Stellamar Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stellamar 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	14% 11% 4 10% 5% 4 4 4 5% 6% 32%	126335
	Night	14% 13% 5% 11% 6% 5% 4 3 5% 6% 29%	124198
Transparency	Day	52% 5% 10% 21% 12% 1	129840
	Night	55% 5% 11% 19% 10% 1	128780
Seeing	Day	22% 18% 32% 19% 8% 3	16968
	Night	21% 15% 31% 25% 8% 1	26465

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	10% 16% 9% 25% 8% 4 3 3 3 4 13%	3617
	11th to 20th	Day	10% 19% 8% 24% 11% 8% 5% 3 3 3 6%	3589
	21st to end	Day	14% 18% 9% 18% 7% 5% 5% 3 4 5% 12%	3982
February	1st to 10th	Day	10% 17% 8% 14% 12% 7% 3 3 5% 5% 15%	1770
	11th to 20th	Day	11% 16% 7% 16% 8% 6% 5% 3 5% 4 20%	1880
	21st to end	Day	11% 19% 7% 13% 7% 5% 4 3 5% 5% 20%	1650
March	1st to 10th	Day	12% 11% 3 12% 5% 4 4 4 5% 6% 33%	4478
	11th to 20th	Day	11% 14% 5% 10% 5% 5% 6% 5% 6% 6% 27%	4952
	21st to end	Day	20% 13% 8% 12% 5% 4 4 3 4 5% 22%	5510
April	1st to 10th	Day	17% 17% 4 9% 4 4 4 4 5% 6% 27%	2400
	11th to 20th	Day	18% 15% 3 6% 3 4 4 4 5% 6% 33%	2530
	21st to end	Day	16% 12% 4 8% 5% 4 4 3 5% 6% 31%	2600
May	1st to 10th	Day	17% 10% 3 6% 4 4 3 3 5% 7% 38%	5888
	11th to 20th	Day	13% 7% 3 7% 5% 4 5% 3 5% 6% 43%	5925
	21st to end	Day	15% 8% 3 6% 4 5% 4 3 5% 7% 40%	7128
June	1st to 10th	Day	13% 6% 3 6% 5% 4 4 4 6% 8% 41%	3000
	11th to 20th	Day	13% 6% 2 7% 5% 5% 6% 4 7% 8% 37%	3000
	21st to end	Day	15% 7% 3 6% 5% 4 4 4 5% 7% 39%	3000
July	1st to 10th	Day	15% 6% 2 6% 4 4 5% 3 5% 9% 41%	6508
	11th to 20th	Day	11% 5% 2 6% 4 4 5% 4 5% 8% 46%	6360
	21st to end	Day	9% 5% 2 6% 4 4 4 4 6% 10% 46%	6926
August	1st to 10th	Day	9% 6% 2 7% 5% 6% 6% 5% 7% 9% 38%	4282
	11th to 20th	Day	15% 6% 2 6% 4 4 4 3 5% 6% 43%	4174
	21st to end	Day	13% 6% 3 7% 5% 4 5% 4 6% 6% 42%	4614
September	1st to 10th	Day	13% 8% 2 7% 5% 5% 5% 4 5% 8% 39%	2330
	11th to 20th	Day	12% 5% 2 5% 4 5% 5% 4 6% 8% 44%	2200
	21st to end	Day	17% 8% 2 7% 4 4 4 4 5% 8% 36%	2200
October	1st to 10th	Day	18% 9% 3 7% 5% 4 4 3 5% 7% 36%	2164
	11th to 20th	Day	23% 14% 3 8% 5% 5% 4 4 5% 5% 23%	1940
	21st to end	Day	19% 17% 5% 10% 6% 5% 5% 3 5% 6% 20%	1980
November	1st to 10th	Day	15% 18% 5% 8% 5% 4 4 4 5% 8% 24%	1852
	11th to 20th	Day	14% 19% 6% 12% 6% 5% 5% 4 4 5% 18%	1843
	21st to end	Day	17% 20% 9% 12% 5% 4 4 3 5% 6% 15%	1600
December	1st to 10th	Day	13% 26% 9% 17% 6% 5% 4 3 4 3 9%	2294
	11th to 20th	Day	14% 23% 8% 16% 8% 6% 4 3 4 4 11%	2621
	21st to end	Day	22% 23% 9% 17% 5% 3 4 2 4 3 8%	3548
January	1st to 10th	Night	10% 15% 6% 26% 9% 5% 4 3 4 4 13%	6348
	11th to 20th	Night	11% 15% 7% 22% 13% 7% 5% 3 4 4 10%	6316
	21st to end	Night	10% 18% 9% 17% 8% 6% 5% 3 5% 5% 14%	6996
February	1st to 10th	Night	9% 17% 6% 15% 9% 7% 5% 3 5% 5% 17%	2629
	11th to 20th	Night	10% 15% 7% 14% 8% 5% 5% 4 5% 5% 22%	2405
	21st to end	Night	14% 16% 5% 12% 6% 5% 4 4 5% 5% 24%	1980
March	1st to 10th	Night	14% 10% 4 8% 6% 5% 4 3 5% 6% 36%	5362
	11th to 20th	Night	10% 12% 5% 9% 5% 4 5% 4 6% 7% 32%	5006
	21st to end	Night	19% 14% 7% 10% 4 4 3 3 4 5% 29%	5452
April	1st to 10th	Night	17% 14% 3 7% 5% 4 5% 3 5% 6% 30%	2000
	11th to 20th	Night	18% 11% 4 6% 4 4 3 4 5% 7% 34%	1817
	21st to end	Night	16% 11% 3 7% 5% 6% 4 3 5% 6% 35%	1800
May	1st to 10th	Night	14% 10% 3 7% 5% 5% 4 4 4 6% 38%	4053
	11th to 20th	Night	14% 8% 3 6% 4 4 5% 4 5% 7% 41%	3955
	21st to end	Night	18% 9% 2 5% 3 3 3 3 5% 5% 43%	3765
June	1st to 10th	Night	14% 6% 3 6% 4 4 4 3 6% 9% 42%	1400
	11th to 20th	Night	9% 5% 2 7% 5% 4 6% 5% 7% 8% 41%	1400
	21st to end	Night	13% 7% 2 5% 4 3 4 4 5% 8% 45%	1400
July	1st to 10th	Night	11% 6% 2 6% 5% 4 4 4 6% 8% 43%	3452
	11th to 20th	Night	11% 6% 1 4 4 4 4 3 5% 6% 52%	3620
	21st to end	Night	8% 5% 2 5% 3 4 4 4 5% 8% 52%	3948
August	1st to 10th	Night	10% 6% 2 6% 4 5% 4 4 5% 7% 46%	2866
	11th to 20th	Night	17% 5% 1 4 3 3 3 3 5% 7% 49%	2875
	21st to end	Night	11% 5% 1 5% 4 4 5% 4 5% 7% 50%	3281
September	1st to 10th	Night	11% 6% 2 6% 4 4 4 3 5% 7% 48%	2050
	11th to 20th	Night	14% 5% 2 6% 4 3 3 4 4 7% 49%	2200
	21st to end	Night	18% 6% 2 6% 5% 4 4 3 5% 8% 39%	2200
October	1st to 10th	Night	18% 7% 2 6% 4 3 4 3 5% 6% 41%	2235
	11th to 20th	Night	20% 14% 3 7% 5% 5% 4 4 5% 5% 28%	2458
	21st to end	Night	22% 12% 3 8% 5% 4 4 3 5% 5% 28%	2860
November	1st to 10th	Night	14% 16% 4 8% 6% 5% 4 4 4 6% 29%	2666
	11th to 20th	Night	13% 17% 6% 10% 6% 6% 5% 4 5% 7% 22%	2709
	21st to end	Night	17% 20% 7% 11% 5% 4 5% 3 5% 5% 18%	2800
December	1st to 10th	Night	12% 19% 8% 15% 6% 6% 5% 4 5% 6% 14%	4895
	11th to 20th	Night	12% 23% 7% 17% 7% 5% 4 3 3 4 13%	5586
	21st to end	Night	22% 23% 11% 14% 5% 4 3 2 3 3 9%	7413

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% 7% 9% 3 2	3618
	11th to 20th	Day	83% 6% 3 6% 2	3620
	21st to end	Day	74% 4 11% 7% 4	3982
February	1st to 10th	Day	70% 5% 9% 12% 3	1770
	11th to 20th	Day	67% 5% 8% 11% 8% 1	1889
	21st to end	Day	65% 5% 9% 15% 6%	1650
March	1st to 10th	Day	52% 4 15% 17% 11% 3	4480
	11th to 20th	Day	56% 4 12% 18% 8% 1	4952
	21st to end	Day	64% 5% 7% 16% 8% 1	5510
April	1st to 10th	Day	58% 5% 9% 19% 9% 1	2400
	11th to 20th	Day	52% 4 9% 19% 14% 1	2530
	21st to end	Day	53% 5% 9% 17% 14% 3	2600
May	1st to 10th	Day	46% 5% 8% 20% 19% 3	5897
	11th to 20th	Day	42% 5% 9% 23% 18% 3	5929
	21st to end	Day	44% 5% 8% 20% 19% 4	7128
June	1st to 10th	Day	41% 5% 10% 27% 15% 2	3000
	11th to 20th	Day	42% 7% 11% 22% 16% 2	3000
	21st to end	Day	44% 5% 12% 24% 13% 2	3000
July	1st to 10th	Day	41% 5% 11% 26% 16% 1	6440
	11th to 20th	Day	36% 5% 9% 35% 13% 1	6285
	21st to end	Day	34% 5% 11% 31% 18% 1	6728
August	1st to 10th	Day	39% 7% 13% 27% 14% 1	4267
	11th to 20th	Day	41% 5% 11% 28% 14% 1	4213
	21st to end	Day	42% 5% 13% 29% 10% 1	4613
September	1st to 10th	Day	43% 5% 12% 24% 15% 1	2330
	11th to 20th	Day	37% 5% 13% 30% 14% 1	2200
	21st to end	Day	46% 6% 13% 24% 11%	2200
October	1st to 10th	Day	36% 4 8% 34% 18% 1	3528
	11th to 20th	Day	55% 5% 7% 20% 13%	3143
	21st to end	Day	62% 4 10% 18% 6%	3180
November	1st to 10th	Day	57% 6% 10% 20% 7%	1852
	11th to 20th	Day	65% 6% 11% 12% 5%	1843
	21st to end	Day	70% 5% 12% 10% 3	1600
December	1st to 10th	Day	80% 4 7% 8% 1	2294
	11th to 20th	Day	77% 4 8% 9% 3	2621
	21st to end	Day	83% 3 4 6% 5%	3548
January	1st to 10th	Night	74% 5% 7% 9% 4 1	6348
	11th to 20th	Night	78% 5% 7% 8% 3	6360
	21st to end	Night	71% 5% 7% 12% 5%	6996
February	1st to 10th	Night	68% 5% 9% 12% 5%	2629
	11th to 20th	Night	62% 6% 10% 16% 6% 1	2407
	21st to end	Night	61% 5% 12% 15% 6%	1980
March	1st to 10th	Night	50% 3 16% 20% 11% 1	5373
	11th to 20th	Night	50% 5% 19% 19% 7% 1	5007
	21st to end	Night	58% 4 9% 16% 11% 1	5452
April	1st to 10th	Night	53% 6% 8% 20% 11% 2	2000
	11th to 20th	Night	49% 5% 7% 23% 13% 2	1817
	21st to end	Night	50% 5% 11% 17% 14% 3	1800
May	1st to 10th	Night	46% 5% 9% 19% 18% 3	4060
	11th to 20th	Night	42% 6% 9% 25% 17% 1	3956
	21st to end	Night	44% 4 11% 23% 14% 4	3766
June	1st to 10th	Night	39% 6% 11% 26% 17% 2	1400
	11th to 20th	Night	38% 7% 13% 26% 14% 3	1400
	21st to end	Night	38% 6% 13% 25% 15% 3	1400
July	1st to 10th	Night	38% 5% 15% 26% 15% 2	3412
	11th to 20th	Night	33% 5% 10% 33% 18%	3572
	21st to end	Night	31% 5% 15% 31% 18%	3832
August	1st to 10th	Night	37% 5% 15% 28% 14% 1	2857
	11th to 20th	Night	34% 4 17% 31% 13%	2902
	21st to end	Night	34% 5% 15% 31% 14% 1	3281
September	1st to 10th	Night	35% 6% 14% 27% 16% 1	2050
	11th to 20th	Night	35% 6% 15% 31% 13%	2200
	21st to end	Night	44% 6% 15% 25% 11%	2200
October	1st to 10th	Night	35% 4 12% 33% 15%	3651
	11th to 20th	Night	48% 5% 10% 23% 14%	3994
	21st to end	Night	53% 5% 13% 23% 6%	4610
November	1st to 10th	Night	55% 6% 12% 20% 7% 1	2666
	11th to 20th	Night	61% 6% 12% 15% 6% 1	2709
	21st to end	Night	67% 7% 13% 11% 3	2800
December	1st to 10th	Night	69% 6% 13% 10% 2	4895
	11th to 20th	Night	75% 5% 7% 8% 4	5585
	21st to end	Night	82% 4 4 7% 3	7413

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	22% 22% 42% 11% 3	300
	11th to 20th	Day	26% 21% 39% 11% 3 1	300
	21st to end	Day	30% 19% 36% 9% 5% 1	330
February	1st to 10th	Day	25% 30% 33% 8% 4 1	300
	11th to 20th	Day	26% 35% 30% 8% 1	300
	21st to end	Day	33% 27% 35% 3 2	249
March	1st to 10th	Day	26% 30% 33% 10% 1	300
	11th to 20th	Day	27% 21% 34% 14% 4 1	485
	21st to end	Day	32% 19% 27% 16% 5% 1	550
April	1st to 10th	Day	32% 21% 28% 14% 5% 1	500
	11th to 20th	Day	33% 20% 25% 13% 7% 2	492
	21st to end	Day	27% 19% 31% 15% 5% 2	500
May	1st to 10th	Day	26% 21% 31% 13% 6% 3	503
	11th to 20th	Day	18% 21% 31% 20% 7% 3	552
	21st to end	Day	19% 16% 32% 23% 8% 2	789
June	1st to 10th	Day	17% 20% 36% 19% 6% 1	730
	11th to 20th	Day	19% 15% 37% 21% 6% 2	730
	21st to end	Day	19% 13% 38% 21% 7% 2	730
July	1st to 10th	Day	13% 12% 35% 25% 11% 3	721
	11th to 20th	Day	15% 13% 33% 26% 10% 3	718
	21st to end	Day	13% 12% 34% 27% 10% 4	794
August	1st to 10th	Day	12% 8% 28% 30% 14% 8%	562
	11th to 20th	Day	15% 7% 24% 29% 14% 10%	519
	21st to end	Day	17% 11% 28% 26% 13% 5%	583
September	1st to 10th	Day	18% 12% 28% 22% 14% 6%	530
	11th to 20th	Day	14% 8% 25% 28% 16% 9%	530
	21st to end	Day	21% 11% 24% 27% 12% 5%	530
October	1st to 10th	Day	24% 14% 29% 19% 12% 2	530
	11th to 20th	Day	28% 19% 34% 15% 3 1	470
	21st to end	Day	29% 24% 33% 10% 4	363
November	1st to 10th	Day	29% 24% 30% 14% 3	330
	11th to 20th	Day	27% 28% 29% 12% 3 1	311
	21st to end	Day	25% 30% 32% 13% 1 1	200
December	1st to 10th	Day	30% 22% 33% 10% 7%	200
	11th to 20th	Day	28% 27% 35% 9% 2	200
	21st to end	Day	34% 28% 25% 10% 3 1	237
January	1st to 10th	Night	22% 22% 33% 18% 5%	1000
	11th to 20th	Night	26% 20% 29% 19% 6% 1	1000
	21st to end	Night	20% 16% 35% 23% 5% 1	1100
February	1st to 10th	Night	19% 24% 34% 19% 3	829
	11th to 20th	Night	22% 24% 31% 18% 5%	783
	21st to end	Night	25% 19% 35% 19% 2	660
March	1st to 10th	Night	19% 21% 33% 22% 5% 1	800
	11th to 20th	Night	22% 21% 33% 19% 6%	800
	21st to end	Night	23% 14% 32% 25% 5%	877
April	1st to 10th	Night	25% 16% 34% 21% 4 1	600
	11th to 20th	Night	27% 14% 28% 24% 6% 1	593
	21st to end	Night	23% 13% 31% 26% 7% 1	600
May	1st to 10th	Night	15% 12% 36% 29% 7% 1	600
	11th to 20th	Night	19% 12% 31% 29% 8%	591
	21st to end	Night	14% 10% 32% 32% 12% 1	570
June	1st to 10th	Night	15% 11% 34% 31% 10% 1	400
	11th to 20th	Night	10% 10% 34% 36% 11%	400
	21st to end	Night	16% 8% 29% 37% 10% 1	400
July	1st to 10th	Night	7% 4 30% 42% 16% 1	508
	11th to 20th	Night	10% 7% 26% 35% 21% 2	582
	21st to end	Night	9% 6% 25% 42% 17% 1	657
August	1st to 10th	Night	11% 4 26% 37% 21%	597
	11th to 20th	Night	13% 5% 19% 36% 23% 4	594
	21st to end	Night	15% 7% 23% 36% 17% 2	660
September	1st to 10th	Night	13% 9% 26% 29% 19% 4	650
	11th to 20th	Night	16% 7% 23% 33% 19% 3	800
	21st to end	Night	20% 8% 24% 30% 15% 3	800
October	1st to 10th	Night	21% 11% 30% 27% 11% 1	800
	11th to 20th	Night	27% 15% 32% 20% 7%	800
	21st to end	Night	30% 13% 30% 22% 4 1	880
November	1st to 10th	Night	26% 16% 32% 20% 5% 1	820
	11th to 20th	Night	25% 20% 33% 18% 4	829
	21st to end	Night	28% 16% 40% 14% 2	800
December	1st to 10th	Night	23% 20% 34% 18% 5%	985
	11th to 20th	Night	24% 25% 32% 16% 3 1	1000
	21st to end	Night	29% 18% 33% 16% 3	1100

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