Robert Reeves Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Robert Reeves Observatory clear sky chart from 2005-11-06 to 2025-02-21.

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% 6% 2 5% 4 3 4 3 5% 7% 48%	109224
	Night	14% 6% 2 4 3 3 3 3 4 6% 52%	109617
Transparency	Day	35% 6% 9% 20% 19% 11%	113108
	Night	32% 5% 10% 19% 20% 14%	113848
Seeing	Day	16% 9% 25% 30% 17% 3	15087
	Night	17% 8% 26% 35% 14% 1	25194

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	14% 6% 2 4 2 2 2 2 3 5% 57%	4077
	11th to 20th	Day	12% 7% 1 4 2 2 2 3 4 5% 56%	4236
	21st to end	Day	22% 6% 2 4 2 2 3 2 4 6% 46%	4994
February	1st to 10th	Day	15% 8% 2 5% 4 3 3 2 4 4 51%	2000
	11th to 20th	Day	17% 7% 2 5% 4 3 4 3 5% 5% 47%	1949
	21st to end	Day	15% 6% 1 3 3 3 3 3 4 6% 53%	1570
March	1st to 10th	Day	16% 8% 2 7% 3 4 3 2 4 4 46%	3247
	11th to 20th	Day	22% 11% 3 7% 4 3 4 3 3 4 36%	3480
	21st to end	Day	15% 6% 2 4 3 4 3 3 5% 7% 47%	3814
April	1st to 10th	Day	15% 7% 2 5% 3 3 3 4 5% 7% 46%	2090
	11th to 20th	Day	19% 5% 2 5% 4 3 3 3 5% 7% 45%	2225
	21st to end	Day	17% 6% 3 6% 3 3 4 3 4 6% 47%	2304
May	1st to 10th	Day	23% 7% 2 6% 4 3 3 3 5% 6% 36%	4082
	11th to 20th	Day	16% 5% 2 6% 4 4 4 3 6% 7% 44%	4082
	21st to end	Day	15% 6% 2 6% 4 4 5% 4 7% 7% 39%	4477
June	1st to 10th	Day	8% 5% 2 5% 4 3 5% 3 7% 10% 48%	2470
	11th to 20th	Day	10% 5% 2 5% 4 5% 5% 4 6% 8% 46%	2470
	21st to end	Day	11% 5% 2 6% 5% 5% 7% 5% 7% 9% 38%	2470
July	1st to 10th	Day	9% 5% 2 5% 5% 5% 5% 4 5% 8% 48%	4120
	11th to 20th	Day	7% 4 1 4 4 5% 5% 5% 6% 9% 50%	4120
	21st to end	Day	10% 5% 2 5% 5% 5% 5% 4 7% 10% 42%	4441
August	1st to 10th	Day	2 2 1 3 3 3 3 3 5% 8% 67%	3785
	11th to 20th	Day	4 2 1 3 3 4 4 3 6% 9% 61%	3726
	21st to end	Day	5% 4 2 5% 5% 4 5% 5% 6% 10% 48%	4154
September	1st to 10th	Day	13% 6% 3 7% 5% 5% 6% 3 5% 7% 40%	2090
	11th to 20th	Day	11% 6% 3 6% 5% 5% 5% 5% 7% 7% 42%	2090
	21st to end	Day	10% 5% 2 6% 5% 5% 4 4 6% 7% 47%	2090
October	1st to 10th	Day	9% 4 2 5% 4 4 4 3 5% 6% 54%	2090
	11th to 20th	Day	15% 5% 2 4 3 3 3 3 5% 6% 51%	2069
	21st to end	Day	15% 5% 1 5% 3 2 3 2 3 5% 57%	2090
November	1st to 10th	Day	17% 7% 2 6% 3 3 4 3 4 6% 46%	1903
	11th to 20th	Day	13% 8% 1 5% 3 3 3 3 4 6% 51%	1800
	21st to end	Day	17% 6% 3 5% 4 3 3 2 4 5% 49%	1800
December	1st to 10th	Day	23% 9% 3 6% 3 3 3 2 4 4 40%	2940
	11th to 20th	Day	17% 10% 2 5% 4 4 3 2 4 5% 45%	3391
	21st to end	Day	14% 6% 2 3 2 2 2 2 3 4 61%	4488
January	1st to 10th	Night	15% 4 2 4 2 2 2 2 4 7% 56%	5888
	11th to 20th	Night	11% 4 1 2 2 2 2 2 5% 5% 64%	5785
	21st to end	Night	27% 6% 2 5% 2 2 2 2 3 4 43%	5984
February	1st to 10th	Night	14% 7% 2 3 2 2 2 3 5% 6% 54%	2400
	11th to 20th	Night	17% 7% 2 4 3 2 3 3 3 5% 51%	2338
	21st to end	Night	16% 5% 1 5% 3 3 3 2 5% 7% 49%	1884
March	1st to 10th	Night	15% 8% 2 5% 3 3 3 3 5% 6% 46%	3611
	11th to 20th	Night	19% 10% 3 7% 5% 4 3 3 3 5% 38%	3480
	21st to end	Night	16% 8% 2 5% 3 3 3 3 4 5% 49%	3817
April	1st to 10th	Night	12% 7% 1 5% 4 3 3 4 5% 6% 51%	2090
	11th to 20th	Night	18% 5% 2 6% 3 3 3 3 5% 5% 48%	1901
	21st to end	Night	19% 6% 2 4 4 3 3 2 4 5% 48%	1875
May	1st to 10th	Night	25% 9% 2 4 2 3 2 3 4 6% 40%	2823
	11th to 20th	Night	13% 5% 2 4 3 3 3 2 4 6% 52%	2823
	21st to end	Night	15% 6% 2 4 3 3 3 3 5% 8% 47%	3090
June	1st to 10th	Night	5% 4 1 3 2 2 3 4 7% 9% 61%	1710
	11th to 20th	Night	7% 4 2 3 3 3 4 4 5% 8% 57%	1710
	21st to end	Night	9% 5% 2 4 5% 3 3 4 5% 10% 52%	1710
July	1st to 10th	Night	7% 4 2 4 4 4 4 4 6% 10% 53%	2850
	11th to 20th	Night	6% 4 2 4 3 2 4 4 6% 9% 54%	2850
	21st to end	Night	13% 6% 2 5% 3 3 4 4 5% 9% 45%	3117
August	1st to 10th	Night	2 2 1 3 2 2 3 3 4 8% 71%	3155
	11th to 20th	Night	4 2 1 3 3 3 3 4 6% 10% 63%	3113
	21st to end	Night	6% 5% 2 4 3 3 4 3 7% 11% 52%	3509
September	1st to 10th	Night	11% 6% 2 5% 4 4 4 3 6% 8% 47%	2090
	11th to 20th	Night	12% 6% 2 6% 4 3 4 3 5% 8% 47%	2090
	21st to end	Night	9% 5% 1 5% 4 4 4 3 5% 7% 53%	2090
October	1st to 10th	Night	8% 5% 2 4 3 2 3 3 4 6% 61%	2090
	11th to 20th	Night	15% 5% 1 4 2 3 3 3 4 6% 55%	2090
	21st to end	Night	16% 4 1 3 2 2 2 1 2 4 63%	2508
November	1st to 10th	Night	15% 6% 2 4 3 2 3 2 4 6% 54%	2390
	11th to 20th	Night	11% 6% 2 4 3 3 3 2 5% 5% 56%	2600
	21st to end	Night	18% 5% 2 4 3 3 3 3 5% 6% 49%	2600
December	1st to 10th	Night	25% 7% 2 4 3 3 3 3 4 5% 40%	4250
	11th to 20th	Night	21% 7% 1 4 3 3 3 2 3 4 49%	4816
	21st to end	Night	17% 6% 1 2 1 2 2 2 3 5% 58%	6490

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	31% 3 5% 18% 26% 17%	4078
	11th to 20th	Day	31% 2 4 15% 25% 22%	4236
	21st to end	Day	41% 4 5% 11% 26% 14%	4994
February	1st to 10th	Day	39% 4 5% 12% 21% 19%	2000
	11th to 20th	Day	42% 3 5% 16% 19% 14%	1959
	21st to end	Day	35% 4 7% 17% 20% 18%	1570
March	1st to 10th	Day	43% 3 6% 12% 21% 15%	3247
	11th to 20th	Day	53% 5% 5% 10% 16% 11%	3480
	21st to end	Day	35% 5% 9% 16% 17% 17%	3814
April	1st to 10th	Day	39% 5% 8% 15% 18% 15%	2090
	11th to 20th	Day	39% 5% 7% 18% 17% 14%	2225
	21st to end	Day	41% 4 7% 18% 18% 12%	2304
May	1st to 10th	Day	48% 7% 9% 14% 14% 8%	4120
	11th to 20th	Day	39% 7% 8% 20% 19% 8%	4086
	21st to end	Day	40% 8% 11% 26% 13% 3	4489
June	1st to 10th	Day	31% 7% 13% 26% 20% 3	2470
	11th to 20th	Day	35% 7% 11% 25% 17% 5%	2470
	21st to end	Day	40% 8% 11% 22% 15% 4	2470
July	1st to 10th	Day	31% 8% 17% 35% 8% 1	4056
	11th to 20th	Day	28% 9% 17% 30% 15% 1	4051
	21st to end	Day	31% 10% 19% 28% 12% 1	4301
August	1st to 10th	Day	16% 6% 11% 40% 27%	3772
	11th to 20th	Day	20% 7% 22% 38% 12% 2	3762
	21st to end	Day	29% 11% 16% 24% 19% 2	4153
September	1st to 10th	Day	43% 7% 10% 17% 16% 7%	2090
	11th to 20th	Day	39% 9% 11% 19% 17% 5%	2090
	21st to end	Day	36% 6% 9% 19% 18% 12%	2090
October	1st to 10th	Day	20% 3 6% 14% 24% 34%	3480
	11th to 20th	Day	30% 4 7% 16% 21% 23%	3413
	21st to end	Day	29% 4 5% 15% 34% 13%	3428
November	1st to 10th	Day	40% 5% 8% 18% 17% 12%	1903
	11th to 20th	Day	37% 3 6% 17% 19% 18%	1800
	21st to end	Day	41% 4 4 17% 20% 14%	1800
December	1st to 10th	Day	50% 4 6% 15% 16% 10%	2940
	11th to 20th	Day	42% 5% 6% 14% 18% 15%	3389
	21st to end	Day	30% 3 4 15% 29% 19%	4488
January	1st to 10th	Night	30% 4 5% 17% 26% 18%	5888
	11th to 20th	Night	23% 2 4 18% 27% 26%	5785
	21st to end	Night	46% 3 3 9% 21% 17%	5984
February	1st to 10th	Night	33% 3 7% 14% 22% 22%	2400
	11th to 20th	Night	37% 3 6% 17% 19% 17%	2338
	21st to end	Night	36% 4 8% 14% 20% 18%	1884
March	1st to 10th	Night	39% 7% 6% 14% 20% 14%	3611
	11th to 20th	Night	48% 7% 7% 11% 14% 13%	3480
	21st to end	Night	38% 6% 8% 15% 16% 17%	3817
April	1st to 10th	Night	34% 5% 10% 17% 17% 17%	2090
	11th to 20th	Night	39% 5% 8% 14% 16% 17%	1901
	21st to end	Night	40% 5% 9% 17% 16% 13%	1875
May	1st to 10th	Night	44% 7% 9% 16% 12% 11%	2850
	11th to 20th	Night	33% 7% 10% 23% 19% 9%	2824
	21st to end	Night	35% 8% 13% 27% 12% 5%	3099
June	1st to 10th	Night	19% 8% 17% 32% 20% 2	1710
	11th to 20th	Night	25% 6% 15% 27% 21% 5%	1710
	21st to end	Night	29% 7% 18% 27% 16% 3	1710
July	1st to 10th	Night	26% 8% 23% 36% 6% 1	2805
	11th to 20th	Night	24% 10% 20% 28% 17% 1	2796
	21st to end	Night	35% 9% 20% 27% 9%	3018
August	1st to 10th	Night	13% 6% 15% 43% 22%	3144
	11th to 20th	Night	18% 8% 27% 36% 10% 2	3143
	21st to end	Night	26% 11% 19% 26% 16% 2	3509
September	1st to 10th	Night	35% 7% 13% 22% 17% 7%	2090
	11th to 20th	Night	36% 7% 17% 20% 14% 6%	2090
	21st to end	Night	31% 7% 11% 21% 19% 12%	2090
October	1st to 10th	Night	17% 4 7% 16% 21% 35%	3480
	11th to 20th	Night	26% 3 8% 13% 29% 20%	3460
	21st to end	Night	26% 3 5% 16% 35% 15%	4120
November	1st to 10th	Night	34% 4 7% 19% 22% 14%	2390
	11th to 20th	Night	30% 4 8% 19% 21% 18%	2600
	21st to end	Night	35% 4 8% 16% 20% 16%	2600
December	1st to 10th	Night	47% 4 8% 14% 14% 13%	4250
	11th to 20th	Night	41% 4 5% 12% 20% 18%	4817
	21st to end	Night	32% 3 4 15% 28% 19%	6490

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% 15% 37% 17% 6% 1	300
	11th to 20th	Day	25% 15% 33% 19% 7% 1	300
	21st to end	Day	23% 21% 31% 19% 6%	330
February	1st to 10th	Day	31% 13% 29% 20% 7%	300
	11th to 20th	Day	28% 17% 34% 15% 5%	300
	21st to end	Day	26% 21% 37% 14% 2	225
March	1st to 10th	Day	25% 20% 40% 14% 1	270
	11th to 20th	Day	27% 21% 36% 14% 3	270
	21st to end	Day	23% 20% 43% 12% 1 1	297
April	1st to 10th	Day	26% 18% 37% 19% 1	270
	11th to 20th	Day	19% 20% 40% 17% 3	423
	21st to end	Day	18% 16% 45% 17% 4	484
May	1st to 10th	Day	23% 11% 39% 21% 5%	653
	11th to 20th	Day	21% 9% 35% 28% 6% 1	675
	21st to end	Day	21% 9% 34% 27% 9% 1	734
June	1st to 10th	Day	9% 4 22% 45% 19% 1	680
	11th to 20th	Day	11% 4 23% 45% 16% 2	680
	21st to end	Day	12% 3 17% 38% 25% 5%	680
July	1st to 10th	Day	13% 4 12% 37% 29% 6%	671
	11th to 20th	Day	7% 1 8% 43% 35% 6%	668
	21st to end	Day	6% 2 9% 40% 36% 6%	727
August	1st to 10th	Day	2 1 6% 47% 36% 8%	487
	11th to 20th	Day	8% 1 8% 42% 34% 8%	477
	21st to end	Day	6% 3 5% 38% 39% 9%	526
September	1st to 10th	Day	11% 1 11% 31% 37% 9%	300
	11th to 20th	Day	7% 1 9% 33% 44% 7%	300
	21st to end	Day	10% 3 11% 36% 36% 6%	300
October	1st to 10th	Day	8% 4 14% 43% 25% 6%	300
	11th to 20th	Day	13% 5% 22% 40% 17% 2	300
	21st to end	Day	24% 11% 26% 25% 12% 2	330
November	1st to 10th	Day	23% 6% 35% 25% 10% 1	300
	11th to 20th	Day	23% 9% 31% 23% 9% 4	300
	21st to end	Day	26% 16% 33% 16% 8% 2	300
December	1st to 10th	Day	24% 11% 30% 25% 9% 1	300
	11th to 20th	Day	25% 15% 31% 20% 8% 1	300
	21st to end	Day	22% 13% 33% 24% 7% 1	330
January	1st to 10th	Night	18% 12% 28% 34% 8% 1	1000
	11th to 20th	Night	22% 11% 29% 31% 7%	950
	21st to end	Night	26% 9% 27% 30% 8%	880
February	1st to 10th	Night	23% 10% 31% 29% 7% 1	800
	11th to 20th	Night	25% 13% 31% 27% 4	784
	21st to end	Night	22% 13% 39% 24% 2	628
March	1st to 10th	Night	24% 12% 33% 26% 4	689
	11th to 20th	Night	22% 10% 35% 28% 4	570
	21st to end	Night	20% 12% 41% 22% 4	627
April	1st to 10th	Night	14% 14% 43% 26% 3	570
	11th to 20th	Night	20% 15% 42% 20% 2	563
	21st to end	Night	20% 11% 43% 21% 5%	570
May	1st to 10th	Night	17% 12% 42% 24% 4	570
	11th to 20th	Night	19% 8% 34% 32% 6%	561
	21st to end	Night	18% 9% 33% 32% 8%	617
June	1st to 10th	Night	6% 3 29% 47% 15%	570
	11th to 20th	Night	9% 3 36% 45% 8%	570
	21st to end	Night	10% 4 25% 42% 19% 1	570
July	1st to 10th	Night	10% 3 15% 50% 21% 1	555
	11th to 20th	Night	6% 2 14% 50% 26% 2	552
	21st to end	Night	7% 1 11% 51% 28% 1	624
August	1st to 10th	Night	3 1 9% 52% 34% 2	567
	11th to 20th	Night	5% 1 7% 50% 35% 2	564
	21st to end	Night	7% 2 9% 43% 34% 3	627
September	1st to 10th	Night	10% 2 8% 43% 34% 3	570
	11th to 20th	Night	13% 2 9% 41% 33% 2	570
	21st to end	Night	10% 3 14% 39% 31% 4	570
October	1st to 10th	Night	9% 3 14% 45% 28% 2	570
	11th to 20th	Night	16% 5% 21% 35% 20% 2	570
	21st to end	Night	17% 8% 24% 28% 21% 2	836
November	1st to 10th	Night	18% 6% 20% 36% 17% 2	830
	11th to 20th	Night	15% 7% 24% 37% 15% 2	1000
	21st to end	Night	22% 10% 23% 33% 11% 1	1000
December	1st to 10th	Night	20% 7% 26% 35% 12% 1	1000
	11th to 20th	Night	26% 10% 24% 32% 9% 1	1000
	21st to end	Night	20% 10% 26% 36% 8%	1100

Page generated at 2025-02-18 02:26:28. Version ac07

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