Robert's Hill
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Robert's Hill clear sky chart from 2005-11-06 to 2024-12-02.

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	13% 7% 2 7% 5% 5% 5% 4 6% 8% 38%	101256
	Night	13% 7% 2 6% 4 4 4 4 6% 8% 42%	98311
Transparency	Day	41% 7% 12% 22% 14% 4	104919
	Night	38% 7% 13% 23% 16% 4	102787
Seeing	Day	22% 15% 25% 22% 12% 5%	15263
	Night	18% 14% 22% 28% 15% 2	25461

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	15% 7% 3 6% 4 4 5% 3 5% 7% 40%	2836
	11th to 20th	Day	14% 8% 4 8% 4 5% 4 4 6% 5% 38%	2840
	21st to end	Day	18% 9% 3 8% 5% 4 4 3 4 6% 36%	3124
February	1st to 10th	Day	11% 7% 3 8% 5% 5% 4 5% 5% 7% 38%	1710
	11th to 20th	Day	15% 7% 3 8% 5% 5% 4 3 6% 7% 37%	1718
	21st to end	Day	17% 8% 3 7% 5% 4 5% 3 5% 6% 37%	1632
March	1st to 10th	Day	21% 8% 3 6% 4 4 4 4 6% 7% 33%	3435
	11th to 20th	Day	10% 5% 2 5% 4 4 4 3 6% 6% 49%	3480
	21st to end	Day	16% 10% 3 7% 5% 4 4 3 5% 6% 38%	3815
April	1st to 10th	Day	10% 6% 3 7% 5% 4 4 4 5% 8% 45%	2150
	11th to 20th	Day	12% 6% 2 7% 6% 4 5% 3 6% 7% 43%	2254
	21st to end	Day	16% 7% 2 6% 4 4 5% 3 5% 7% 40%	2427
May	1st to 10th	Day	15% 8% 3 9% 6% 6% 6% 5% 6% 8% 28%	4082
	11th to 20th	Day	20% 8% 3 7% 5% 5% 5% 4 6% 7% 30%	4081
	21st to end	Day	9% 5% 2 6% 5% 5% 6% 5% 8% 11% 39%	4476
June	1st to 10th	Day	11% 5% 2 6% 4 6% 5% 4 8% 10% 38%	2470
	11th to 20th	Day	9% 5% 2 6% 6% 6% 7% 5% 8% 10% 38%	2470
	21st to end	Day	8% 6% 2 6% 6% 5% 7% 5% 8% 10% 38%	2470
July	1st to 10th	Day	7% 4 2 6% 6% 6% 7% 5% 9% 11% 37%	4120
	11th to 20th	Day	8% 5% 2 6% 6% 6% 6% 5% 8% 11% 36%	4120
	21st to end	Day	13% 9% 4 10% 8% 7% 7% 5% 7% 8% 23%	4466
August	1st to 10th	Day	14% 6% 3 8% 6% 6% 7% 6% 8% 11% 27%	4104
	11th to 20th	Day	9% 5% 2 7% 5% 6% 8% 5% 9% 11% 33%	4044
	21st to end	Day	7% 5% 2 6% 4 5% 5% 4 7% 11% 44%	4244
September	1st to 10th	Day	13% 6% 2 5% 5% 5% 5% 4 6% 8% 42%	2228
	11th to 20th	Day	16% 7% 2 7% 5% 3 4 3 6% 7% 39%	2090
	21st to end	Day	23% 7% 2 5% 4 4 4 3 6% 5% 35%	2090
October	1st to 10th	Day	17% 5% 2 5% 3 4 3 2 4 7% 47%	1922
	11th to 20th	Day	13% 6% 2 6% 3 3 4 4 5% 7% 47%	1900
	21st to end	Day	14% 6% 2 6% 4 3 3 3 4 6% 49%	2090
November	1st to 10th	Day	11% 6% 2 4 4 4 4 3 5% 7% 51%	1957
	11th to 20th	Day	12% 7% 2 4 3 4 5% 4 5% 7% 47%	1823
	21st to end	Day	13% 7% 3 6% 4 4 3 3 5% 6% 47%	1800
December	1st to 10th	Day	15% 8% 3 7% 4 6% 5% 4 6% 7% 35%	1530
	11th to 20th	Day	14% 6% 2 6% 3 3 4 3 5% 8% 45%	2134
	21st to end	Day	18% 8% 3 7% 5% 4 6% 4 6% 7% 31%	3124
January	1st to 10th	Night	12% 8% 3 7% 5% 5% 5% 5% 7% 7% 37%	4109
	11th to 20th	Night	14% 8% 3 7% 4 4 4 4 5% 5% 43%	4120
	21st to end	Night	19% 11% 3 6% 4 3 4 3 4 6% 38%	4532
February	1st to 10th	Night	13% 8% 3 5% 4 4 4 4 6% 8% 41%	2470
	11th to 20th	Night	18% 8% 2 6% 5% 4 5% 5% 5% 5% 38%	2346
	21st to end	Night	14% 6% 3 6% 5% 4 5% 3 6% 7% 40%	1822
March	1st to 10th	Night	22% 9% 3 7% 5% 4 4 4 5% 5% 32%	3418
	11th to 20th	Night	10% 5% 2 6% 4 4 5% 5% 6% 7% 47%	3480
	21st to end	Night	15% 7% 2 5% 3 4 3 3 5% 6% 47%	3817
April	1st to 10th	Night	12% 8% 2 5% 5% 4 4 4 7% 7% 42%	2028
	11th to 20th	Night	13% 5% 2 5% 4 4 4 3 6% 8% 47%	1875
	21st to end	Night	14% 6% 3 5% 3 4 4 5% 4 7% 46%	1768
May	1st to 10th	Night	15% 8% 3 7% 5% 5% 5% 3 6% 8% 34%	2813
	11th to 20th	Night	22% 9% 3 6% 3 4 3 3 5% 7% 36%	2812
	21st to end	Night	9% 4 2 5% 4 4 5% 5% 8% 11% 44%	3079
June	1st to 10th	Night	9% 4 2 5% 4 5% 5% 5% 8% 11% 41%	1710
	11th to 20th	Night	9% 5% 2 5% 4 4 4 4 7% 9% 47%	1710
	21st to end	Night	7% 4 1 4 4 5% 4 5% 7% 11% 47%	1710
July	1st to 10th	Night	5% 4 2 6% 5% 6% 6% 5% 8% 10% 42%	2840
	11th to 20th	Night	7% 6% 2 5% 4 5% 5% 4 7% 11% 44%	2840
	21st to end	Night	8% 9% 3 9% 6% 6% 6% 5% 7% 10% 30%	3087
August	1st to 10th	Night	16% 5% 3 7% 4 5% 4 4 6% 10% 34%	2825
	11th to 20th	Night	8% 4 2 7% 5% 6% 6% 5% 7% 10% 42%	2785
	21st to end	Night	7% 4 2 5% 4 4 4 4 6% 9% 51%	3380
September	1st to 10th	Night	11% 6% 2 6% 4 4 4 4 5% 7% 47%	1947
	11th to 20th	Night	16% 6% 2 6% 3 3 5% 3 5% 8% 43%	2090
	21st to end	Night	19% 7% 3 6% 3 4 5% 3 5% 6% 39%	2090
October	1st to 10th	Night	17% 6% 2 4 4 3 4 3 5% 7% 47%	2256
	11th to 20th	Night	13% 6% 2 5% 4 4 4 3 5% 5% 50%	2280
	21st to end	Night	13% 4 2 4 3 3 3 3 5% 7% 52%	2508
November	1st to 10th	Night	13% 7% 2 5% 3 3 4 4 5% 5% 50%	2341
	11th to 20th	Night	11% 5% 2 5% 5% 4 5% 4 5% 6% 49%	2575
	21st to end	Night	12% 6% 2 5% 5% 4 4 4 5% 6% 48%	2600
December	1st to 10th	Night	15% 9% 2 8% 4 4 4 3 4 7% 39%	2669
	11th to 20th	Night	13% 7% 3 6% 5% 4 5% 4 6% 8% 40%	3047
	21st to end	Night	17% 6% 2 8% 5% 5% 6% 4 6% 8% 32%	4532

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	39% 8% 8% 27% 14% 3	2837
	11th to 20th	Day	45% 6% 7% 21% 18% 3	2840
	21st to end	Day	50% 4 7% 19% 17% 2	3124
February	1st to 10th	Day	45% 6% 9% 21% 16% 2	1710
	11th to 20th	Day	46% 5% 10% 20% 15% 4	1727
	21st to end	Day	48% 5% 10% 15% 17% 5%	1632
March	1st to 10th	Day	47% 6% 10% 20% 12% 5%	3435
	11th to 20th	Day	35% 6% 9% 24% 18% 7%	3480
	21st to end	Day	47% 5% 9% 22% 13% 3	3815
April	1st to 10th	Day	37% 6% 11% 23% 17% 6%	2150
	11th to 20th	Day	39% 6% 11% 23% 17% 4	2254
	21st to end	Day	44% 5% 12% 23% 13% 3	2427
May	1st to 10th	Day	51% 7% 12% 18% 10% 1	4120
	11th to 20th	Day	50% 8% 11% 22% 8% 1	4085
	21st to end	Day	36% 9% 17% 20% 15% 3	4489
June	1st to 10th	Day	39% 9% 16% 24% 12%	2470
	11th to 20th	Day	39% 10% 16% 19% 14% 2	2470
	21st to end	Day	39% 10% 15% 23% 13% 1	2470
July	1st to 10th	Day	36% 12% 21% 19% 8% 4	4055
	11th to 20th	Day	38% 8% 20% 24% 8% 1	4051
	21st to end	Day	55% 9% 12% 18% 6% 1	4324
August	1st to 10th	Day	46% 9% 19% 21% 5%	4091
	11th to 20th	Day	37% 11% 18% 23% 10% 1	4083
	21st to end	Day	32% 9% 18% 28% 11% 1	4244
September	1st to 10th	Day	39% 8% 11% 20% 19% 4	2228
	11th to 20th	Day	44% 6% 9% 23% 16% 2	2090
	21st to end	Day	50% 5% 10% 18% 15% 1	2090
October	1st to 10th	Day	31% 5% 9% 21% 26% 8%	3195
	11th to 20th	Day	26% 4 10% 24% 22% 15%	3140
	21st to end	Day	26% 5% 11% 27% 20% 13%	3428
November	1st to 10th	Day	33% 5% 9% 23% 25% 5%	1957
	11th to 20th	Day	35% 5% 10% 22% 23% 6%	1823
	21st to end	Day	38% 5% 10% 23% 22% 3	1800
December	1st to 10th	Day	48% 6% 12% 17% 14% 3	1530
	11th to 20th	Day	35% 6% 11% 22% 21% 5%	2131
	21st to end	Day	48% 7% 8% 22% 12% 2	3124
January	1st to 10th	Night	40% 10% 10% 21% 16% 4	4109
	11th to 20th	Night	41% 5% 8% 21% 19% 5%	4120
	21st to end	Night	48% 4 7% 24% 14% 4	4532
February	1st to 10th	Night	40% 5% 14% 23% 16% 1	2470
	11th to 20th	Night	46% 5% 9% 19% 16% 5%	2348
	21st to end	Night	42% 7% 11% 24% 11% 5%	1822
March	1st to 10th	Night	50% 7% 10% 18% 12% 2	3418
	11th to 20th	Night	32% 7% 9% 25% 22% 5%	3480
	21st to end	Night	38% 6% 12% 24% 17% 4	3817
April	1st to 10th	Night	38% 7% 12% 22% 17% 4	2028
	11th to 20th	Night	36% 4 14% 21% 21% 5%	1875
	21st to end	Night	38% 6% 12% 27% 15% 2	1768
May	1st to 10th	Night	47% 9% 14% 20% 10% 1	2840
	11th to 20th	Night	48% 6% 14% 24% 8% 1	2813
	21st to end	Night	31% 9% 21% 25% 9% 4	3087
June	1st to 10th	Night	34% 10% 17% 25% 14% 1	1710
	11th to 20th	Night	32% 8% 20% 25% 13% 1	1710
	21st to end	Night	29% 10% 19% 25% 15% 2	1710
July	1st to 10th	Night	34% 13% 20% 20% 11% 3	2795
	11th to 20th	Night	31% 10% 22% 28% 8% 1	2786
	21st to end	Night	43% 12% 16% 21% 7%	2988
August	1st to 10th	Night	43% 9% 20% 22% 6%	2814
	11th to 20th	Night	34% 10% 19% 26% 10% 1	2812
	21st to end	Night	28% 7% 17% 34% 12% 1	3380
September	1st to 10th	Night	36% 8% 14% 21% 16% 5%	1947
	11th to 20th	Night	39% 5% 12% 24% 18% 1	2090
	21st to end	Night	45% 5% 10% 21% 16% 2	2090
October	1st to 10th	Night	35% 5% 8% 22% 26% 5%	3763
	11th to 20th	Night	25% 4 10% 24% 24% 13%	3779
	21st to end	Night	20% 4 10% 30% 24% 12%	4120
November	1st to 10th	Night	36% 4 9% 22% 23% 5%	2341
	11th to 20th	Night	34% 5% 8% 23% 23% 6%	2575
	21st to end	Night	36% 5% 9% 26% 19% 5%	2600
December	1st to 10th	Night	45% 5% 9% 24% 14% 3	2669
	11th to 20th	Night	39% 8% 9% 22% 20% 3	3049
	21st to end	Night	45% 8% 11% 21% 14% 1	4532

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	36% 30% 21% 10% 2	270
	11th to 20th	Day	39% 24% 30% 7%	270
	21st to end	Day	34% 29% 26% 7% 3	297
February	1st to 10th	Day	30% 29% 29% 11% 2	270
	11th to 20th	Day	44% 27% 21% 6% 1	270
	21st to end	Day	39% 24% 27% 9% 2	287
March	1st to 10th	Day	27% 27% 27% 15% 4	460
	11th to 20th	Day	31% 28% 23% 14% 3 1	460
	21st to end	Day	28% 20% 30% 16% 5% 1	506
April	1st to 10th	Day	25% 25% 25% 16% 8%	460
	11th to 20th	Day	22% 18% 31% 18% 8% 3	452
	21st to end	Day	26% 19% 28% 17% 8% 2	460
May	1st to 10th	Day	24% 19% 32% 15% 7% 4	463
	11th to 20th	Day	22% 13% 28% 19% 14% 3	487
	21st to end	Day	17% 10% 23% 29% 16% 7%	529
June	1st to 10th	Day	15% 9% 29% 24% 17% 6%	490
	11th to 20th	Day	11% 11% 24% 29% 18% 7%	490
	21st to end	Day	11% 11% 29% 29% 15% 6%	490
July	1st to 10th	Day	12% 5% 19% 32% 22% 10%	485
	11th to 20th	Day	7% 4 22% 36% 22% 10%	484
	21st to end	Day	14% 5% 24% 27% 20% 11%	532
August	1st to 10th	Day	17% 6% 21% 25% 21% 11%	486
	11th to 20th	Day	13% 4 20% 28% 24% 11%	479
	21st to end	Day	10% 3 18% 31% 22% 16%	539
September	1st to 10th	Day	16% 4 17% 29% 24% 10%	490
	11th to 20th	Day	19% 5% 13% 30% 24% 9%	490
	21st to end	Day	30% 9% 19% 24% 15% 3	490
October	1st to 10th	Day	21% 10% 20% 28% 18% 3	490
	11th to 20th	Day	17% 19% 22% 26% 14% 2	490
	21st to end	Day	20% 15% 30% 23% 9% 2	539
November	1st to 10th	Day	20% 16% 26% 25% 11% 1	495
	11th to 20th	Day	26% 20% 29% 18% 7% 1	323
	21st to end	Day	26% 24% 27% 17% 4 1	300
December	1st to 10th	Day	32% 27% 25% 14% 2	182
	11th to 20th	Day	25% 25% 32% 15% 3 1	261
	21st to end	Day	29% 25% 29% 12% 5%	297
January	1st to 10th	Night	24% 29% 26% 17% 4	760
	11th to 20th	Night	25% 22% 29% 22% 2	760
	21st to end	Night	22% 26% 33% 18% 1	836
February	1st to 10th	Night	20% 23% 31% 23% 2	760
	11th to 20th	Night	29% 28% 27% 16% 1	743
	21st to end	Night	25% 23% 31% 19% 2	628
March	1st to 10th	Night	20% 22% 30% 24% 4	760
	11th to 20th	Night	26% 22% 26% 19% 6% 1	760
	21st to end	Night	19% 19% 31% 23% 7%	836
April	1st to 10th	Night	22% 21% 26% 25% 6%	760
	11th to 20th	Night	19% 16% 29% 24% 11%	751
	21st to end	Night	18% 14% 26% 32% 9%	628
May	1st to 10th	Night	19% 10% 29% 28% 12% 2	570
	11th to 20th	Night	20% 8% 19% 36% 16% 1	561
	21st to end	Night	16% 4 14% 34% 27% 6%	617
June	1st to 10th	Night	10% 3 17% 41% 27% 3	570
	11th to 20th	Night	11% 3 17% 34% 31% 4	570
	21st to end	Night	9% 4 18% 38% 26% 5%	570
July	1st to 10th	Night	10% 2 12% 39% 32% 5%	555
	11th to 20th	Night	6% 1 11% 36% 40% 5%	552
	21st to end	Night	7% 3 11% 40% 34% 3	624
August	1st to 10th	Night	15% 4 10% 37% 29% 5%	567
	11th to 20th	Night	12% 3 8% 37% 37% 4	564
	21st to end	Night	11% 2 8% 32% 38% 7%	779
September	1st to 10th	Night	14% 4 9% 32% 34% 6%	760
	11th to 20th	Night	17% 4 11% 31% 32% 6%	760
	21st to end	Night	24% 7% 16% 30% 21% 2	760
October	1st to 10th	Night	20% 9% 15% 27% 26% 4	760
	11th to 20th	Night	19% 13% 23% 28% 16% 1	760
	21st to end	Night	18% 11% 23% 33% 13% 2	836
November	1st to 10th	Night	19% 10% 23% 33% 13% 1	780
	11th to 20th	Night	18% 18% 24% 29% 10% 1	800
	21st to end	Night	20% 18% 25% 27% 9% 1	800
December	1st to 10th	Night	23% 22% 26% 23% 6%	768
	11th to 20th	Night	17% 23% 28% 28% 5%	760
	21st to end	Night	23% 20% 25% 28% 3	836

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