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 2026-09-29.

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	13% 9% 3 8% 5% 5% 5% 4 6% 8% 33%	148433
	Night	14% 10% 4 8% 5% 4 5% 4 5% 6% 34%	143076
Transparency	Day	32% 35% 9% 15% 9% 1	152005
	Night	33% 35% 8% 14% 9% 1	147564
Seeing	Day	18% 12% 29% 23% 13% 5%	17945
	Night	19% 10% 28% 29% 12% 1	27690

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	13% 15% 6% 16% 7% 5% 4 3 4 5% 20%	4691
	11th to 20th	Day	12% 16% 6% 17% 9% 8% 7% 4 4 5% 12%	4763
	21st to end	Day	18% 15% 6% 12% 7% 6% 7% 4 5% 5% 15%	5780
February	1st to 10th	Day	14% 15% 5% 13% 8% 6% 5% 4 5% 5% 21%	1890
	11th to 20th	Day	13% 13% 4 12% 7% 6% 5% 3 5% 6% 24%	1981
	21st to end	Day	16% 12% 5% 12% 5% 5% 4 4 6% 6% 26%	1730
March	1st to 10th	Day	16% 12% 5% 11% 5% 4 4 3 4 5% 29%	4716
	11th to 20th	Day	12% 12% 4 9% 5% 5% 4 3 6% 7% 34%	5090
	21st to end	Day	17% 14% 4 10% 6% 4 4 3 4 6% 27%	5586
April	1st to 10th	Day	17% 12% 3 7% 5% 4 5% 5% 5% 7% 30%	2420
	11th to 20th	Day	17% 9% 3 7% 5% 4 5% 4 5% 7% 33%	2671
	21st to end	Day	17% 9% 3 8% 4 5% 5% 4 5% 7% 36%	2730
May	1st to 10th	Day	17% 9% 3 8% 5% 5% 5% 4 5% 7% 32%	7616
	11th to 20th	Day	14% 7% 2 7% 5% 5% 6% 5% 6% 9% 33%	7626
	21st to end	Day	14% 6% 2 6% 4 4 4 3 5% 7% 44%	8391
June	1st to 10th	Day	10% 6% 2 6% 6% 5% 5% 5% 7% 9% 41%	2915
	11th to 20th	Day	10% 6% 2 6% 5% 5% 6% 5% 6% 9% 39%	3141
	21st to end	Day	9% 6% 2 6% 5% 5% 6% 4 8% 10% 38%	2940
July	1st to 10th	Day	10% 6% 2 6% 4 5% 5% 4 7% 10% 41%	8280
	11th to 20th	Day	5% 4 2 5% 4 5% 5% 4 8% 12% 46%	8280
	21st to end	Day	7% 6% 2 7% 4 5% 5% 4 8% 10% 43%	8364
August	1st to 10th	Day	13% 6% 2 7% 4 4 5% 4 5% 8% 41%	6014
	11th to 20th	Day	9% 7% 3 8% 5% 5% 7% 5% 9% 10% 32%	5951
	21st to end	Day	7% 5% 2 7% 5% 6% 6% 4 8% 11% 39%	6552
September	1st to 10th	Day	9% 7% 2 7% 4 5% 5% 4 7% 8% 42%	2360
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 5% 6% 9% 47%	2310
	21st to end	Day	15% 6% 3 7% 4 5% 4 4 5% 7% 39%	2288
October	1st to 10th	Day	11% 6% 3 7% 5% 4 4 3 6% 9% 43%	2136
	11th to 20th	Day	16% 8% 2 7% 6% 5% 5% 4 5% 8% 35%	2000
	21st to end	Day	18% 10% 2 7% 6% 6% 4 3 6% 6% 32%	2059
November	1st to 10th	Day	12% 7% 3 9% 5% 4 5% 3 6% 6% 40%	1852
	11th to 20th	Day	14% 13% 3 8% 6% 5% 4 3 5% 8% 31%	1890
	21st to end	Day	17% 13% 5% 9% 5% 6% 5% 3 5% 6% 27%	1781
December	1st to 10th	Day	13% 13% 6% 14% 6% 5% 5% 4 5% 6% 21%	2680
	11th to 20th	Day	20% 19% 5% 9% 5% 5% 4 3 6% 6% 18%	2936
	21st to end	Day	27% 18% 8% 13% 5% 4 3 2 3 4 12%	4023
January	1st to 10th	Night	15% 15% 6% 13% 6% 5% 5% 4 4 5% 24%	8232
	11th to 20th	Night	10% 17% 8% 14% 8% 7% 6% 4 6% 5% 16%	8220
	21st to end	Night	15% 15% 6% 10% 5% 5% 5% 3 4 5% 24%	8401
February	1st to 10th	Night	16% 15% 4 11% 7% 5% 4 4 5% 5% 24%	2730
	11th to 20th	Night	15% 13% 4 10% 6% 5% 6% 4 5% 6% 27%	2520
	21st to end	Night	18% 12% 4 10% 5% 5% 4 3 4 6% 29%	2076
March	1st to 10th	Night	15% 12% 4 8% 4 4 5% 3 5% 6% 35%	5355
	11th to 20th	Night	10% 8% 3 7% 4 5% 5% 4 6% 8% 41%	5090
	21st to end	Night	16% 9% 4 8% 5% 5% 4 3 5% 6% 36%	5588
April	1st to 10th	Night	17% 11% 2 7% 6% 4 3 4 5% 6% 35%	2216
	11th to 20th	Night	16% 9% 2 7% 4 4 4 4 7% 7% 36%	1897
	21st to end	Night	17% 9% 2 6% 4 4 4 4 5% 6% 39%	1890
May	1st to 10th	Night	19% 9% 3 8% 5% 5% 5% 3 5% 6% 31%	5237
	11th to 20th	Night	15% 7% 3 7% 5% 5% 4 3 4 7% 39%	5234
	21st to end	Night	13% 6% 2 6% 4 4 4 3 5% 7% 44%	5761
June	1st to 10th	Night	12% 5% 2 4 4 3 5% 4 5% 9% 47%	1685
	11th to 20th	Night	9% 5% 3 7% 5% 4 5% 5% 6% 9% 43%	1470
	21st to end	Night	10% 5% 2 5% 4 5% 5% 3 5% 9% 47%	1672
July	1st to 10th	Night	7% 5% 2 6% 4 4 5% 4 6% 9% 48%	4708
	11th to 20th	Night	7% 4 2 5% 4 4 4 4 7% 9% 49%	4707
	21st to end	Night	7% 5% 2 6% 4 4 4 4 6% 8% 49%	5712
August	1st to 10th	Night	15% 7% 3 6% 4 4 3 3 5% 7% 44%	4135
	11th to 20th	Night	9% 7% 3 8% 5% 4 6% 4 5% 8% 42%	4095
	21st to end	Night	6% 5% 3 7% 5% 5% 5% 4 7% 8% 46%	4571
September	1st to 10th	Night	9% 5% 2 6% 5% 4 6% 4 6% 6% 48%	2238
	11th to 20th	Night	9% 4 2 5% 3 3 4 3 5% 7% 55%	2310
	21st to end	Night	14% 7% 3 6% 4 3 4 4 5% 6% 43%	2289
October	1st to 10th	Night	12% 6% 2 6% 4 3 4 3 5% 7% 47%	2262
	11th to 20th	Night	16% 9% 3 7% 4 4 4 4 4 6% 38%	2400
	21st to end	Night	19% 8% 2 7% 5% 5% 4 4 5% 5% 36%	2779
November	1st to 10th	Night	13% 7% 2 6% 3 4 5% 3 5% 7% 43%	2666
	11th to 20th	Night	15% 11% 3 8% 6% 4 5% 4 7% 7% 31%	2730
	21st to end	Night	20% 14% 4 8% 4 4 3 3 5% 6% 27%	2834
December	1st to 10th	Night	13% 13% 6% 13% 6% 6% 5% 3 5% 6% 24%	4720
	11th to 20th	Night	16% 17% 6% 12% 6% 5% 4 3 4 6% 20%	5468
	21st to end	Night	27% 20% 6% 10% 5% 3 4 3 4 4 15%	7178

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	52% 23% 9% 10% 4 1	4692
	11th to 20th	Day	61% 23% 5% 6% 5%	4763
	21st to end	Day	53% 25% 9% 8% 4	5780
February	1st to 10th	Day	27% 59% 4 7% 3	1890
	11th to 20th	Day	26% 58% 4 8% 4	1991
	21st to end	Day	21% 60% 6% 9% 3 1	1730
March	1st to 10th	Day	42% 30% 8% 12% 6%	4716
	11th to 20th	Day	34% 30% 12% 17% 6% 1	5090
	21st to end	Day	42% 29% 8% 10% 10% 1	5586
April	1st to 10th	Day	25% 59% 5% 7% 4	2420
	11th to 20th	Day	21% 60% 5% 9% 5% 1	2671
	21st to end	Day	20% 59% 5% 10% 5%	2730
May	1st to 10th	Day	38% 26% 9% 18% 8% 2	7651
	11th to 20th	Day	35% 26% 11% 18% 9% 1	7630
	21st to end	Day	31% 23% 6% 15% 19% 5%	8405
June	1st to 10th	Day	15% 60% 4 12% 8% 1	2915
	11th to 20th	Day	19% 60% 5% 8% 6% 1	3141
	21st to end	Day	16% 59% 6% 11% 7%	2940
July	1st to 10th	Day	30% 24% 12% 21% 13% 1	8211
	11th to 20th	Day	21% 24% 11% 27% 16% 1	8205
	21st to end	Day	26% 24% 11% 19% 19% 1	8180
August	1st to 10th	Day	29% 29% 9% 16% 14% 2	6001
	11th to 20th	Day	31% 32% 12% 21% 4	5990
	21st to end	Day	26% 31% 15% 22% 6%	6551
September	1st to 10th	Day	16% 56% 8% 14% 5%	2360
	11th to 20th	Day	15% 56% 6% 13% 9% 1	2310
	21st to end	Day	20% 56% 5% 12% 7%	2288
October	1st to 10th	Day	16% 37% 9% 21% 17% 1	3474
	11th to 20th	Day	20% 38% 8% 20% 14%	3240
	21st to end	Day	25% 37% 9% 17% 10% 1	3296
November	1st to 10th	Day	24% 55% 5% 11% 5% 1	1852
	11th to 20th	Day	23% 55% 6% 11% 5% 1	1890
	21st to end	Day	29% 54% 6% 7% 3	1781
December	1st to 10th	Day	41% 39% 10% 7% 3	2680
	11th to 20th	Day	48% 33% 7% 10% 4	2932
	21st to end	Day	59% 26% 5% 6% 4	4023
January	1st to 10th	Night	49% 23% 9% 12% 6% 1	8233
	11th to 20th	Night	53% 24% 9% 9% 5%	8220
	21st to end	Night	48% 24% 9% 12% 7%	8401
February	1st to 10th	Night	25% 60% 4 7% 3 1	2730
	11th to 20th	Night	23% 58% 4 11% 4	2521
	21st to end	Night	24% 59% 5% 8% 4	2076
March	1st to 10th	Night	36% 29% 11% 15% 7% 1	5355
	11th to 20th	Night	27% 30% 11% 20% 11% 1	5090
	21st to end	Night	35% 30% 8% 14% 12% 1	5588
April	1st to 10th	Night	25% 59% 4 8% 4 1	2216
	11th to 20th	Night	21% 60% 5% 7% 6%	1897
	21st to end	Night	18% 59% 6% 10% 6% 1	1890
May	1st to 10th	Night	41% 26% 8% 11% 11% 2	5264
	11th to 20th	Night	35% 25% 8% 22% 9% 1	5235
	21st to end	Night	32% 24% 8% 16% 16% 4	5771
June	1st to 10th	Night	13% 60% 4 12% 11% 1	1685
	11th to 20th	Night	18% 60% 4 11% 5% 1	1470
	21st to end	Night	15% 58% 6% 12% 9%	1672
July	1st to 10th	Night	25% 24% 11% 26% 11% 3	4670
	11th to 20th	Night	23% 24% 10% 25% 17% 1	4662
	21st to end	Night	25% 23% 10% 18% 22% 2	5583
August	1st to 10th	Night	31% 29% 8% 14% 17% 2	4124
	11th to 20th	Night	31% 31% 12% 21% 5%	4122
	21st to end	Night	25% 31% 13% 24% 7%	4571
September	1st to 10th	Night	14% 56% 6% 16% 7% 1	2238
	11th to 20th	Night	13% 55% 5% 14% 11% 2	2310
	21st to end	Night	19% 56% 6% 10% 8% 1	2289
October	1st to 10th	Night	12% 36% 6% 25% 21%	3704
	11th to 20th	Night	24% 36% 7% 17% 16%	3899
	21st to end	Night	21% 36% 7% 22% 12% 1	4479
November	1st to 10th	Night	24% 54% 5% 9% 6% 1	2666
	11th to 20th	Night	21% 54% 5% 12% 7% 1	2730
	21st to end	Night	28% 55% 6% 8% 4	2834
December	1st to 10th	Night	38% 38% 11% 10% 3	4720
	11th to 20th	Night	46% 33% 8% 8% 4 1	5471
	21st to end	Night	56% 27% 6% 6% 5%	7178

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	23% 17% 39% 14% 6% 1	330
	11th to 20th	Day	25% 19% 39% 15% 2 1	330
	21st to end	Day	29% 18% 33% 17% 4	363
February	1st to 10th	Day	24% 19% 38% 14% 5%	330
	11th to 20th	Day	25% 21% 35% 17% 3	330
	21st to end	Day	23% 27% 38% 9% 3	273
March	1st to 10th	Day	27% 19% 35% 15% 4	388
	11th to 20th	Day	26% 16% 30% 22% 6% 1	540
	21st to end	Day	26% 18% 31% 19% 5% 1	594
April	1st to 10th	Day	24% 17% 32% 20% 7% 1	540
	11th to 20th	Day	27% 18% 30% 15% 7% 4	532
	21st to end	Day	25% 14% 31% 19% 8% 2	540
May	1st to 10th	Day	22% 17% 31% 19% 6% 4	543
	11th to 20th	Day	19% 13% 30% 22% 13% 4	567
	21st to end	Day	15% 9% 26% 26% 17% 6%	617
June	1st to 10th	Day	13% 9% 31% 30% 14% 3	718
	11th to 20th	Day	14% 10% 31% 30% 12% 3	780
	21st to end	Day	11% 9% 32% 29% 15% 4	780
July	1st to 10th	Day	9% 6% 25% 28% 23% 9%	771
	11th to 20th	Day	10% 7% 27% 30% 18% 8%	768
	21st to end	Day	8% 5% 25% 30% 24% 9%	631
August	1st to 10th	Day	8% 4 23% 32% 20% 13%	566
	11th to 20th	Day	12% 5% 23% 29% 21% 10%	559
	21st to end	Day	10% 6% 21% 29% 23% 10%	627
September	1st to 10th	Day	11% 7% 24% 27% 19% 12%	570
	11th to 20th	Day	8% 6% 19% 26% 30% 12%	570
	21st to end	Day	18% 9% 24% 23% 17% 9%	562
October	1st to 10th	Day	13% 9% 26% 24% 20% 7%	530
	11th to 20th	Day	18% 15% 31% 26% 7% 2	530
	21st to end	Day	26% 16% 33% 19% 5% 1	442
November	1st to 10th	Day	18% 18% 35% 19% 8% 2	330
	11th to 20th	Day	22% 23% 32% 16% 5% 1	330
	21st to end	Day	28% 16% 35% 17% 5%	272
December	1st to 10th	Day	24% 19% 34% 16% 5% 1	220
	11th to 20th	Day	28% 22% 35% 11% 3	220
	21st to end	Day	30% 22% 31% 13% 3	352
January	1st to 10th	Night	23% 15% 35% 21% 5% 1	1050
	11th to 20th	Night	26% 15% 33% 21% 4	1029
	21st to end	Night	20% 16% 34% 26% 4	924
February	1st to 10th	Night	26% 12% 35% 23% 4	840
	11th to 20th	Night	23% 14% 32% 27% 4	823
	21st to end	Night	26% 16% 37% 19% 2	692
March	1st to 10th	Night	21% 13% 33% 28% 5%	840
	11th to 20th	Night	20% 15% 35% 24% 5% 1	840
	21st to end	Night	23% 12% 31% 27% 7%	924
April	1st to 10th	Night	23% 12% 34% 26% 4	746
	11th to 20th	Night	21% 13% 33% 24% 8% 1	623
	21st to end	Night	20% 10% 30% 33% 7%	630
May	1st to 10th	Night	20% 11% 30% 29% 9% 1	630
	11th to 20th	Night	21% 5% 28% 33% 12% 1	621
	21st to end	Night	11% 5% 24% 40% 19% 1	683
June	1st to 10th	Night	14% 3 25% 43% 14%	578
	11th to 20th	Night	10% 5% 24% 46% 14%	420
	21st to end	Night	11% 5% 22% 40% 21% 1	622
July	1st to 10th	Night	8% 4 17% 41% 27% 3	615
	11th to 20th	Night	8% 4 20% 40% 24% 4	612
	21st to end	Night	7% 3 16% 47% 25% 2	690
August	1st to 10th	Night	8% 4 17% 40% 27% 3	627
	11th to 20th	Night	9% 3 18% 37% 30% 3	624
	21st to end	Night	9% 2 16% 40% 30% 2	693
September	1st to 10th	Night	12% 6% 19% 33% 26% 4	768
	11th to 20th	Night	9% 4 18% 34% 31% 4	840
	21st to end	Night	15% 6% 22% 32% 21% 3	833
October	1st to 10th	Night	16% 7% 24% 33% 18% 3	800
	11th to 20th	Night	21% 12% 29% 26% 10% 2	800
	21st to end	Night	22% 14% 30% 28% 6%	880
November	1st to 10th	Night	17% 13% 31% 27% 11% 1	820
	11th to 20th	Night	22% 16% 31% 23% 7% 1	840
	21st to end	Night	28% 12% 33% 21% 6%	840
December	1st to 10th	Night	26% 14% 34% 19% 6%	840
	11th to 20th	Night	24% 15% 35% 22% 4	898
	21st to end	Night	31% 13% 32% 21% 3	1155

Page generated at 2026-09-26 04:18:43. Version ac07

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