4	600
May	1st to 10th	Night	30% 13% 33% 20% 4	600
	11th to 20th	Night	21% 15% 40% 20% 4	592
	21st to end	Night	22% 7% 32% 32% 6%	469
June	1st to 10th	Night	18% 9% 32% 33% 9% 1	400
	11th to 20th	Night	20% 6% 33% 33% 8%	400
	21st to end	Night	19% 7% 29% 39% 6%	400
July	1st to 10th	Night	17% 4 29% 40% 9% 1	390
	11th to 20th	Night	16% 5% 27% 41% 11% 1	388
	21st to end	Night	14% 4 28% 44% 10%	438
August	1st to 10th	Night	13% 6% 32% 39% 9%	593
	11th to 20th	Night	13% 6% 26% 42% 12%	608
	21st to end	Night	14% 4 25% 39% 16% 2	880
September	1st to 10th	Night	15% 6% 30% 35% 13% 1	800
	11th to 20th	Night	16% 7% 25% 31% 18% 3	800
	21st to end	Night	23% 7% 27% 30% 13% 2	800
October	1st to 10th	Night	23% 9% 26% 27% 14% 2	800
	11th to 20th	Night	19% 12% 33% 24% 12% 1	800
	21st to end	Night	24% 13% 35% 20% 7%	880
November	1st to 10th	Night	19% 15% 33% 24% 9% 1	820
	11th to 20th	Night	20% 22% 34% 18% 7%	829
	21st to end	Night	21% 21% 33% 21% 4 1	800
December	1st to 10th	Night	25% 18% 33% 18% 5%	800
	11th to 20th	Night	25% 20% 32% 20% 3	800
	21st to end	Night	25% 20% 34% 19% 2	880

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