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 2024-11-02.

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% 9% 3 8% 5% 5% 5% 4 6% 8% 33%	100935
	Night	15% 10% 4 8% 5% 4 4 4 5% 7% 35%	97990
Transparency	Day	26% 49% 7% 11% 6% 1	100736
	Night	26% 49% 6% 11% 6% 1	97860
Seeing	Day	17% 12% 29% 24% 13% 5%	15379
	Night	19% 10% 28% 30% 12% 1	25166

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	16% 17% 7% 11% 6% 6% 5% 3 4 5% 20%	2527
	11th to 20th	Day	15% 18% 6% 12% 8% 7% 7% 4 5% 6% 11%	2557
	21st to end	Day	27% 18% 8% 12% 5% 3 4 3 4 3 12%	3124
February	1st to 10th	Day	15% 15% 4 12% 7% 5% 5% 4 5% 5% 21%	1710
	11th to 20th	Day	14% 13% 4 12% 7% 7% 5% 4 5% 6% 24%	1787
	21st to end	Day	17% 12% 4 11% 5% 4 4 4 5% 6% 27%	1570
March	1st to 10th	Day	18% 15% 5% 9% 5% 4 4 3 4 6% 27%	3211
	11th to 20th	Day	13% 12% 4 9% 6% 5% 5% 3 5% 6% 32%	3480
	21st to end	Day	19% 13% 4 10% 6% 5% 4 3 5% 6% 27%	3815
April	1st to 10th	Day	16% 13% 3 7% 5% 4 5% 5% 6% 7% 31%	2190
	11th to 20th	Day	18% 9% 3 7% 5% 4 5% 4 5% 8% 33%	2413
	21st to end	Day	18% 9% 3 8% 4 4 5% 4 4 6% 35%	2470
May	1st to 10th	Day	17% 9% 3 7% 5% 5% 5% 4 5% 7% 33%	4082
	11th to 20th	Day	16% 8% 2 7% 5% 5% 5% 4 6% 8% 34%	4081
	21st to end	Day	9% 6% 2 7% 4 5% 5% 4 5% 8% 45%	4476
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%	1681
	11th to 20th	Day	14% 13% 3 7% 6% 5% 4 3 5% 8% 32%	1710
	21st to end	Day	18% 12% 4 8% 4 5% 5% 3 5% 7% 28%	1612
December	1st to 10th	Day	16% 14% 6% 12% 5% 6% 6% 4 5% 6% 20%	1520
	11th to 20th	Day	16% 15% 5% 10% 5% 5% 5% 4 6% 7% 22%	1818
	21st to end	Day	27% 19% 7% 13% 6% 4 4 2 4 5% 10%	2751
January	1st to 10th	Night	16% 16% 6% 10% 5% 4 4 4 4 5% 25%	4428
	11th to 20th	Night	15% 18% 8% 13% 5% 5% 5% 4 6% 5% 15%	4408
	21st to end	Night	25% 18% 8% 11% 5% 4 3 2 3 4 17%	4532
February	1st to 10th	Night	17% 16% 4 10% 7% 5% 4 4 5% 4 23%	2470
	11th to 20th	Night	15% 14% 4 10% 6% 4 5% 4 5% 6% 27%	2274
	21st to end	Night	19% 12% 4 10% 5% 5% 4 3 4 5% 30%	1884
March	1st to 10th	Night	16% 13% 4 8% 4 3 4 3 4 5% 34%	3640
	11th to 20th	Night	10% 8% 3 6% 4 5% 5% 4 6% 8% 41%	3480
	21st to end	Night	16% 7% 4 8% 5% 5% 4 3 5% 6% 36%	3817
April	1st to 10th	Night	16% 11% 2 7% 5% 4 3 4 5% 6% 36%	2005
	11th to 20th	Night	17% 8% 2 6% 4 3 4 4 7% 7% 36%	1715
	21st to end	Night	17% 9% 2 6% 4 4 4 4 5% 6% 38%	1710
May	1st to 10th	Night	17% 9% 3 7% 5% 4 4 3 5% 7% 35%	2813
	11th to 20th	Night	15% 6% 3 7% 5% 5% 5% 3 5% 8% 38%	2812
	21st to end	Night	9% 6% 2 6% 4 4 4 3 5% 7% 48%	3079
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%	2420
	11th to 20th	Night	16% 10% 3 8% 6% 4 4 4 6% 7% 31%	2470
	21st to end	Night	21% 14% 4 7% 4 4 3 3 5% 6% 29%	2564
December	1st to 10th	Night	18% 15% 5% 10% 6% 5% 5% 3 5% 5% 22%	2660
	11th to 20th	Night	15% 14% 4 11% 6% 6% 5% 4 5% 6% 24%	3363
	21st to end	Night	28% 21% 6% 10% 5% 4 4 3 4 4 12%	4912

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	42% 38% 8% 8% 4	2528
	11th to 20th	Day	49% 38% 5% 6% 2	2557
	21st to end	Day	49% 39% 6% 4 2	3124
February	1st to 10th	Day	23% 65% 3 7% 3	1710
	11th to 20th	Day	22% 64% 3 8% 4	1797
	21st to end	Day	18% 65% 5% 8% 3 1	1570
March	1st to 10th	Day	39% 42% 5% 11% 3	3211
	11th to 20th	Day	31% 42% 10% 14% 4 1	3480
	21st to end	Day	34% 41% 8% 8% 8% 1	3815
April	1st to 10th	Day	21% 65% 4 6% 4	2190
	11th to 20th	Day	18% 66% 4 8% 4 1	2413
	21st to end	Day	18% 64% 5% 7% 5%	2470
May	1st to 10th	Day	29% 41% 7% 16% 6% 1	4120
	11th to 20th	Day	28% 43% 10% 16% 3	4085
	21st to end	Day	22% 41% 6% 10% 16% 5%	4489
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	16% 61% 6% 10% 7% 1	2029
	11th to 20th	Day	17% 61% 5% 10% 6%	1900
	21st to end	Day	23% 59% 5% 8% 4	1956
November	1st to 10th	Day	21% 60% 5% 9% 5% 1	1681
	11th to 20th	Day	19% 60% 6% 10% 5%	1710
	21st to end	Day	24% 59% 7% 6% 3	1612
December	1st to 10th	Day	27% 60% 3 7% 3	1520
	11th to 20th	Day	31% 50% 6% 10% 4	1814
	21st to end	Day	52% 37% 6% 4 2	2751
January	1st to 10th	Night	40% 37% 8% 10% 4 1	4428
	11th to 20th	Night	46% 38% 7% 6% 3	4408
	21st to end	Night	49% 40% 4 5% 2	4532
February	1st to 10th	Night	23% 66% 3 5% 3	2470
	11th to 20th	Night	19% 64% 3 10% 4	2275
	21st to end	Night	21% 65% 3 7% 4	1884
March	1st to 10th	Night	32% 41% 8% 15% 4	3640
	11th to 20th	Night	21% 41% 11% 13% 14% 1	3480
	21st to end	Night	29% 41% 7% 15% 8% 1	3817
April	1st to 10th	Night	20% 65% 3 7% 3 1	2005
	11th to 20th	Night	18% 66% 4 6% 5% 1	1715
	21st to end	Night	16% 65% 5% 8% 5% 1	1710
May	1st to 10th	Night	29% 41% 6% 13% 9% 2	2840
	11th to 20th	Night	27% 41% 8% 19% 4	2813
	21st to end	Night	21% 41% 6% 13% 16% 4	3087
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	15% 60% 4 12% 8%	2149
	11th to 20th	Night	19% 60% 4 8% 9%	2280
	21st to end	Night	21% 59% 4 10% 5%	2640
November	1st to 10th	Night	22% 59% 5% 8% 5% 1	2420
	11th to 20th	Night	19% 59% 5% 11% 6%	2470
	21st to end	Night	23% 60% 6% 7% 4	2564
December	1st to 10th	Night	25% 61% 5% 6% 4	2660
	11th to 20th	Night	29% 50% 7% 8% 4 1	3366
	21st to end	Night	51% 37% 6% 5% 2	4912

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% 19% 35% 16% 7% 1	270
	11th to 20th	Day	24% 15% 40% 17% 3 1	270
	21st to end	Day	30% 16% 32% 17% 5%	297
February	1st to 10th	Day	25% 19% 38% 14% 4	270
	11th to 20th	Day	24% 20% 34% 18% 3	270
	21st to end	Day	24% 28% 36% 8% 4	225
March	1st to 10th	Day	27% 19% 32% 17% 5%	322
	11th to 20th	Day	24% 14% 31% 23% 7% 1	460
	21st to end	Day	26% 16% 31% 21% 6% 1	506
April	1st to 10th	Day	19% 17% 33% 22% 8% 1	460
	11th to 20th	Day	27% 18% 28% 16% 7% 4	452
	21st to end	Day	26% 14% 31% 19% 8% 2	460
May	1st to 10th	Day	22% 15% 30% 21% 7% 5%	463
	11th to 20th	Day	20% 10% 29% 23% 15% 4	487
	21st to end	Day	14% 6% 25% 29% 18% 7%	529
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% 14% 35% 20% 5% 1	405
November	1st to 10th	Day	15% 19% 34% 20% 10% 2	273
	11th to 20th	Day	20% 22% 33% 18% 7% 1	270
	21st to end	Day	27% 14% 37% 16% 6%	223
December	1st to 10th	Day	23% 21% 34% 15% 5% 2	180
	11th to 20th	Day	24% 23% 36% 13% 3 1	180
	21st to end	Day	28% 22% 33% 14% 3	288
January	1st to 10th	Night	24% 15% 34% 21% 5% 1	950
	11th to 20th	Night	27% 13% 32% 22% 4 1	931
	21st to end	Night	20% 15% 33% 27% 5%	836
February	1st to 10th	Night	27% 11% 35% 23% 3	760
	11th to 20th	Night	24% 13% 32% 27% 4	743
	21st to end	Night	26% 16% 35% 21% 2	628
March	1st to 10th	Night	21% 12% 33% 29% 6%	760
	11th to 20th	Night	19% 15% 36% 25% 5% 1	760
	21st to end	Night	23% 11% 30% 28% 8%	836
April	1st to 10th	Night	20% 12% 35% 28% 5%	675
	11th to 20th	Night	21% 11% 34% 25% 8% 1	563
	21st to end	Night	20% 9% 30% 32% 8%	570
May	1st to 10th	Night	19% 10% 29% 31% 10% 1	570
	11th to 20th	Night	21% 4 27% 35% 13% 1	561
	21st to end	Night	10% 5% 23% 40% 21% 2	617
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	745
	11th to 20th	Night	22% 15% 31% 23% 8% 1	760
	21st to end	Night	28% 11% 32% 22% 7%	760
December	1st to 10th	Night	27% 13% 35% 19% 7%	760
	11th to 20th	Night	24% 14% 35% 22% 4	812
	21st to end	Night	31% 12% 32% 22% 3	1045

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