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 2024-11-08.

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 9% 5% 5% 4 4 5% 6% 32%	101283
	Night	14% 12% 5% 10% 6% 5% 4 3 5% 6% 30%	97902
Transparency	Day	51% 5% 10% 21% 12% 1	104805
	Night	53% 6% 11% 20% 9% 1	102468
Seeing	Day	21% 18% 31% 19% 8% 3	15588
	Night	21% 15% 31% 25% 8% 1	25102

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	13% 21% 10% 19% 7% 5% 4 3 3 4 12%	2527
	11th to 20th	Day	12% 19% 8% 18% 13% 9% 5% 3 3 4 7%	2530
	21st to end	Day	18% 22% 9% 13% 6% 5% 5% 2 4 5% 11%	2783
February	1st to 10th	Day	10% 17% 9% 14% 12% 8% 3 3 4 5% 15%	1681
	11th to 20th	Day	11% 16% 7% 16% 8% 5% 5% 4 5% 4 20%	1783
	21st to end	Day	11% 19% 7% 13% 8% 5% 4 3 5% 5% 20%	1570
March	1st to 10th	Day	12% 13% 4 10% 5% 4 4 4 5% 5% 35%	3118
	11th to 20th	Day	9% 14% 7% 12% 6% 5% 5% 4 5% 5% 28%	3466
	21st to end	Day	16% 13% 8% 13% 6% 4 5% 3 4 5% 23%	3846
April	1st to 10th	Day	17% 17% 4 9% 5% 4 4 4 5% 6% 27%	2280
	11th to 20th	Day	19% 15% 3 6% 3 4 4 4 5% 6% 32%	2402
	21st to end	Day	17% 12% 4 8% 5% 4 4 3 5% 6% 32%	2470
May	1st to 10th	Day	15% 9% 3 7% 5% 4 4 4 5% 7% 37%	4108
	11th to 20th	Day	12% 6% 3 6% 5% 4 5% 4 5% 7% 45%	4131
	21st to end	Day	13% 6% 2 6% 4 5% 4 3 6% 7% 45%	4960
June	1st to 10th	Day	14% 6% 3 6% 5% 4 4 4 6% 8% 41%	2850
	11th to 20th	Day	13% 6% 2 6% 5% 5% 6% 4 7% 8% 38%	2850
	21st to end	Day	15% 7% 3 6% 5% 4 4 4 5% 7% 39%	2850
July	1st to 10th	Day	17% 8% 3 7% 4 4 5% 4 5% 8% 37%	4547
	11th to 20th	Day	9% 5% 2 6% 5% 4 6% 5% 6% 8% 44%	4440
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 4 6% 9% 44%	4814
August	1st to 10th	Day	9% 6% 2 7% 5% 6% 6% 5% 7% 9% 38%	4150
	11th to 20th	Day	15% 6% 2 6% 4 4 4 3 5% 6% 43%	4044
	21st to end	Day	14% 6% 3 7% 5% 4 5% 4 5% 6% 42%	4473
September	1st to 10th	Day	13% 7% 2 6% 4 5% 5% 4 5% 8% 40%	2214
	11th to 20th	Day	12% 6% 2 5% 4 5% 4 4 6% 8% 44%	2090
	21st to end	Day	17% 8% 2 7% 4 3 4 4 5% 8% 36%	2090
October	1st to 10th	Day	19% 10% 3 7% 5% 4 3 3 5% 7% 35%	2056
	11th to 20th	Day	23% 14% 4 9% 5% 5% 4 4 5% 5% 22%	1843
	21st to end	Day	20% 18% 5% 10% 6% 5% 5% 3 5% 6% 18%	1881
November	1st to 10th	Day	15% 18% 5% 8% 5% 4 4 4 5% 7% 24%	1735
	11th to 20th	Day	13% 19% 7% 11% 7% 5% 5% 4 4 5% 18%	1691
	21st to end	Day	17% 20% 8% 12% 5% 4 4 3 5% 7% 15%	1520
December	1st to 10th	Day	13% 22% 10% 16% 7% 5% 4 3 4 4 12%	1344
	11th to 20th	Day	15% 18% 8% 16% 8% 6% 3 3 4 4 13%	1671
	21st to end	Day	22% 25% 9% 18% 6% 3 4 2 3 3 7%	2475
January	1st to 10th	Night	13% 18% 8% 20% 7% 6% 4 3 4 4 14%	4428
	11th to 20th	Night	12% 16% 8% 20% 12% 6% 5% 3 3 4 12%	4440
	21st to end	Night	14% 20% 9% 15% 9% 6% 4 3 4 4 10%	4884
February	1st to 10th	Night	9% 18% 6% 15% 9% 7% 5% 3 5% 5% 18%	2498
	11th to 20th	Night	10% 14% 8% 14% 8% 5% 5% 4 5% 5% 22%	2282
	21st to end	Night	14% 17% 5% 11% 6% 5% 4 4 5% 4 24%	1884
March	1st to 10th	Night	14% 11% 3 7% 3 4 3 3 5% 6% 40%	3722
	11th to 20th	Night	8% 14% 6% 8% 5% 4 4 3 6% 6% 36%	3492
	21st to end	Night	13% 12% 7% 11% 5% 4 4 3 5% 6% 30%	3802
April	1st to 10th	Night	18% 14% 3 7% 5% 4 5% 3 5% 6% 29%	1900
	11th to 20th	Night	18% 10% 3 7% 4 4 3 4 6% 7% 34%	1725
	21st to end	Night	16% 10% 3 6% 5% 6% 4 3 5% 6% 36%	1710
May	1st to 10th	Night	14% 9% 2 7% 6% 5% 4 4 4 6% 39%	2833
	11th to 20th	Night	12% 7% 3 7% 4 4 5% 4 6% 7% 42%	2761
	21st to end	Night	16% 6% 2 6% 4 3 4 3 6% 6% 44%	2622
June	1st to 10th	Night	14% 6% 3 6% 4 4 4 3 6% 8% 42%	1330
	11th to 20th	Night	9% 5% 2 7% 5% 4 6% 5% 7% 8% 43%	1330
	21st to end	Night	13% 7% 2 5% 4 3 3 4 5% 8% 46%	1330
July	1st to 10th	Night	11% 7% 2 6% 5% 4 4 4 6% 8% 44%	2418
	11th to 20th	Night	10% 6% 1 5% 4 4 4 3 5% 6% 51%	2530
	21st to end	Night	9% 5% 2 5% 3 4 4 4 6% 8% 50%	2750
August	1st to 10th	Night	11% 6% 2 6% 4 5% 4 4 5% 7% 46%	2778
	11th to 20th	Night	17% 5% 1 4 3 3 3 3 5% 7% 49%	2785
	21st to end	Night	11% 5% 1 5% 4 4 5% 4 5% 7% 49%	3180
September	1st to 10th	Night	12% 6% 2 6% 4 4 4 3 5% 7% 49%	1947
	11th to 20th	Night	14% 5% 2 6% 4 4 3 3 4 7% 49%	2090
	21st to end	Night	18% 6% 2 6% 5% 4 4 3 5% 8% 39%	2090
October	1st to 10th	Night	18% 7% 2 6% 4 3 4 3 5% 6% 40%	2123
	11th to 20th	Night	21% 14% 3 7% 4 4 4 4 5% 6% 28%	2335
	21st to end	Night	23% 13% 3 9% 5% 4 4 3 5% 5% 26%	2717
November	1st to 10th	Night	14% 16% 4 7% 6% 5% 4 4 4 7% 28%	2504
	11th to 20th	Night	13% 16% 6% 11% 6% 6% 5% 4 5% 7% 22%	2486
	21st to end	Night	17% 19% 7% 11% 5% 5% 4 3 5% 5% 19%	2660
December	1st to 10th	Night	13% 19% 8% 15% 7% 6% 5% 4 5% 6% 14%	2835
	11th to 20th	Night	13% 19% 7% 17% 8% 5% 6% 4 4 5% 13%	3526
	21st to end	Night	22% 25% 10% 14% 5% 4 3 3 3 3 7%	5175

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% 8% 9% 8% 2	2528
	11th to 20th	Day	80% 7% 4 7% 2	2530
	21st to end	Day	76% 4 11% 5% 2	2783
February	1st to 10th	Day	71% 5% 9% 12% 3	1681
	11th to 20th	Day	66% 5% 8% 12% 8% 1	1792
	21st to end	Day	65% 5% 9% 15% 6%	1570
March	1st to 10th	Day	51% 4 13% 22% 9% 1	3120
	11th to 20th	Day	56% 5% 11% 20% 6% 2	3466
	21st to end	Day	62% 6% 8% 14% 8% 1	3846
April	1st to 10th	Day	58% 5% 9% 18% 9% 1	2280
	11th to 20th	Day	52% 4 9% 19% 14% 1	2402
	21st to end	Day	53% 5% 9% 16% 14% 3	2470
May	1st to 10th	Day	44% 6% 9% 20% 19% 1	4120
	11th to 20th	Day	38% 6% 11% 27% 15% 2	4135
	21st to end	Day	37% 6% 11% 20% 22% 4	4960
June	1st to 10th	Day	41% 5% 10% 27% 15% 3	2850
	11th to 20th	Day	41% 7% 11% 22% 17% 2	2850
	21st to end	Day	43% 5% 12% 25% 13% 2	2850
July	1st to 10th	Day	46% 5% 11% 21% 15% 1	4478
	11th to 20th	Day	35% 6% 10% 33% 14% 1	4365
	21st to end	Day	36% 6% 11% 25% 21% 1	4662
August	1st to 10th	Day	39% 7% 13% 26% 14% 1	4135
	11th to 20th	Day	41% 5% 11% 28% 14% 1	4083
	21st to end	Day	42% 5% 13% 29% 10% 1	4472
September	1st to 10th	Day	43% 5% 11% 24% 15% 1	2214
	11th to 20th	Day	38% 5% 12% 30% 14% 1	2090
	21st to end	Day	46% 6% 13% 25% 10%	2090
October	1st to 10th	Day	36% 4 8% 34% 17% 1	3420
	11th to 20th	Day	56% 5% 7% 19% 13%	3046
	21st to end	Day	63% 4 10% 18% 5%	3081
November	1st to 10th	Day	58% 6% 10% 19% 7%	1735
	11th to 20th	Day	66% 7% 11% 11% 5%	1691
	21st to end	Day	70% 6% 12% 10% 3	1520
December	1st to 10th	Day	76% 6% 7% 10% 2 1	1344
	11th to 20th	Day	74% 6% 9% 9% 2	1671
	21st to end	Day	86% 3 5% 4 2	2475
January	1st to 10th	Night	72% 7% 8% 10% 3	4428
	11th to 20th	Night	76% 5% 6% 9% 3	4440
	21st to end	Night	76% 6% 8% 7% 2	4884
February	1st to 10th	Night	68% 5% 9% 12% 5%	2498
	11th to 20th	Night	62% 6% 10% 16% 6% 1	2284
	21st to end	Night	61% 5% 12% 14% 7%	1884
March	1st to 10th	Night	45% 4 16% 25% 10% 1	3733
	11th to 20th	Night	48% 5% 13% 24% 9% 1	3493
	21st to end	Night	55% 5% 11% 19% 9% 1	3802
April	1st to 10th	Night	53% 7% 8% 20% 10% 2	1900
	11th to 20th	Night	49% 5% 8% 23% 13% 2	1725
	21st to end	Night	49% 5% 11% 17% 14% 3	1710
May	1st to 10th	Night	45% 6% 10% 21% 15% 2	2840
	11th to 20th	Night	38% 7% 11% 27% 16% 1	2762
	21st to end	Night	40% 5% 14% 20% 16% 4	2623
June	1st to 10th	Night	40% 6% 11% 26% 16% 1	1330
	11th to 20th	Night	36% 7% 13% 26% 15% 3	1330
	21st to end	Night	36% 6% 13% 26% 15% 2	1330
July	1st to 10th	Night	38% 4 15% 25% 15% 3	2378
	11th to 20th	Night	32% 5% 12% 33% 17% 1	2482
	21st to end	Night	33% 6% 16% 26% 20% 1	2662
August	1st to 10th	Night	37% 5% 15% 28% 14% 1	2769
	11th to 20th	Night	34% 4 17% 31% 13%	2812
	21st to end	Night	34% 5% 15% 31% 15% 1	3180
September	1st to 10th	Night	35% 6% 13% 28% 17% 1	1947
	11th to 20th	Night	35% 6% 14% 32% 13%	2090
	21st to end	Night	44% 6% 15% 25% 10%	2090
October	1st to 10th	Night	36% 4 12% 33% 15%	3539
	11th to 20th	Night	48% 5% 10% 23% 14%	3871
	21st to end	Night	54% 5% 14% 22% 5%	4467
November	1st to 10th	Night	55% 6% 12% 20% 7% 1	2504
	11th to 20th	Night	61% 6% 12% 15% 6%	2486
	21st to end	Night	66% 7% 13% 11% 3	2660
December	1st to 10th	Night	70% 7% 9% 11% 2	2835
	11th to 20th	Night	72% 8% 9% 8% 3	3525
	21st to end	Night	83% 5% 5% 5% 3	5175

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	24% 21% 40% 12% 3	270
	11th to 20th	Day	26% 20% 39% 11% 3 1	270
	21st to end	Day	32% 18% 36% 9% 5% 1	297
February	1st to 10th	Day	23% 31% 34% 8% 4 1	270
	11th to 20th	Day	24% 35% 30% 9% 1	270
	21st to end	Day	32% 29% 34% 3 2	225
March	1st to 10th	Day	24% 32% 31% 11% 1	270
	11th to 20th	Day	25% 21% 35% 14% 4 1	446
	21st to end	Day	31% 19% 28% 17% 5% 1	506
April	1st to 10th	Day	30% 21% 29% 14% 5%	460
	11th to 20th	Day	33% 19% 25% 13% 7% 2	452
	21st to end	Day	27% 18% 32% 15% 6% 2	460
May	1st to 10th	Day	26% 21% 30% 14% 6% 3	463
	11th to 20th	Day	17% 20% 32% 20% 7% 3	511
	21st to end	Day	16% 16% 33% 24% 8% 2	734
June	1st to 10th	Day	17% 21% 35% 20% 6% 1	680
	11th to 20th	Day	18% 15% 36% 21% 6% 2	680
	21st to end	Day	19% 13% 37% 22% 7% 2	680
July	1st to 10th	Day	13% 13% 35% 25% 11% 3	671
	11th to 20th	Day	14% 13% 32% 26% 11% 4	668
	21st to end	Day	13% 12% 33% 27% 10% 5%	739
August	1st to 10th	Day	13% 9% 28% 28% 13% 8%	520
	11th to 20th	Day	14% 8% 23% 29% 15% 11%	479
	21st to end	Day	18% 11% 27% 27% 13% 4	539
September	1st to 10th	Day	18% 12% 27% 23% 14% 6%	490
	11th to 20th	Day	15% 9% 27% 27% 15% 7%	490
	21st to end	Day	22% 12% 23% 26% 12% 5%	490
October	1st to 10th	Day	25% 14% 29% 18% 12% 2	490
	11th to 20th	Day	28% 20% 33% 15% 3 1	433
	21st to end	Day	32% 25% 32% 8% 3	330
November	1st to 10th	Day	29% 22% 31% 14% 3	291
	11th to 20th	Day	25% 28% 31% 12% 3 1	261
	21st to end	Day	25% 29% 31% 13% 1 1	180
December	1st to 10th	Day	28% 23% 33% 10% 6%	180
	11th to 20th	Day	26% 28% 36% 8% 2	180
	21st to end	Day	31% 31% 24% 10% 2 1	213
January	1st to 10th	Night	23% 22% 33% 18% 5%	950
	11th to 20th	Night	26% 20% 29% 19% 6% 1	950
	21st to end	Night	20% 15% 35% 23% 5% 1	1045
February	1st to 10th	Night	19% 23% 34% 20% 4	788
	11th to 20th	Night	21% 23% 32% 19% 5%	743
	21st to end	Night	26% 18% 35% 19% 2	628
March	1st to 10th	Night	18% 21% 32% 23% 5% 1	760
	11th to 20th	Night	22% 20% 33% 19% 6%	760
	21st to end	Night	22% 14% 32% 26% 6%	833
April	1st to 10th	Night	25% 15% 34% 21% 4 1	570
	11th to 20th	Night	26% 14% 29% 24% 6% 1	563
	21st to end	Night	23% 12% 32% 26% 7% 1	570
May	1st to 10th	Night	15% 12% 36% 29% 8% 1	570
	11th to 20th	Night	18% 12% 32% 29% 8%	561
	21st to end	Night	12% 10% 33% 33% 12% 1	541
June	1st to 10th	Night	14% 11% 33% 31% 10% 1	380
	11th to 20th	Night	10% 10% 33% 37% 11%	380
	21st to end	Night	15% 8% 29% 37% 11% 1	380
July	1st to 10th	Night	7% 4 30% 42% 16% 1	482
	11th to 20th	Night	10% 7% 26% 33% 22% 2	552
	21st to end	Night	9% 6% 24% 42% 17% 1	624
August	1st to 10th	Night	11% 4 26% 37% 21%	567
	11th to 20th	Night	13% 5% 20% 34% 24% 4	564
	21st to end	Night	15% 7% 24% 36% 16% 2	627
September	1st to 10th	Night	13% 9% 26% 29% 19% 4	617
	11th to 20th	Night	16% 8% 24% 33% 17% 3	760
	21st to end	Night	21% 8% 24% 30% 15% 3	760
October	1st to 10th	Night	21% 12% 29% 26% 10% 1	760
	11th to 20th	Night	27% 16% 32% 19% 7%	760
	21st to end	Night	31% 14% 30% 20% 4 1	836
November	1st to 10th	Night	25% 16% 33% 20% 6% 1	771
	11th to 20th	Night	24% 19% 34% 19% 5%	760
	21st to end	Night	28% 16% 40% 14% 3	760
December	1st to 10th	Night	23% 20% 34% 17% 5%	935
	11th to 20th	Night	23% 24% 32% 17% 3 1	950
	21st to end	Night	29% 18% 33% 16% 3	1045

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