The Brower Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Brower Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	19% 8% 2 6% 4 4 4 3 5% 7% 37%	130518
	Night	18% 9% 3 7% 4 4 4 3 5% 6% 37%	131631
Transparency	Day	47% 4 11% 22% 14% 2	134039
	Night	47% 4 11% 22% 14% 2	136198
Seeing	Day	24% 18% 31% 18% 7% 2	17191
	Night	23% 14% 31% 25% 7% 1	25942

By Month

Percentage Cloud Cover NE (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	13% 7% 3 8% 6% 5% 5% 4 5% 7% 37%	4661
	11th to 20th	Day	16% 10% 3 7% 4 4 4 3 4 5% 42%	4650
	21st to end	Day	10% 10% 3 6% 4 4 4 2 4 5% 48%	5124
February	1st to 10th	Day	16% 11% 2 6% 5% 3 4 3 6% 5% 37%	1884
	11th to 20th	Day	13% 9% 4 8% 5% 5% 5% 4 5% 6% 38%	2011
	21st to end	Day	15% 10% 4 7% 5% 4 5% 3 5% 5% 37%	1730
March	1st to 10th	Day	24% 11% 3 9% 4 4 4 3 4 4 31%	4576
	11th to 20th	Day	20% 10% 3 7% 4 4 4 3 4 6% 35%	5043
	21st to end	Day	28% 8% 3 6% 4 4 3 3 6% 6% 31%	5650
April	1st to 10th	Day	20% 10% 3 7% 4 4 4 3 4 5% 35%	2520
	11th to 20th	Day	20% 7% 2 5% 4 3 5% 3 4 7% 38%	2580
	21st to end	Day	23% 8% 2 7% 4 4 4 3 5% 6% 35%	2600
May	1st to 10th	Day	32% 11% 3 8% 4 3 3 3 4 5% 23%	5886
	11th to 20th	Day	19% 9% 3 8% 5% 5% 5% 3 5% 7% 31%	5894
	21st to end	Day	21% 7% 2 6% 4 4 4 4 6% 8% 34%	6949
June	1st to 10th	Day	22% 7% 2 7% 4 5% 4 3 6% 8% 32%	2925
	11th to 20th	Day	18% 7% 2 6% 5% 3 4 4 5% 7% 38%	3000
	21st to end	Day	23% 8% 2 6% 5% 4 5% 5% 5% 7% 31%	2903
July	1st to 10th	Day	18% 8% 2 7% 5% 4 5% 4 6% 9% 31%	6360
	11th to 20th	Day	16% 7% 2 6% 4 4 5% 4 6% 9% 37%	6360
	21st to end	Day	17% 5% 2 4 4 4 4 4 6% 8% 43%	6891
August	1st to 10th	Day	17% 6% 3 7% 4 4 5% 4 6% 8% 35%	4235
	11th to 20th	Day	17% 8% 3 7% 5% 5% 4 4 5% 7% 35%	4175
	21st to end	Day	11% 5% 2 5% 3 4 5% 4 6% 9% 46%	4552
September	1st to 10th	Day	13% 4 2 5% 3 4 5% 3 6% 7% 48%	2330
	11th to 20th	Day	12% 5% 2 5% 3 4 3 3 3 6% 55%	2200
	21st to end	Day	17% 6% 2 4 3 3 3 3 5% 7% 45%	2200
October	1st to 10th	Day	17% 6% 2 7% 4 5% 4 3 4 6% 42%	2184
	11th to 20th	Day	19% 7% 1 6% 4 4 4 3 4 6% 42%	1945
	21st to end	Day	21% 8% 2 6% 4 4 3 3 4 5% 39%	1980
November	1st to 10th	Day	17% 6% 2 6% 4 5% 3 3 5% 6% 44%	1851
	11th to 20th	Day	20% 5% 2 5% 3 3 4 3 5% 6% 44%	1890
	21st to end	Day	21% 8% 2 5% 3 3 3 3 4 6% 41%	1766
December	1st to 10th	Day	16% 11% 2 5% 3 3 4 3 5% 7% 41%	2434
	11th to 20th	Day	19% 8% 2 4 3 3 3 3 3 6% 46%	2688
	21st to end	Day	22% 11% 3 6% 4 4 4 3 4 5% 35%	3891
January	1st to 10th	Night	12% 9% 3 8% 5% 5% 4 3 5% 6% 41%	8234
	11th to 20th	Night	15% 12% 4 9% 5% 5% 5% 4 5% 5% 31%	8235
	21st to end	Night	11% 10% 3 7% 4 4 4 3 4 6% 42%	9057
February	1st to 10th	Night	15% 9% 3 7% 5% 5% 5% 3 6% 6% 35%	2735
	11th to 20th	Night	13% 9% 3 7% 5% 4 5% 3 5% 5% 40%	2491
	21st to end	Night	14% 10% 3 6% 5% 4 4 3 5% 5% 40%	2076
March	1st to 10th	Night	27% 10% 3 7% 4 4 3 3 4 5% 30%	5484
	11th to 20th	Night	17% 10% 3 7% 5% 4 4 4 6% 6% 32%	5136
	21st to end	Night	25% 9% 3 6% 4 3 4 3 6% 6% 30%	5509
April	1st to 10th	Night	23% 10% 2 6% 4 4 4 3 6% 5% 34%	2100
	11th to 20th	Night	20% 7% 2 7% 5% 4 3 5% 5% 7% 35%	1900
	21st to end	Night	22% 9% 3 7% 4 5% 4 3 4 6% 35%	1800
May	1st to 10th	Night	32% 10% 3 7% 4 4 3 3 5% 7% 23%	4054
	11th to 20th	Night	21% 9% 3 7% 5% 4 6% 4 6% 7% 28%	4033
	21st to end	Night	18% 8% 3 8% 4 4 5% 4 5% 8% 32%	3935
June	1st to 10th	Night	23% 7% 2 6% 5% 5% 4 3 6% 7% 32%	1470
	11th to 20th	Night	16% 7% 2 6% 4 4 4 4 6% 8% 39%	1400
	21st to end	Night	23% 9% 2 5% 5% 4 4 4 6% 8% 30%	1488
July	1st to 10th	Night	15% 8% 3 6% 4 4 5% 5% 7% 9% 34%	3600
	11th to 20th	Night	16% 10% 3 7% 5% 5% 4 4 5% 7% 35%	3600
	21st to end	Night	12% 5% 2 5% 4 4 4 4 7% 9% 44%	3926
August	1st to 10th	Night	22% 7% 2 8% 5% 5% 5% 3 6% 7% 31%	2903
	11th to 20th	Night	16% 9% 3 8% 4 5% 5% 4 5% 8% 34%	2872
	21st to end	Night	10% 6% 2 6% 5% 4 4 3 5% 8% 47%	3343
September	1st to 10th	Night	12% 5% 2 6% 4 4 4 4 4 5% 50%	2070
	11th to 20th	Night	15% 6% 1 4 2 3 3 3 5% 6% 52%	2200
	21st to end	Night	16% 6% 3 5% 3 3 3 3 4 7% 46%	2200
October	1st to 10th	Night	18% 6% 2 6% 4 4 4 4 4 6% 43%	2215
	11th to 20th	Night	18% 5% 2 6% 4 4 5% 3 5% 6% 42%	2450
	21st to end	Night	24% 7% 2 5% 4 3 4 3 5% 6% 37%	2860
November	1st to 10th	Night	17% 6% 2 5% 4 3 4 3 4 6% 44%	2667
	11th to 20th	Night	18% 6% 2 5% 4 4 4 3 5% 6% 44%	2730
	21st to end	Night	17% 8% 2 5% 4 4 4 3 5% 7% 40%	2850
December	1st to 10th	Night	16% 10% 3 6% 4 4 4 3 5% 7% 38%	4958
	11th to 20th	Night	23% 7% 2 4 3 3 3 3 4 5% 41%	5741
	21st to end	Night	22% 11% 3 6% 4 3 3 3 4 5% 37%	7309

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	47% 3 11% 25% 13% 1	4662
	11th to 20th	Day	46% 4 9% 25% 13% 3	4680
	21st to end	Day	40% 3 9% 22% 20% 5%	5124
February	1st to 10th	Day	47% 4 10% 21% 15% 3	1884
	11th to 20th	Day	47% 5% 11% 20% 14% 3	2020
	21st to end	Day	48% 5% 10% 20% 13% 5%	1730
March	1st to 10th	Day	57% 4 7% 10% 14% 7%	4579
	11th to 20th	Day	50% 4 14% 19% 11% 2	5043
	21st to end	Day	54% 2 6% 17% 16% 5%	5650
April	1st to 10th	Day	52% 4 8% 17% 15% 4	2520
	11th to 20th	Day	46% 4 10% 22% 15% 3	2580
	21st to end	Day	51% 5% 8% 18% 15% 4	2600
May	1st to 10th	Day	63% 6% 9% 14% 7% 1	5897
	11th to 20th	Day	52% 5% 13% 19% 7% 3	5898
	21st to end	Day	47% 5% 14% 21% 12% 1	6949
June	1st to 10th	Day	50% 7% 12% 21% 10%	2925
	11th to 20th	Day	45% 5% 12% 20% 15% 2	3000
	21st to end	Day	50% 8% 11% 19% 11% 1	2903
July	1st to 10th	Day	49% 4 14% 22% 10%	6290
	11th to 20th	Day	43% 4 15% 23% 13% 1	6285
	21st to end	Day	40% 3 14% 26% 16% 1	6693
August	1st to 10th	Day	46% 6% 15% 25% 8%	4222
	11th to 20th	Day	48% 5% 16% 23% 8%	4214
	21st to end	Day	34% 6% 13% 27% 19% 1	4552
September	1st to 10th	Day	34% 6% 13% 27% 19% 1	2330
	11th to 20th	Day	32% 4 9% 30% 22% 2	2200
	21st to end	Day	39% 5% 10% 27% 17% 1	2200
October	1st to 10th	Day	50% 3 8% 23% 13% 1	3561
	11th to 20th	Day	35% 3 9% 30% 22% 1	3148
	21st to end	Day	37% 3 11% 26% 20% 3	3180
November	1st to 10th	Day	41% 5% 12% 23% 16% 2	1851
	11th to 20th	Day	40% 5% 9% 23% 19% 4	1890
	21st to end	Day	45% 4 15% 23% 12% 1	1766
December	1st to 10th	Day	44% 3 13% 26% 13% 1	2434
	11th to 20th	Day	42% 3 12% 19% 17% 8%	2688
	21st to end	Day	53% 3 8% 18% 14% 4	3891
January	1st to 10th	Night	45% 3 11% 27% 13% 1	8234
	11th to 20th	Night	54% 4 11% 17% 12% 2	8280
	21st to end	Night	44% 3 6% 23% 19% 5%	9057
February	1st to 10th	Night	48% 5% 11% 20% 14% 3	2735
	11th to 20th	Night	46% 4 10% 21% 16% 3	2493
	21st to end	Night	45% 5% 10% 21% 16% 4	2076
March	1st to 10th	Night	57% 3 6% 12% 14% 7%	5494
	11th to 20th	Night	50% 5% 17% 18% 8% 2	5137
	21st to end	Night	54% 3 6% 15% 19% 3	5509
April	1st to 10th	Night	52% 4 8% 17% 16% 4	2100
	11th to 20th	Night	47% 5% 9% 22% 15% 1	1900
	21st to end	Night	51% 5% 10% 17% 13% 3	1800
May	1st to 10th	Night	61% 5% 11% 14% 7% 1	4060
	11th to 20th	Night	53% 7% 11% 20% 8% 1	4034
	21st to end	Night	50% 6% 13% 21% 9% 1	3936
June	1st to 10th	Night	52% 7% 10% 21% 9% 1	1470
	11th to 20th	Night	43% 6% 17% 20% 13% 2	1400
	21st to end	Night	51% 7% 12% 19% 9% 2	1488
July	1st to 10th	Night	46% 4 15% 24% 10%	3560
	11th to 20th	Night	49% 3 13% 23% 11%	3552
	21st to end	Night	37% 5% 17% 30% 11% 1	3809
August	1st to 10th	Night	53% 6% 13% 21% 7%	2892
	11th to 20th	Night	48% 5% 14% 22% 10%	2899
	21st to end	Night	36% 5% 14% 26% 18% 1	3343
September	1st to 10th	Night	36% 4 13% 29% 17% 1	2070
	11th to 20th	Night	33% 4 12% 28% 21% 2	2200
	21st to end	Night	39% 5% 9% 29% 16% 2	2200
October	1st to 10th	Night	43% 4 13% 24% 15% 1	3618
	11th to 20th	Night	35% 4 9% 28% 22% 1	3986
	21st to end	Night	39% 3 10% 28% 17% 2	4610
November	1st to 10th	Night	41% 5% 11% 26% 16% 2	2667
	11th to 20th	Night	42% 5% 10% 23% 18% 2	2730
	21st to end	Night	44% 5% 15% 20% 15% 2	2850
December	1st to 10th	Night	46% 3 8% 30% 11%	4958
	11th to 20th	Night	47% 3 10% 18% 18% 3	5742
	21st to end	Night	51% 4 9% 18% 17% 1	7309

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	28% 25% 29% 15% 3	220
	11th to 20th	Day	29% 28% 25% 14% 4 1	220
	21st to end	Day	22% 30% 31% 15% 2	242
February	1st to 10th	Day	25% 22% 30% 20% 2	424
	11th to 20th	Day	26% 29% 31% 13% 2	517
	21st to end	Day	26% 22% 34% 14% 4	446
March	1st to 10th	Day	29% 26% 30% 12% 3	540
	11th to 20th	Day	24% 29% 30% 13% 3 1	540
	21st to end	Day	31% 25% 27% 11% 4 1	594
April	1st to 10th	Day	34% 24% 27% 10% 5% 1	540
	11th to 20th	Day	30% 27% 25% 13% 3 1	516
	21st to end	Day	28% 23% 32% 12% 3 1	500
May	1st to 10th	Day	34% 21% 28% 13% 4 1	503
	11th to 20th	Day	22% 21% 33% 17% 6% 1	527
	21st to end	Day	22% 18% 36% 18% 5% 1	573
June	1st to 10th	Day	24% 16% 32% 17% 9% 2	655
	11th to 20th	Day	21% 16% 30% 20% 12% 1	730
	21st to end	Day	24% 14% 31% 21% 8% 2	633
July	1st to 10th	Day	21% 9% 39% 22% 8% 2	525
	11th to 20th	Day	17% 10% 37% 24% 9% 2	524
	21st to end	Day	17% 11% 36% 24% 9% 2	576
August	1st to 10th	Day	14% 8% 34% 29% 12% 3	526
	11th to 20th	Day	18% 8% 31% 26% 13% 5%	519
	21st to end	Day	13% 13% 34% 26% 12% 3	583
September	1st to 10th	Day	15% 11% 34% 26% 11% 3	530
	11th to 20th	Day	16% 12% 30% 21% 13% 8%	530
	21st to end	Day	19% 15% 31% 20% 10% 5%	530
October	1st to 10th	Day	21% 13% 29% 22% 11% 4	521
	11th to 20th	Day	23% 16% 31% 19% 7% 3	420
	21st to end	Day	30% 15% 27% 21% 6% 1	462
November	1st to 10th	Day	23% 19% 30% 20% 6% 2	425
	11th to 20th	Day	28% 19% 30% 18% 4 1	430
	21st to end	Day	28% 22% 26% 18% 6%	430
December	1st to 10th	Day	27% 18% 35% 16% 4	278
	11th to 20th	Day	34% 25% 29% 10% 1	220
	21st to end	Day	33% 21% 26% 13% 6%	242
January	1st to 10th	Night	21% 19% 35% 22% 3	840
	11th to 20th	Night	27% 20% 30% 19% 4	840
	21st to end	Night	19% 20% 37% 20% 4	924
February	1st to 10th	Night	22% 20% 33% 22% 4	840
	11th to 20th	Night	21% 21% 38% 17% 3	823
	21st to end	Night	22% 17% 37% 20% 3	692
March	1st to 10th	Night	28% 17% 36% 17% 2	840
	11th to 20th	Night	23% 17% 34% 19% 6%	840
	21st to end	Night	25% 18% 33% 20% 4	924
April	1st to 10th	Night	30% 19% 31% 17% 4	840
	11th to 20th	Night	26% 16% 32% 23% 4	680
	21st to end	Night	30% 14% 31% 21% 4	600
May	1st to 10th	Night	26% 11% 32% 25% 6% 1	600
	11th to 20th	Night	24% 12% 34% 25% 4 1	592
	21st to end	Night	25% 7% 27% 32% 7%	442
June	1st to 10th	Night	26% 7% 26% 31% 9% 1	400
	11th to 20th	Night	18% 6% 30% 34% 13% 1	400
	21st to end	Night	25% 5% 31% 33% 7%	400
July	1st to 10th	Night	21% 5% 23% 40% 11%	390
	11th to 20th	Night	21% 2 23% 40% 14% 1	388
	21st to end	Night	16% 4 26% 41% 12% 1	438
August	1st to 10th	Night	17% 4 27% 39% 13%	586
	11th to 20th	Night	16% 5% 26% 37% 15% 1	592
	21st to end	Night	15% 6% 27% 35% 16% 1	728
September	1st to 10th	Night	14% 6% 32% 32% 15% 1	800
	11th to 20th	Night	18% 7% 25% 29% 18% 4	800
	21st to end	Night	19% 9% 28% 29% 14% 1	800
October	1st to 10th	Night	22% 10% 24% 28% 15% 1	800
	11th to 20th	Night	22% 11% 32% 24% 10% 2	800
	21st to end	Night	30% 13% 29% 23% 6%	880
November	1st to 10th	Night	23% 13% 31% 25% 7% 1	820
	11th to 20th	Night	23% 15% 33% 21% 7% 1	840
	21st to end	Night	22% 21% 31% 21% 5%	903
December	1st to 10th	Night	27% 14% 35% 18% 6%	950
	11th to 20th	Night	25% 21% 32% 18% 3	950
	21st to end	Night	27% 19% 31% 20% 3	960

Page generated at 2026-04-14 02:44:27. Version ac07

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