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 2026-03-03.

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	12% 7% 2 7% 5% 5% 5% 4 6% 8% 38%	130975
	Night	13% 6% 2 6% 4 4 5% 4 6% 8% 42%	130168
Transparency	Day	40% 7% 12% 21% 15% 4	134594
	Night	37% 7% 13% 23% 16% 4	134619
Seeing	Day	22% 15% 25% 21% 12% 4	16874
	Night	19% 14% 22% 28% 15% 2	27160

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% 8% 3 6% 4 4 4 3 5% 6% 44%	5258
	11th to 20th	Day	10% 7% 3 7% 4 5% 5% 3 5% 5% 46%	5280
	21st to end	Day	14% 8% 3 8% 4 3 3 3 4 5% 44%	5780
February	1st to 10th	Day	11% 8% 3 8% 5% 5% 4 5% 5% 7% 38%	1890
	11th to 20th	Day	17% 7% 3 7% 5% 5% 4 3 5% 7% 36%	1904
	21st to end	Day	16% 8% 3 7% 5% 4 5% 3 5% 6% 37%	1798
March	1st to 10th	Day	16% 7% 3 8% 5% 5% 5% 4 6% 7% 34%	4957
	11th to 20th	Day	10% 6% 2 5% 4 4 4 3 6% 7% 49%	4980
	21st to end	Day	13% 9% 3 8% 5% 5% 4 3 6% 6% 40%	5465
April	1st to 10th	Day	10% 7% 3 7% 5% 4 4 4 5% 7% 45%	2263
	11th to 20th	Day	12% 6% 2 7% 5% 4 4 3 6% 7% 43%	2374
	21st to end	Day	15% 7% 3 6% 5% 5% 5% 3 5% 7% 40%	2555
May	1st to 10th	Day	12% 7% 3 8% 6% 7% 6% 5% 8% 10% 29%	5862
	11th to 20th	Day	20% 8% 3 7% 5% 5% 5% 5% 7% 8% 27%	5861
	21st to end	Day	13% 6% 2 6% 4 5% 5% 5% 7% 10% 36%	6433
June	1st to 10th	Day	11% 5% 2 6% 5% 5% 6% 4 8% 10% 39%	2600
	11th to 20th	Day	9% 5% 2 7% 6% 6% 7% 5% 8% 10% 37%	2600
	21st to end	Day	8% 6% 2 6% 6% 5% 7% 5% 8% 10% 39%	2600
July	1st to 10th	Day	7% 5% 2 7% 5% 5% 6% 5% 9% 11% 37%	5900
	11th to 20th	Day	6% 5% 2 8% 7% 8% 7% 6% 9% 12% 29%	5900
	21st to end	Day	10% 7% 3 9% 7% 8% 7% 6% 8% 10% 25%	6424
August	1st to 10th	Day	14% 6% 3 8% 6% 6% 6% 6% 8% 11% 26%	4234
	11th to 20th	Day	9% 5% 2 7% 5% 6% 7% 5% 9% 11% 33%	4174
	21st to end	Day	7% 5% 2 6% 4 5% 5% 4 7% 11% 44%	4378
September	1st to 10th	Day	12% 5% 2 5% 5% 4 5% 4 6% 8% 43%	2345
	11th to 20th	Day	16% 7% 2 7% 5% 3 4 3 6% 7% 39%	2200
	21st to end	Day	23% 8% 2 6% 4 5% 4 4 6% 5% 33%	2200
October	1st to 10th	Day	16% 5% 2 5% 4 4 3 2 4 7% 48%	2023
	11th to 20th	Day	13% 6% 2 6% 3 3 4 4 5% 7% 48%	2000
	21st to end	Day	14% 6% 2 6% 4 3 3 3 4 6% 48%	2200
November	1st to 10th	Day	11% 6% 2 4 4 4 4 4 5% 7% 51%	2057
	11th to 20th	Day	12% 7% 2 4 3 4 5% 4 5% 7% 48%	1914
	21st to end	Day	13% 7% 2 7% 4 3 3 3 5% 6% 46%	1890
December	1st to 10th	Day	10% 6% 2 6% 4 5% 6% 3 5% 6% 45%	2682
	11th to 20th	Day	17% 8% 3 6% 3 4 4 4 5% 7% 39%	3429
	21st to end	Day	18% 8% 3 8% 5% 5% 6% 4 6% 7% 31%	4565
January	1st to 10th	Night	12% 7% 2 6% 4 5% 5% 4 6% 7% 42%	7635
	11th to 20th	Night	10% 7% 3 7% 4 4 4 4 5% 5% 47%	7680
	21st to end	Night	15% 8% 3 5% 3 3 4 3 4 6% 45%	8401
February	1st to 10th	Night	12% 8% 3 5% 4 4 4 4 6% 8% 41%	2730
	11th to 20th	Night	18% 8% 2 6% 5% 4 5% 5% 5% 5% 37%	2599
	21st to end	Night	14% 6% 3 7% 5% 4 5% 3 6% 7% 40%	2007
March	1st to 10th	Night	18% 7% 2 6% 5% 4 4 4 5% 6% 41%	4941
	11th to 20th	Night	10% 4 2 5% 4 3 4 4 6% 8% 50%	4980
	21st to end	Night	14% 8% 2 7% 5% 5% 4 3 5% 6% 42%	5467
April	1st to 10th	Night	12% 8% 2 6% 5% 4 4 3 7% 7% 41%	2135
	11th to 20th	Night	13% 5% 2 6% 4 4 4 3 6% 8% 46%	1975
	21st to end	Night	13% 6% 3 5% 4 4 5% 5% 4 7% 45%	1861
May	1st to 10th	Night	14% 7% 2 7% 4 5% 5% 3 7% 9% 36%	4033
	11th to 20th	Night	21% 9% 3 8% 4 4 4 3 6% 7% 31%	4032
	21st to end	Night	17% 6% 2 6% 4 4 5% 4 7% 9% 37%	4421
June	1st to 10th	Night	9% 4 2 5% 4 5% 5% 5% 8% 11% 42%	1800
	11th to 20th	Night	9% 5% 3 5% 4 4 4 4 7% 9% 46%	1800
	21st to end	Night	7% 4 1 5% 4 5% 4 5% 7% 11% 47%	1800
July	1st to 10th	Night	7% 5% 2 6% 5% 6% 6% 5% 8% 11% 40%	4058
	11th to 20th	Night	7% 6% 2 7% 6% 6% 6% 6% 8% 13% 34%	4057
	21st to end	Night	7% 8% 3 9% 6% 6% 7% 5% 8% 10% 32%	4427
August	1st to 10th	Night	16% 6% 3 7% 5% 5% 4 4 6% 10% 34%	2915
	11th to 20th	Night	8% 5% 1 7% 5% 5% 6% 5% 7% 10% 41%	2875
	21st to end	Night	7% 4 2 5% 4 4 4 4 6% 9% 51%	3487
September	1st to 10th	Night	11% 6% 2 5% 4 4 4 4 5% 7% 48%	2050
	11th to 20th	Night	15% 6% 2 6% 3 3 5% 3 5% 7% 44%	2200
	21st to end	Night	19% 8% 3 7% 4 5% 5% 3 5% 6% 37%	2200
October	1st to 10th	Night	17% 6% 2 4 4 3 4 3 4 6% 49%	2375
	11th to 20th	Night	13% 6% 2 5% 4 4 4 3 5% 6% 50%	2400
	21st to end	Night	14% 5% 2 4 3 3 3 3 5% 7% 52%	2640
November	1st to 10th	Night	13% 7% 2 5% 3 3 4 3 5% 5% 50%	2461
	11th to 20th	Night	11% 5% 2 5% 4 4 4 4 5% 7% 49%	2704
	21st to end	Night	12% 6% 2 5% 5% 4 4 4 5% 6% 48%	2730
December	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 5% 7% 48%	4702
	11th to 20th	Night	16% 8% 3 8% 5% 5% 5% 3 5% 7% 35%	4957
	21st to end	Night	17% 6% 2 7% 5% 5% 6% 4 6% 8% 34%	6633

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% 6% 11% 20% 22% 3	5259
	11th to 20th	Day	35% 6% 7% 20% 24% 8%	5280
	21st to end	Day	42% 4 7% 25% 18% 4	5780
February	1st to 10th	Day	45% 6% 9% 22% 16% 3	1890
	11th to 20th	Day	47% 5% 9% 21% 15% 4	1913
	21st to end	Day	47% 5% 10% 17% 16% 5%	1798
March	1st to 10th	Day	44% 7% 10% 19% 13% 8%	4957
	11th to 20th	Day	33% 6% 8% 22% 22% 9%	4980
	21st to end	Day	44% 5% 9% 21% 16% 4	5465
April	1st to 10th	Day	38% 6% 11% 23% 17% 6%	2263
	11th to 20th	Day	38% 6% 11% 24% 17% 4	2374
	21st to end	Day	44% 6% 12% 24% 13% 3	2555
May	1st to 10th	Day	47% 9% 14% 20% 10% 1	5897
	11th to 20th	Day	52% 8% 11% 20% 9% 1	5865
	21st to end	Day	40% 8% 15% 18% 14% 4	6447
June	1st to 10th	Day	38% 9% 16% 23% 14%	2600
	11th to 20th	Day	40% 10% 16% 19% 14% 1	2600
	21st to end	Day	38% 10% 15% 24% 13% 1	2600
July	1st to 10th	Day	36% 11% 19% 20% 10% 3	5835
	11th to 20th	Day	42% 13% 20% 19% 6%	5831
	21st to end	Day	48% 11% 15% 18% 7%	6240
August	1st to 10th	Day	47% 9% 19% 21% 5%	4221
	11th to 20th	Day	38% 11% 18% 23% 10% 1	4213
	21st to end	Day	32% 9% 18% 28% 12% 1	4378
September	1st to 10th	Day	38% 8% 11% 20% 19% 4	2345
	11th to 20th	Day	43% 6% 9% 24% 16% 2	2200
	21st to end	Day	51% 5% 10% 18% 14% 1	2200
October	1st to 10th	Day	30% 5% 9% 21% 27% 8%	3296
	11th to 20th	Day	26% 4 9% 24% 22% 14%	3240
	21st to end	Day	26% 5% 11% 26% 20% 12%	3538
November	1st to 10th	Day	33% 5% 10% 23% 25% 5%	2057
	11th to 20th	Day	34% 5% 10% 23% 23% 5%	1914
	21st to end	Day	38% 5% 10% 24% 21% 3	1890
December	1st to 10th	Day	37% 7% 10% 20% 21% 5%	2682
	11th to 20th	Day	42% 6% 11% 19% 17% 3	3426
	21st to end	Day	49% 8% 9% 20% 12% 1	4565
January	1st to 10th	Night	36% 8% 13% 21% 17% 5%	7636
	11th to 20th	Night	35% 6% 9% 21% 23% 7%	7680
	21st to end	Night	38% 5% 9% 26% 18% 4	8401
February	1st to 10th	Night	40% 5% 14% 23% 17% 2	2730
	11th to 20th	Night	46% 6% 9% 19% 16% 4	2601
	21st to end	Night	42% 7% 11% 23% 12% 5%	2007
March	1st to 10th	Night	40% 7% 9% 22% 16% 6%	4941
	11th to 20th	Night	30% 6% 8% 22% 26% 7%	4980
	21st to end	Night	42% 6% 10% 22% 18% 3	5467
April	1st to 10th	Night	39% 7% 12% 22% 17% 4	2135
	11th to 20th	Night	36% 4 14% 22% 20% 5%	1975
	21st to end	Night	39% 6% 12% 27% 15% 2	1861
May	1st to 10th	Night	43% 10% 16% 21% 10% 1	4060
	11th to 20th	Night	51% 7% 13% 20% 8%	4033
	21st to end	Night	41% 9% 18% 22% 7% 3	4429
June	1st to 10th	Night	34% 9% 18% 24% 15% 1	1800
	11th to 20th	Night	33% 9% 19% 24% 13% 1	1800
	21st to end	Night	29% 10% 19% 25% 15% 2	1800
July	1st to 10th	Night	35% 13% 19% 21% 11% 2	4015
	11th to 20th	Night	39% 12% 22% 20% 6% 1	4006
	21st to end	Night	42% 12% 17% 20% 9%	4297
August	1st to 10th	Night	43% 9% 20% 22% 6%	2904
	11th to 20th	Night	34% 10% 19% 26% 10% 1	2902
	21st to end	Night	29% 7% 17% 34% 13% 1	3487
September	1st to 10th	Night	35% 8% 14% 21% 17% 5%	2050
	11th to 20th	Night	39% 5% 12% 24% 19% 1	2200
	21st to end	Night	47% 6% 10% 20% 15% 2	2200
October	1st to 10th	Night	34% 5% 8% 22% 26% 5%	3882
	11th to 20th	Night	25% 4 10% 24% 24% 12%	3899
	21st to end	Night	21% 4 10% 29% 24% 12%	4252
November	1st to 10th	Night	36% 4 9% 23% 23% 5%	2461
	11th to 20th	Night	34% 5% 8% 23% 23% 6%	2704
	21st to end	Night	36% 5% 9% 26% 19% 4	2730
December	1st to 10th	Night	33% 7% 10% 28% 20% 2	4702
	11th to 20th	Night	45% 7% 12% 19% 15% 3	4959
	21st to end	Night	43% 9% 12% 22% 13% 1	6633

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	40% 28% 20% 9% 2	330
	11th to 20th	Day	43% 24% 27% 6%	330
	21st to end	Day	37% 31% 24% 6% 3	363
February	1st to 10th	Day	29% 32% 28% 9% 2	330
	11th to 20th	Day	48% 25% 21% 5% 1	330
	21st to end	Day	37% 25% 28% 9% 1	341
March	1st to 10th	Day	29% 26% 26% 15% 4	508
	11th to 20th	Day	31% 28% 24% 14% 3 1	500
	21st to end	Day	29% 21% 30% 15% 4 1	550
April	1st to 10th	Day	26% 27% 24% 16% 8%	500
	11th to 20th	Day	23% 19% 31% 18% 7% 3	492
	21st to end	Day	24% 19% 29% 18% 8% 2	500
May	1st to 10th	Day	23% 20% 32% 15% 6% 3	503
	11th to 20th	Day	23% 14% 28% 18% 13% 3	527
	21st to end	Day	20% 10% 23% 27% 14% 6%	573
June	1st to 10th	Day	15% 10% 29% 24% 16% 6%	530
	11th to 20th	Day	11% 11% 25% 29% 17% 7%	530
	21st to end	Day	10% 10% 27% 29% 17% 7%	530
July	1st to 10th	Day	11% 4 19% 34% 21% 10%	525
	11th to 20th	Day	6% 4 22% 35% 22% 11%	524
	21st to end	Day	13% 5% 23% 28% 21% 11%	576
August	1st to 10th	Day	18% 5% 21% 25% 20% 10%	526
	11th to 20th	Day	13% 4 19% 29% 24% 11%	519
	21st to end	Day	10% 4 20% 31% 21% 15%	583
September	1st to 10th	Day	15% 4 19% 28% 24% 10%	530
	11th to 20th	Day	18% 5% 14% 30% 24% 9%	530
	21st to end	Day	29% 9% 21% 24% 14% 3	530
October	1st to 10th	Day	20% 10% 20% 28% 19% 3	530
	11th to 20th	Day	17% 18% 23% 25% 14% 2	530
	21st to end	Day	24% 15% 29% 22% 9% 2	583
November	1st to 10th	Day	20% 17% 27% 24% 10% 1	535
	11th to 20th	Day	27% 20% 31% 16% 6% 1	354
	21st to end	Day	28% 25% 26% 16% 4 1	330
December	1st to 10th	Day	33% 28% 25% 12% 1	220
	11th to 20th	Day	29% 23% 33% 12% 2 1	319
	21st to end	Day	30% 24% 29% 12% 4	363
January	1st to 10th	Night	26% 29% 25% 16% 3	840
	11th to 20th	Night	27% 22% 29% 20% 2	840
	21st to end	Night	22% 26% 33% 17% 1	924
February	1st to 10th	Night	21% 23% 32% 22% 2	840
	11th to 20th	Night	29% 28% 27% 15% 1	823
	21st to end	Night	25% 24% 31% 18% 2	692
March	1st to 10th	Night	21% 22% 30% 23% 3	809
	11th to 20th	Night	26% 22% 27% 19% 6% 1	800
	21st to end	Night	20% 19% 32% 22% 6%	880
April	1st to 10th	Night	24% 21% 26% 24% 6%	800
	11th to 20th	Night	19% 17% 30% 24% 10%	791
	21st to end	Night	18% 15% 26% 32% 9%	661
May	1st to 10th	Night	19% 10% 29% 28% 12% 2	600
	11th to 20th	Night	20% 8% 19% 36% 15% 1	591
	21st to end	Night	17% 5% 15% 32% 26% 5%	650
June	1st to 10th	Night	10% 4 17% 41% 26% 3	600
	11th to 20th	Night	11% 4 17% 34% 30% 4	600
	21st to end	Night	8% 4 17% 38% 28% 5%	600
July	1st to 10th	Night	10% 2 12% 39% 32% 5%	585
	11th to 20th	Night	6% 2 12% 35% 41% 5%	582
	21st to end	Night	7% 3 12% 39% 35% 3	657
August	1st to 10th	Night	16% 4 10% 36% 29% 5%	597
	11th to 20th	Night	12% 3 8% 36% 37% 4	594
	21st to end	Night	11% 3 10% 33% 36% 7%	820
September	1st to 10th	Night	13% 4 11% 33% 34% 6%	800
	11th to 20th	Night	16% 4 11% 31% 32% 6%	800
	21st to end	Night	24% 8% 16% 31% 20% 2	800
October	1st to 10th	Night	19% 9% 15% 27% 27% 4	800
	11th to 20th	Night	19% 12% 24% 29% 16% 1	800
	21st to end	Night	20% 11% 23% 32% 12% 2	880
November	1st to 10th	Night	19% 10% 24% 33% 13% 1	820
	11th to 20th	Night	19% 19% 24% 29% 9% 1	840
	21st to end	Night	20% 18% 25% 27% 8% 1	840
December	1st to 10th	Night	23% 22% 28% 22% 5%	840
	11th to 20th	Night	19% 22% 28% 26% 5%	840
	21st to end	Night	24% 20% 25% 28% 3	924

Page generated at 2026-02-28 01:40:23. Version ac07

Copyright © 2026 Attilla Danko.