Hector J. Robinson Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hector J. Robinson Observatory clear sky chart from 2005-11-06 to 2025-05-26.

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% 10% 4 9% 5% 5% 5% 4 6% 7% 32%	113549
	Night	14% 11% 4 8% 5% 4 4 4 5% 6% 34%	115763
Transparency	Day	29% 43% 8% 12% 7% 1	117165
	Night	30% 41% 7% 13% 8% 1	120274
Seeing	Day	18% 12% 29% 23% 13% 5%	16046
	Night	19% 10% 28% 29% 12% 1	25970

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	11% 14% 7% 16% 7% 5% 4 3 4 5% 23%	3617
	11th to 20th	Day	14% 16% 5% 17% 8% 7% 8% 4 5% 6% 11%	3660
	21st to end	Day	22% 16% 7% 13% 5% 4 6% 4 5% 4 14%	4466
February	1st to 10th	Day	15% 15% 5% 13% 7% 6% 5% 4 5% 5% 21%	1800
	11th to 20th	Day	14% 13% 4 12% 7% 7% 5% 3 5% 6% 24%	1884
	21st to end	Day	17% 12% 4 11% 5% 4 4 4 6% 6% 26%	1650
March	1st to 10th	Day	16% 12% 5% 11% 6% 4 4 3 4 5% 29%	4613
	11th to 20th	Day	12% 11% 4 9% 5% 5% 4 3 6% 7% 34%	4980
	21st to end	Day	17% 14% 4 10% 5% 4 4 3 4 6% 27%	5465
April	1st to 10th	Day	17% 12% 3 7% 5% 4 5% 5% 5% 7% 30%	2305
	11th to 20th	Day	17% 9% 3 7% 5% 4 5% 4 5% 7% 33%	2542
	21st to end	Day	18% 9% 3 8% 4 4 5% 4 4 6% 36%	2600
May	1st to 10th	Day	17% 9% 3 8% 5% 5% 5% 4 5% 7% 33%	4212
	11th to 20th	Day	16% 8% 2 7% 5% 5% 5% 4 6% 8% 34%	4211
	21st to end	Day	10% 6% 2 7% 4 5% 5% 4 5% 8% 45%	4541
June	1st to 10th	Day	10% 6% 2 6% 6% 5% 5% 5% 7% 8% 39%	2637
	11th to 20th	Day	10% 6% 2 6% 5% 4 6% 5% 6% 9% 40%	2843
	21st to end	Day	9% 7% 2 6% 5% 5% 6% 4 8% 10% 37%	2660
July	1st to 10th	Day	10% 6% 2 6% 5% 5% 5% 5% 7% 10% 39%	4440
	11th to 20th	Day	7% 4 2 6% 5% 5% 6% 4 8% 11% 44%	4440
	21st to end	Day	6% 5% 2 7% 5% 5% 5% 5% 8% 9% 41%	4476
August	1st to 10th	Day	11% 5% 2 6% 5% 4 6% 5% 6% 9% 41%	4104
	11th to 20th	Day	9% 6% 2 7% 5% 5% 6% 5% 9% 11% 35%	4044
	21st to end	Day	8% 5% 2 6% 6% 6% 6% 4 8% 10% 38%	4505
September	1st to 10th	Day	9% 7% 2 7% 4 5% 5% 3 7% 7% 42%	2136
	11th to 20th	Day	8% 5% 2 5% 4 4 5% 5% 6% 9% 47%	2090
	21st to end	Day	15% 7% 2 7% 5% 5% 4 4 6% 8% 38%	2090
October	1st to 10th	Day	12% 6% 3 7% 5% 4 4 3 6% 9% 42%	2029
	11th to 20th	Day	16% 8% 2 8% 6% 5% 5% 4 5% 8% 34%	1900
	21st to end	Day	18% 10% 2 7% 6% 6% 4 3 5% 6% 31%	1956
November	1st to 10th	Day	12% 6% 3 9% 5% 4 5% 3 6% 6% 41%	1762
	11th to 20th	Day	14% 13% 3 8% 6% 5% 4 3 5% 8% 31%	1800
	21st to end	Day	18% 12% 5% 9% 4 5% 5% 3 5% 6% 27%	1696
December	1st to 10th	Day	13% 13% 6% 14% 6% 5% 6% 4 5% 6% 21%	2600
	11th to 20th	Day	20% 19% 5% 9% 5% 5% 4 3 5% 6% 18%	2859
	21st to end	Day	27% 18% 8% 13% 5% 4 3 2 3 4 12%	3936
January	1st to 10th	Night	14% 15% 6% 11% 6% 5% 5% 4 5% 5% 26%	6348
	11th to 20th	Night	13% 17% 8% 14% 7% 6% 5% 4 6% 5% 15%	6314
	21st to end	Night	19% 16% 7% 10% 5% 4 4 3 3 5% 24%	6490
February	1st to 10th	Night	17% 15% 4 11% 7% 5% 5% 4 5% 5% 23%	2600
	11th to 20th	Night	15% 13% 4 10% 6% 5% 5% 4 5% 6% 27%	2397
	21st to end	Night	19% 12% 4 10% 5% 5% 4 3 5% 5% 29%	1980
March	1st to 10th	Night	15% 12% 4 8% 4 4 4 3 5% 6% 35%	5238
	11th to 20th	Night	10% 8% 3 7% 4 4 5% 4 6% 8% 41%	4980
	21st to end	Night	16% 9% 4 8% 5% 5% 4 3 4 6% 37%	5467
April	1st to 10th	Night	17% 11% 2 7% 5% 4 3 4 5% 6% 36%	2110
	11th to 20th	Night	17% 8% 2 6% 4 3 4 4 7% 7% 37%	1806
	21st to end	Night	16% 8% 2 6% 4 4 4 5% 5% 7% 39%	1800
May	1st to 10th	Night	17% 9% 3 7% 5% 4 4 3 5% 7% 35%	2903
	11th to 20th	Night	15% 6% 3 7% 5% 5% 5% 3 5% 8% 38%	2902
	21st to end	Night	9% 6% 2 6% 4 4 4 3 5% 7% 48%	3130
June	1st to 10th	Night	12% 5% 2 3 4 3 5% 4 5% 10% 46%	1525
	11th to 20th	Night	8% 5% 3 7% 5% 4 5% 5% 6% 9% 43%	1330
	21st to end	Night	10% 5% 2 6% 4 4 5% 3 5% 9% 47%	1512
July	1st to 10th	Night	8% 3 1 5% 4 3 5% 4 6% 10% 50%	2530
	11th to 20th	Night	8% 3 2 5% 4 4 4 4 7% 9% 51%	2530
	21st to end	Night	4 4 2 6% 4 5% 5% 5% 8% 9% 47%	3076
August	1st to 10th	Night	12% 6% 3 6% 4 4 4 3 6% 7% 45%	2825
	11th to 20th	Night	7% 6% 3 6% 4 4 5% 4 5% 7% 50%	2785
	21st to end	Night	7% 5% 2 6% 4 5% 5% 4 7% 8% 48%	3148
September	1st to 10th	Night	9% 5% 2 6% 5% 4 6% 4 5% 6% 48%	2024
	11th to 20th	Night	9% 4 2 4 3 3 4 3 6% 7% 55%	2090
	21st to end	Night	14% 7% 3 6% 4 4 4 4 5% 6% 42%	2090
October	1st to 10th	Night	13% 7% 2 6% 4 4 4 3 5% 7% 46%	2149
	11th to 20th	Night	16% 9% 3 8% 4 4 4 4 4 7% 37%	2280
	21st to end	Night	20% 8% 2 7% 5% 5% 4 4 5% 5% 34%	2640
November	1st to 10th	Night	13% 7% 2 6% 4 4 5% 3 5% 7% 43%	2536
	11th to 20th	Night	16% 11% 3 8% 6% 4 4 4 7% 7% 31%	2600
	21st to end	Night	20% 14% 4 8% 4 4 3 3 5% 6% 28%	2699
December	1st to 10th	Night	13% 13% 6% 13% 6% 6% 5% 3 5% 6% 24%	4580
	11th to 20th	Night	16% 17% 6% 12% 6% 5% 4 3 4 6% 20%	5325
	21st to end	Night	27% 20% 6% 10% 5% 3 4 3 4 4 15%	7024

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	47% 28% 9% 11% 5% 1	3618
	11th to 20th	Day	56% 28% 6% 5% 4	3660
	21st to end	Day	52% 30% 8% 6% 4	4466
February	1st to 10th	Day	25% 62% 3 7% 3	1800
	11th to 20th	Day	25% 61% 3 8% 3	1894
	21st to end	Day	19% 62% 5% 9% 3 1	1650
March	1st to 10th	Day	42% 31% 9% 12% 6%	4613
	11th to 20th	Day	33% 31% 12% 18% 6% 1	4980
	21st to end	Day	41% 30% 8% 10% 10% 1	5465
April	1st to 10th	Day	23% 62% 4 6% 4	2305
	11th to 20th	Day	19% 63% 4 9% 4 1	2542
	21st to end	Day	19% 62% 5% 9% 5%	2600
May	1st to 10th	Day	30% 40% 7% 16% 6% 1	4250
	11th to 20th	Day	28% 42% 10% 16% 3	4215
	21st to end	Day	23% 40% 6% 10% 16% 5%	4554
June	1st to 10th	Day	14% 66% 4 10% 5% 1	2637
	11th to 20th	Day	16% 66% 4 7% 6% 1	2843
	21st to end	Day	15% 65% 5% 9% 7%	2660
July	1st to 10th	Day	27% 38% 10% 13% 10% 1	4371
	11th to 20th	Day	18% 38% 10% 23% 10%	4365
	21st to end	Day	23% 40% 9% 11% 15% 2	4334
August	1st to 10th	Day	22% 40% 9% 18% 10% 1	4091
	11th to 20th	Day	24% 42% 9% 21% 5%	4083
	21st to end	Day	23% 41% 11% 18% 6%	4504
September	1st to 10th	Day	14% 61% 8% 12% 5%	2136
	11th to 20th	Day	13% 61% 5% 12% 9% 1	2090
	21st to end	Day	18% 61% 5% 12% 5%	2090
October	1st to 10th	Day	15% 38% 9% 21% 16% 1	3367
	11th to 20th	Day	19% 39% 8% 20% 14%	3140
	21st to end	Day	25% 38% 9% 17% 9% 1	3193
November	1st to 10th	Day	22% 57% 5% 10% 5% 1	1762
	11th to 20th	Day	22% 57% 6% 10% 5%	1800
	21st to end	Day	27% 57% 7% 6% 3	1696
December	1st to 10th	Day	40% 40% 10% 7% 3	2600
	11th to 20th	Day	47% 33% 6% 9% 3	2855
	21st to end	Day	58% 27% 5% 6% 4	3936
January	1st to 10th	Night	43% 28% 8% 14% 6% 1	6348
	11th to 20th	Night	51% 29% 8% 8% 4	6314
	21st to end	Night	47% 29% 8% 8% 8%	6490
February	1st to 10th	Night	25% 63% 4 5% 3	2600
	11th to 20th	Night	21% 61% 3 10% 4	2398
	21st to end	Night	22% 62% 4 8% 4	1980
March	1st to 10th	Night	36% 30% 11% 15% 7% 1	5238
	11th to 20th	Night	26% 30% 11% 21% 11% 1	4980
	21st to end	Night	35% 30% 8% 14% 12% 1	5467
April	1st to 10th	Night	23% 62% 3 7% 3 1	2110
	11th to 20th	Night	19% 63% 5% 7% 6%	1806
	21st to end	Night	16% 62% 5% 9% 7% 1	1800
May	1st to 10th	Night	30% 40% 6% 13% 9% 2	2930
	11th to 20th	Night	28% 40% 8% 19% 4	2903
	21st to end	Night	21% 40% 6% 13% 16% 4	3138
June	1st to 10th	Night	11% 66% 4 12% 7% 1	1525
	11th to 20th	Night	16% 66% 3 10% 4 1	1330
	21st to end	Night	13% 64% 5% 10% 8%	1512
July	1st to 10th	Night	18% 38% 10% 22% 7% 5%	2490
	11th to 20th	Night	17% 38% 10% 22% 12% 1	2482
	21st to end	Night	19% 40% 13% 12% 14% 2	2978
August	1st to 10th	Night	24% 40% 8% 17% 12% 1	2814
	11th to 20th	Night	22% 40% 7% 24% 7%	2812
	21st to end	Night	22% 41% 8% 21% 8%	3148
September	1st to 10th	Night	12% 61% 6% 13% 7% 1	2024
	11th to 20th	Night	11% 60% 4 12% 11% 2	2090
	21st to end	Night	18% 61% 5% 10% 6%	2090
October	1st to 10th	Night	12% 37% 6% 24% 21%	3591
	11th to 20th	Night	24% 37% 7% 17% 15%	3779
	21st to end	Night	21% 38% 7% 22% 12% 1	4340
November	1st to 10th	Night	23% 56% 5% 9% 6% 1	2536
	11th to 20th	Night	21% 56% 5% 10% 7%	2600
	21st to end	Night	26% 57% 6% 7% 4	2699
December	1st to 10th	Night	37% 39% 11% 10% 3	4580
	11th to 20th	Night	46% 34% 8% 8% 4 1	5328
	21st to end	Night	56% 27% 7% 6% 5%	7024

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% 18% 38% 15% 6% 1	300
	11th to 20th	Day	23% 17% 40% 16% 2 1	300
	21st to end	Day	28% 18% 33% 16% 4	330
February	1st to 10th	Day	26% 20% 38% 13% 4	300
	11th to 20th	Day	25% 21% 34% 17% 3	300
	21st to end	Day	25% 27% 36% 9% 3	249
March	1st to 10th	Day	26% 20% 34% 16% 4	355
	11th to 20th	Day	24% 15% 31% 23% 6% 1	500
	21st to end	Day	26% 17% 31% 20% 5% 1	550
April	1st to 10th	Day	22% 17% 32% 21% 8% 1	500
	11th to 20th	Day	26% 18% 29% 16% 7% 4	492
	21st to end	Day	25% 14% 32% 18% 8% 2	500
May	1st to 10th	Day	22% 16% 30% 20% 7% 4	503
	11th to 20th	Day	20% 11% 29% 23% 14% 4	527
	21st to end	Day	15% 7% 26% 28% 18% 7%	549
June	1st to 10th	Day	14% 9% 29% 30% 15% 3	624
	11th to 20th	Day	14% 8% 29% 32% 14% 3	680
	21st to end	Day	12% 9% 31% 28% 16% 4	680
July	1st to 10th	Day	9% 6% 25% 27% 24% 9%	671
	11th to 20th	Day	11% 6% 27% 29% 18% 8%	668
	21st to end	Day	8% 5% 24% 29% 26% 9%	542
August	1st to 10th	Day	8% 4 23% 30% 21% 14%	486
	11th to 20th	Day	11% 5% 22% 29% 21% 11%	479
	21st to end	Day	11% 6% 19% 29% 24% 11%	539
September	1st to 10th	Day	11% 6% 23% 27% 21% 12%	490
	11th to 20th	Day	8% 4 19% 26% 30% 13%	490
	21st to end	Day	18% 9% 24% 22% 17% 9%	490
October	1st to 10th	Day	14% 9% 27% 24% 19% 7%	490
	11th to 20th	Day	19% 16% 29% 26% 7% 2	490
	21st to end	Day	26% 15% 34% 20% 5% 1	405
November	1st to 10th	Day	15% 19% 35% 19% 9% 2	300
	11th to 20th	Day	22% 22% 33% 16% 6% 1	300
	21st to end	Day	26% 15% 37% 16% 5%	247
December	1st to 10th	Day	23% 21% 34% 17% 5% 2	200
	11th to 20th	Day	27% 23% 35% 13% 3 1	200
	21st to end	Day	29% 22% 32% 14% 3	320
January	1st to 10th	Night	23% 15% 35% 20% 5% 1	1000
	11th to 20th	Night	27% 14% 33% 22% 4 1	980
	21st to end	Night	20% 16% 33% 26% 4	880
February	1st to 10th	Night	27% 12% 36% 23% 3	800
	11th to 20th	Night	24% 14% 32% 27% 4	783
	21st to end	Night	26% 16% 37% 20% 2	660
March	1st to 10th	Night	21% 13% 33% 28% 6%	800
	11th to 20th	Night	19% 15% 36% 25% 5% 1	800
	21st to end	Night	23% 11% 30% 27% 8%	880
April	1st to 10th	Night	22% 12% 35% 27% 4	710
	11th to 20th	Night	21% 12% 33% 25% 8% 1	593
	21st to end	Night	20% 10% 30% 33% 8%	600
May	1st to 10th	Night	19% 11% 29% 30% 10% 1	600
	11th to 20th	Night	21% 5% 27% 33% 13% 1	591
	21st to end	Night	11% 5% 24% 38% 20% 2	634
June	1st to 10th	Night	14% 2 25% 43% 15%	523
	11th to 20th	Night	9% 4 24% 48% 14%	380
	21st to end	Night	11% 5% 22% 40% 20% 2	562
July	1st to 10th	Night	9% 4 17% 39% 28% 3	555
	11th to 20th	Night	8% 3 20% 41% 24% 4	552
	21st to end	Night	6% 2 16% 48% 26% 2	624
August	1st to 10th	Night	8% 4 17% 41% 27% 3	567
	11th to 20th	Night	9% 2 18% 37% 31% 3	564
	21st to end	Night	10% 3 15% 40% 30% 3	627
September	1st to 10th	Night	12% 6% 18% 33% 26% 4	694
	11th to 20th	Night	9% 3 18% 34% 31% 4	760
	21st to end	Night	16% 7% 22% 31% 22% 3	760
October	1st to 10th	Night	16% 8% 24% 32% 17% 3	760
	11th to 20th	Night	22% 11% 29% 26% 10% 2	760
	21st to end	Night	23% 13% 30% 27% 6%	836
November	1st to 10th	Night	17% 12% 30% 27% 12% 1	780
	11th to 20th	Night	22% 16% 31% 23% 8% 1	800
	21st to end	Night	28% 12% 33% 21% 6%	800
December	1st to 10th	Night	27% 13% 34% 19% 7%	800
	11th to 20th	Night	25% 15% 34% 22% 4	855
	21st to end	Night	31% 12% 32% 21% 3	1100

Page generated at 2025-05-23 09:20:58. Version ac07

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