Robert Brownlee Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Robert Brownlee Observatory clear sky chart from 2005-11-06 to 2024-11-25.

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	3 4 1 3 2 2 2 2 4 5% 71%	105665
	Night	3 5% 2 4 2 2 2 2 4 5% 69%	102615
Transparency	Day	15% 4 6% 16% 27% 32%	105428
	Night	17% 5% 6% 16% 26% 31%	102442
Seeing	Day	7% 10% 30% 35% 14% 5%	15758
	Night	7% 4 16% 35% 30% 8%	25705

By Month

Percentage Cloud Cover SW (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	6% 5% 2 4 2 3 2 2 5% 6% 63%	2858
	11th to 20th	Day	7% 5% 2 3 2 3 2 2 5% 8% 61%	2964
	21st to end	Day	9% 8% 2 5% 4 4 3 3 5% 6% 51%	3498
February	1st to 10th	Day	7% 8% 2 3 3 2 3 2 4 5% 60%	1900
	11th to 20th	Day	5% 8% 2 5% 3 2 2 3 5% 6% 58%	1850
	21st to end	Day	7% 11% 3 5% 3 3 3 2 4 6% 53%	1570
March	1st to 10th	Day	8% 10% 2 5% 3 4 3 4 6% 8% 48%	3138
	11th to 20th	Day	3 6% 1 5% 3 3 3 3 6% 8% 58%	3317
	21st to end	Day	6% 8% 2 4 3 4 3 3 5% 7% 55%	4062
April	1st to 10th	Day	6% 6% 2 4 3 3 3 2 5% 6% 61%	2280
	11th to 20th	Day	4 4 2 3 2 3 3 2 4 5% 68%	2249
	21st to end	Day	2 5% 1 4 3 3 3 2 4 7% 68%	2280
May	1st to 10th	Day	2 3 1 3 2 2 3 1 3 5% 76%	4037
	11th to 20th	Day	2 2 1 2 1 1 2 2 4 5% 78%	4071
	21st to end	Day	2 1 1 2 1 1 2 1 3 4 81%	4477
June	1st to 10th	Day	1 2 1 3 2 2 2 2 4 6% 75%	2470
	11th to 20th	Day	1 1 1 1 1 1 3 4 86%	2492
	21st to end	Day	1 2 1 2 1 2 2 1 2 3 84%	2660
July	1st to 10th	Day	1 1 1 1 1 2 1 2 3 87%	4399
	11th to 20th	Day	1 1 1 2 1 2 2 2 3 5% 79%	4110
	21st to end	Day	1 2 1 2 2 2 2 2 4 5% 79%	4339
August	1st to 10th	Day	1 1 1 2 2 2 2 2 3 4 80%	3783
	11th to 20th	Day	1 1 1 1 1 2 2 4 87%	3724
	21st to end	Day	1 1 1 1 1 1 2 2 3 87%	4180
September	1st to 10th	Day	2 1 1 3 2 2 2 2 4 6% 75%	2280
	11th to 20th	Day	1 1 1 1 1 1 2 3 3 85%	2090
	21st to end	Day	1 1 1 3 2 1 2 2 3 4 81%	2090
October	1st to 10th	Day	1 2 1 2 2 2 2 2 4 5% 78%	3490
	11th to 20th	Day	1 1 1 1 1 1 1 3 6% 82%	3409
	21st to end	Day	2 2 1 3 2 2 3 2 4 5% 73%	3263
November	1st to 10th	Day	3 4 2 2 3 2 2 2 4 5% 70%	1764
	11th to 20th	Day	3 4 1 4 2 2 3 3 4 6% 68%	1800
	21st to end	Day	5% 5% 1 4 3 2 4 2 5% 7% 61%	1746
December	1st to 10th	Day	6% 5% 2 5% 4 4 3 3 6% 8% 56%	1710
	11th to 20th	Day	9% 9% 3 6% 3 3 4 5% 6% 9% 43%	2169
	21st to end	Day	8% 8% 3 7% 5% 4 4 3 5% 7% 46%	3146
January	1st to 10th	Night	5% 7% 2 4 3 3 3 2 4 6% 62%	4097
	11th to 20th	Night	5% 5% 2 6% 3 3 3 3 5% 8% 57%	4006
	21st to end	Night	10% 8% 2 5% 3 3 3 4 4 5% 53%	4180
February	1st to 10th	Night	6% 10% 2 5% 3 3 2 3 4 5% 58%	2280
	11th to 20th	Night	5% 7% 2 5% 3 3 3 3 4 5% 60%	2218
	21st to end	Night	6% 12% 4 6% 3 3 3 3 5% 5% 51%	1884
March	1st to 10th	Night	6% 13% 4 8% 3 4 4 3 4 5% 45%	3736
	11th to 20th	Night	3 6% 2 4 3 3 4 3 6% 8% 58%	3661
	21st to end	Night	2 12% 3 5% 3 3 3 3 5% 6% 54%	3586
April	1st to 10th	Night	4 7% 2 5% 4 3 3 3 5% 7% 57%	1900
	11th to 20th	Night	3 5% 2 5% 3 2 3 3 4 5% 66%	1879
	21st to end	Night	2 5% 2 5% 3 2 3 2 4 5% 66%	1900
May	1st to 10th	Night	3 3 1 3 2 1 2 2 3 4 76%	2934
	11th to 20th	Night	1 2 1 2 2 1 2 2 3 4 79%	2834
	21st to end	Night	1 1 1 4 3 2 2 2 3 4 78%	3110
June	1st to 10th	Night	1 1 3 2 2 2 1 4 5% 78%	1710
	11th to 20th	Night	1 1 1 2 1 1 1 1 3 4 85%	1678
	21st to end	Night	1 1 1 1 1 2 2 2 3 86%	1520
July	1st to 10th	Night	1 1 1 1 1 1 2 92%	2540
	11th to 20th	Night	1 1 1 1 1 2 2 3 3 84%	2860
	21st to end	Night	1 2 1 2 2 1 1 1 3 4 83%	3224
August	1st to 10th	Night	1 1 1 2 1 2 1 2 4 86%	3166
	11th to 20th	Night	1 1 1 1 1 1 1 3 4 87%	3126
	21st to end	Night	1 1 1 1 1 1 1 1 2 91%	3498
September	1st to 10th	Night	2 2 1 2 2 1 1 1 2 4 81%	1900
	11th to 20th	Night	1 1 2 1 1 2 1 2 3 86%	2090
	21st to end	Night	1 1 1 2 2 2 2 2 2 3 84%	2090
October	1st to 10th	Night	1 2 1 2 1 1 1 1 2 3 84%	3490
	11th to 20th	Night	1 2 1 2 1 2 2 2 4 3 81%	3524
	21st to end	Night	2 4 1 4 2 2 2 2 2 4 75%	4299
November	1st to 10th	Night	3 5% 2 3 3 2 2 2 4 5% 70%	2535
	11th to 20th	Night	2 4 2 4 2 2 3 2 5% 6% 68%	2600
	21st to end	Night	4 7% 2 5% 4 3 4 3 5% 6% 58%	2528
December	1st to 10th	Night	5% 8% 2 5% 3 3 3 3 5% 7% 55%	2470
	11th to 20th	Night	8% 10% 4 6% 3 3 4 3 7% 8% 45%	3041
	21st to end	Night	5% 8% 3 8% 4 4 4 4 6% 6% 49%	4521

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	19% 5% 5% 21% 34% 16%	2859
	11th to 20th	Day	22% 4 13% 23% 23% 14%	2964
	21st to end	Day	32% 6% 7% 16% 23% 16%	3498
February	1st to 10th	Day	25% 4 5% 14% 25% 27%	1900
	11th to 20th	Day	26% 4 4 16% 27% 23%	1860
	21st to end	Day	31% 6% 7% 16% 22% 19%	1570
March	1st to 10th	Day	28% 12% 7% 18% 22% 13%	3138
	11th to 20th	Day	21% 6% 13% 28% 18% 14%	3317
	21st to end	Day	23% 12% 12% 27% 19% 8%	4062
April	1st to 10th	Day	24% 5% 8% 24% 26% 13%	2280
	11th to 20th	Day	19% 5% 6% 21% 29% 21%	2252
	21st to end	Day	18% 5% 7% 21% 36% 13%	2280
May	1st to 10th	Day	13% 3 7% 21% 35% 20%	4037
	11th to 20th	Day	9% 4 7% 22% 45% 13%	4074
	21st to end	Day	10% 7% 5% 23% 44% 12%	4477
June	1st to 10th	Day	12% 3 6% 19% 37% 22%	2470
	11th to 20th	Day	6% 3 4 12% 30% 45%	2492
	21st to end	Day	8% 2 3 6% 24% 57%	2660
July	1st to 10th	Day	5% 2 2 5% 12% 75%	4330
	11th to 20th	Day	8% 3 4 10% 24% 51%	4041
	21st to end	Day	9% 2 5% 19% 23% 42%	4202
August	1st to 10th	Day	8% 3 3 12% 35% 39%	3770
	11th to 20th	Day	4 2 2 13% 22% 56%	3760
	21st to end	Day	5% 2 2 5% 25% 61%	4179
September	1st to 10th	Day	10% 3 5% 8% 30% 45%	2280
	11th to 20th	Day	6% 2 3 10% 33% 47%	2090
	21st to end	Day	9% 2 3 12% 31% 44%	2090
October	1st to 10th	Day	10% 3 3 11% 18% 56%	3490
	11th to 20th	Day	7% 2 3 11% 33% 44%	3409
	21st to end	Day	14% 3 3 7% 31% 42%	3263
November	1st to 10th	Day	17% 2 5% 14% 31% 32%	1764
	11th to 20th	Day	17% 3 5% 13% 24% 38%	1800
	21st to end	Day	24% 3 5% 13% 23% 31%	1746
December	1st to 10th	Day	25% 4 7% 16% 21% 26%	1710
	11th to 20th	Day	32% 7% 11% 14% 20% 16%	2168
	21st to end	Day	33% 10% 7% 20% 18% 13%	3146
January	1st to 10th	Night	22% 5% 6% 14% 33% 20%	4097
	11th to 20th	Night	23% 7% 11% 25% 19% 15%	4006
	21st to end	Night	32% 6% 9% 15% 24% 15%	4180
February	1st to 10th	Night	28% 4 5% 15% 21% 28%	2280
	11th to 20th	Night	25% 4 6% 16% 23% 25%	2219
	21st to end	Night	33% 6% 4 16% 21% 20%	1884
March	1st to 10th	Night	31% 15% 5% 18% 16% 14%	3736
	11th to 20th	Night	20% 7% 13% 25% 23% 13%	3661
	21st to end	Night	24% 12% 13% 23% 20% 8%	3586
April	1st to 10th	Night	26% 5% 10% 23% 20% 17%	1900
	11th to 20th	Night	19% 6% 6% 22% 26% 21%	1879
	21st to end	Night	21% 5% 5% 21% 33% 15%	1900
May	1st to 10th	Night	14% 3 5% 26% 31% 21%	2934
	11th to 20th	Night	10% 5% 5% 26% 40% 14%	2835
	21st to end	Night	9% 9% 6% 23% 43% 10%	3110
June	1st to 10th	Night	11% 3 5% 20% 36% 25%	1710
	11th to 20th	Night	6% 3 4 13% 29% 44%	1678
	21st to end	Night	6% 1 3 7% 23% 59%	1520
July	1st to 10th	Night	3 1 1 4 14% 77%	2500
	11th to 20th	Night	5% 2 3 11% 22% 55%	2806
	21st to end	Night	8% 2 5% 18% 27% 40%	3123
August	1st to 10th	Night	5% 2 3 14% 36% 40%	3156
	11th to 20th	Night	3 1 3 13% 27% 53%	3156
	21st to end	Night	4 1 2 5% 25% 63%	3498
September	1st to 10th	Night	9% 3 4 7% 29% 48%	1900
	11th to 20th	Night	7% 2 3 9% 35% 44%	2090
	21st to end	Night	9% 2 3 12% 29% 45%	2090
October	1st to 10th	Night	9% 1 3 9% 19% 59%	3490
	11th to 20th	Night	9% 2 3 11% 33% 42%	3524
	21st to end	Night	15% 3 2 7% 26% 47%	4299
November	1st to 10th	Night	17% 4 4 12% 28% 34%	2535
	11th to 20th	Night	17% 3 4 14% 24% 38%	2600
	21st to end	Night	26% 5% 5% 14% 19% 32%	2528
December	1st to 10th	Night	27% 4 6% 14% 21% 28%	2470
	11th to 20th	Night	32% 8% 9% 13% 23% 16%	3041
	21st to end	Night	30% 9% 8% 22% 19% 12%	4521

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	17% 7% 30% 33% 11% 1	270
	11th to 20th	Day	14% 7% 29% 33% 15% 2	335
	21st to end	Day	14% 15% 30% 29% 10% 2	506
February	1st to 10th	Day	15% 16% 31% 24% 12% 3	460
	11th to 20th	Day	13% 15% 35% 30% 6% 1	455
	21st to end	Day	16% 19% 32% 27% 6% 1	382
March	1st to 10th	Day	19% 17% 33% 22% 8% 1	460
	11th to 20th	Day	16% 13% 36% 26% 7% 2	460
	21st to end	Day	13% 22% 36% 22% 7% 1	506
April	1st to 10th	Day	12% 14% 33% 26% 13% 2	460
	11th to 20th	Day	6% 24% 34% 23% 11% 2	452
	21st to end	Day	7% 18% 30% 28% 10% 6%	460
May	1st to 10th	Day	9% 15% 34% 28% 9% 4	463
	11th to 20th	Day	6% 13% 34% 30% 13% 4	488
	21st to end	Day	7% 9% 35% 31% 13% 5%	529
June	1st to 10th	Day	3 8% 27% 37% 18% 8%	490
	11th to 20th	Day	1 8% 32% 38% 15% 7%	490
	21st to end	Day	2 5% 29% 40% 16% 9%	490
July	1st to 10th	Day	1 1 20% 50% 18% 9%	486
	11th to 20th	Day	1 1 18% 46% 22% 12%	484
	21st to end	Day	2 2 17% 47% 18% 14%	531
August	1st to 10th	Day	2 2 27% 43% 15% 11%	487
	11th to 20th	Day	1 2 21% 45% 19% 13%	477
	21st to end	Day	1 2 23% 45% 19% 10%	539
September	1st to 10th	Day	3 3 28% 44% 17% 6%	490
	11th to 20th	Day	3 6% 31% 41% 16% 3	490
	21st to end	Day	2 9% 32% 37% 18% 3	490
October	1st to 10th	Day	3 9% 31% 40% 16% 1	490
	11th to 20th	Day	5% 7% 32% 38% 15% 3	490
	21st to end	Day	5% 9% 28% 37% 18% 3	429
November	1st to 10th	Day	7% 9% 26% 41% 15% 2	300
	11th to 20th	Day	5% 11% 28% 36% 15% 4	300
	21st to end	Day	7% 12% 29% 34% 15% 3	282
December	1st to 10th	Day	10% 8% 32% 35% 14% 1	270
	11th to 20th	Day	7% 10% 31% 33% 16% 2	270
	21st to end	Day	15% 13% 33% 23% 12% 3	297
January	1st to 10th	Night	13% 6% 21% 33% 20% 7%	760
	11th to 20th	Night	10% 5% 22% 39% 19% 6%	760
	21st to end	Night	14% 6% 25% 33% 18% 4	836
February	1st to 10th	Night	12% 7% 23% 33% 18% 6%	760
	11th to 20th	Night	10% 7% 23% 35% 19% 5%	744
	21st to end	Night	15% 9% 24% 32% 16% 4	628
March	1st to 10th	Night	13% 8% 23% 35% 17% 3	760
	11th to 20th	Night	12% 6% 20% 33% 22% 7%	760
	21st to end	Night	10% 7% 24% 36% 19% 4	836
April	1st to 10th	Night	9% 6% 18% 41% 21% 5%	760
	11th to 20th	Night	7% 5% 25% 36% 22% 5%	751
	21st to end	Night	7% 6% 18% 38% 23% 7%	760
May	1st to 10th	Night	8% 4 19% 40% 25% 4	618
	11th to 20th	Night	6% 2 17% 37% 32% 6%	561
	21st to end	Night	5% 5% 17% 40% 28% 6%	617
June	1st to 10th	Night	1 3 12% 40% 39% 5%	570
	11th to 20th	Night	2 1 12% 43% 37% 4	538
	21st to end	Night	1 1 5% 34% 49% 9%	380
July	1st to 10th	Night	1 1 28% 57% 14%	370
	11th to 20th	Night	2 4 35% 51% 8%	552
	21st to end	Night	2 1 3 29% 53% 13%	690
August	1st to 10th	Night	1 1 4 32% 45% 16%	756
	11th to 20th	Night	1 1 2 26% 51% 18%	752
	21st to end	Night	2 3 27% 50% 18%	836
September	1st to 10th	Night	3 1 4 30% 45% 17%	760
	11th to 20th	Night	1 1 9% 35% 42% 13%	760
	21st to end	Night	2 1 11% 36% 38% 11%	760
October	1st to 10th	Night	5% 2 12% 46% 29% 6%	760
	11th to 20th	Night	5% 3 14% 37% 32% 9%	760
	21st to end	Night	6% 4 20% 33% 26% 12%	836
November	1st to 10th	Night	7% 4 14% 40% 27% 8%	780
	11th to 20th	Night	7% 3 21% 35% 25% 9%	800
	21st to end	Night	8% 5% 21% 36% 23% 7%	778
December	1st to 10th	Night	11% 4 21% 35% 23% 6%	760
	11th to 20th	Night	15% 4 23% 34% 19% 5%	760
	21st to end	Night	12% 8% 26% 32% 17% 5%	836

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