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 2024-12-24.

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 4 5% 7% 37%	101431
	Night	19% 8% 2 6% 4 4 4 3 5% 6% 37%	98686
Transparency	Day	46% 5% 12% 22% 13% 2	104970
	Night	46% 5% 12% 22% 13% 2	103237
Seeing	Day	23% 18% 31% 19% 7% 2	15528
	Night	23% 14% 31% 25% 7% 1	24210

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	17% 7% 3 6% 5% 4 3 3 5% 5% 41%	2517
	11th to 20th	Day	18% 6% 2 6% 4 4 4 3 5% 6% 42%	2520
	21st to end	Day	16% 12% 3 8% 4 4 4 2 4 6% 38%	2772
February	1st to 10th	Day	18% 11% 2 6% 5% 3 4 3 6% 5% 37%	1704
	11th to 20th	Day	13% 9% 4 8% 5% 5% 5% 4 4 5% 37%	1814
	21st to end	Day	16% 10% 4 7% 4 4 5% 3 5% 5% 36%	1570
March	1st to 10th	Day	26% 12% 3 7% 3 4 4 3 4 5% 29%	3116
	11th to 20th	Day	18% 8% 3 8% 5% 5% 4 4 4 6% 36%	3448
	21st to end	Day	25% 9% 3 6% 4 3 3 3 6% 6% 32%	3864
April	1st to 10th	Day	20% 10% 3 7% 4 4 4 3 4 5% 35%	2280
	11th to 20th	Day	19% 7% 2 5% 4 4 5% 3 5% 7% 39%	2378
	21st to end	Day	23% 8% 2 7% 4 5% 4 3 5% 6% 33%	2470
May	1st to 10th	Day	23% 10% 3 8% 5% 4 4 4 5% 6% 28%	4106
	11th to 20th	Day	19% 8% 3 7% 5% 5% 5% 4 5% 7% 33%	4100
	21st to end	Day	17% 6% 3 5% 4 4 4 5% 6% 8% 38%	4837
June	1st to 10th	Day	22% 7% 2 7% 4 5% 4 3 6% 7% 32%	2779
	11th to 20th	Day	18% 7% 2 6% 5% 3 4 4 5% 7% 38%	2850
	21st to end	Day	23% 8% 2 6% 5% 4 5% 5% 6% 7% 30%	2758
July	1st to 10th	Day	15% 7% 2 7% 5% 5% 5% 4 6% 9% 36%	4440
	11th to 20th	Day	14% 5% 2 6% 4 4 5% 5% 7% 10% 38%	4440
	21st to end	Day	22% 6% 2 5% 4 4 4 4 6% 7% 36%	4793
August	1st to 10th	Day	18% 7% 3 7% 4 5% 5% 4 6% 8% 34%	4105
	11th to 20th	Day	17% 8% 3 7% 5% 5% 4 4 5% 7% 35%	4045
	21st to end	Day	11% 5% 2 5% 4 4 5% 4 6% 9% 46%	4413
September	1st to 10th	Day	12% 4 2 5% 3 5% 5% 3 5% 7% 48%	2214
	11th to 20th	Day	12% 5% 2 5% 3 4 3 3 3 6% 54%	2090
	21st to end	Day	17% 6% 2 5% 3 3 3 3 5% 7% 44%	2090
October	1st to 10th	Day	18% 7% 2 7% 4 5% 4 3 4 6% 40%	2075
	11th to 20th	Day	19% 6% 1 6% 4 4 4 3 4 6% 43%	1848
	21st to end	Day	22% 8% 2 6% 4 4 3 3 4 5% 39%	1881
November	1st to 10th	Day	17% 6% 2 6% 4 4 3 3 5% 6% 44%	1761
	11th to 20th	Day	20% 5% 2 5% 3 3 4 3 6% 6% 44%	1800
	21st to end	Day	21% 8% 2 5% 3 3 3 3 4 6% 43%	1682
December	1st to 10th	Day	20% 12% 2 6% 4 4 4 3 4 6% 36%	1455
	11th to 20th	Day	22% 10% 3 4 4 3 3 3 3 6% 40%	1738
	21st to end	Day	26% 11% 3 6% 4 4 4 3 4 5% 31%	2678
January	1st to 10th	Night	14% 9% 3 8% 4 4 4 3 5% 5% 41%	4429
	11th to 20th	Night	14% 9% 3 9% 5% 5% 4 3 5% 6% 36%	4440
	21st to end	Night	17% 12% 4 8% 5% 4 4 3 4 5% 34%	4884
February	1st to 10th	Night	16% 9% 3 7% 6% 5% 5% 4 6% 6% 34%	2475
	11th to 20th	Night	13% 10% 3 7% 5% 5% 4 3 5% 5% 40%	2249
	21st to end	Night	15% 11% 3 7% 5% 4 4 3 5% 5% 40%	1884
March	1st to 10th	Night	29% 12% 2 6% 4 3 3 3 4 5% 31%	3724
	11th to 20th	Night	15% 9% 3 8% 6% 5% 5% 4 6% 6% 32%	3511
	21st to end	Night	26% 9% 3 6% 5% 3 4 3 6% 5% 29%	3767
April	1st to 10th	Night	22% 10% 2 6% 5% 4 4 3 5% 5% 34%	1900
	11th to 20th	Night	19% 7% 2 7% 5% 4 3 5% 5% 7% 36%	1749
	21st to end	Night	22% 8% 3 6% 4 5% 4 3 4 6% 35%	1710
May	1st to 10th	Night	23% 9% 2 6% 4 5% 4 3 6% 9% 29%	2834
	11th to 20th	Night	18% 8% 3 8% 6% 4 6% 5% 7% 9% 27%	2813
	21st to end	Night	17% 7% 3 8% 4 4 5% 4 5% 8% 35%	2747
June	1st to 10th	Night	23% 7% 2 7% 5% 5% 4 3 6% 7% 31%	1397
	11th to 20th	Night	16% 7% 2 6% 4 4 4 4 6% 8% 38%	1330
	21st to end	Night	24% 9% 2 6% 5% 4 4 4 6% 7% 29%	1413
July	1st to 10th	Night	14% 8% 3 6% 4 5% 6% 5% 8% 9% 33%	2520
	11th to 20th	Night	15% 6% 2 7% 5% 5% 5% 4 6% 8% 37%	2520
	21st to end	Night	15% 6% 2 6% 4 4 4 4 7% 9% 39%	2739
August	1st to 10th	Night	22% 7% 3 8% 5% 5% 5% 4 6% 7% 31%	2813
	11th to 20th	Night	17% 9% 2 8% 4 5% 5% 4 5% 8% 33%	2782
	21st to end	Night	10% 6% 2 6% 5% 4 4 3 5% 8% 47%	3240
September	1st to 10th	Night	12% 5% 2 6% 5% 4 4 4 5% 5% 49%	1966
	11th to 20th	Night	15% 6% 1 4 2 3 3 3 5% 6% 52%	2090
	21st to end	Night	16% 6% 3 5% 3 3 3 3 4 7% 46%	2090
October	1st to 10th	Night	19% 6% 2 6% 4 5% 4 4 4 6% 41%	2104
	11th to 20th	Night	18% 5% 2 6% 4 4 5% 3 5% 6% 42%	2327
	21st to end	Night	24% 7% 2 5% 4 3 3 3 4 6% 37%	2717
November	1st to 10th	Night	17% 6% 2 5% 4 3 4 3 4 7% 44%	2537
	11th to 20th	Night	18% 6% 2 5% 4 4 4 3 5% 6% 44%	2600
	21st to end	Night	17% 8% 2 5% 4 4 4 3 5% 7% 40%	2714
December	1st to 10th	Night	21% 10% 2 6% 5% 5% 3 3 4 6% 35%	2940
	11th to 20th	Night	23% 8% 2 4 4 4 3 3 4 5% 39%	3681
	21st to end	Night	25% 9% 2 6% 4 4 3 3 4 6% 34%	5050

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	44% 4 13% 28% 11% 1	2518
	11th to 20th	Day	42% 6% 12% 25% 12% 3	2520
	21st to end	Day	49% 4 13% 20% 13% 1	2772
February	1st to 10th	Day	49% 4 11% 19% 15% 2	1704
	11th to 20th	Day	48% 5% 11% 19% 14% 3	1823
	21st to end	Day	49% 5% 10% 20% 12% 5%	1570
March	1st to 10th	Day	58% 5% 8% 12% 14% 3	3119
	11th to 20th	Day	47% 6% 11% 20% 14% 3	3448
	21st to end	Day	52% 3 8% 15% 18% 5%	3864
April	1st to 10th	Day	52% 5% 8% 17% 15% 4	2280
	11th to 20th	Day	45% 5% 10% 22% 15% 3	2378
	21st to end	Day	51% 5% 8% 18% 14% 3	2470
May	1st to 10th	Day	55% 8% 11% 16% 9% 1	4120
	11th to 20th	Day	49% 7% 15% 19% 9% 1	4104
	21st to end	Day	42% 7% 16% 20% 14% 1	4837
June	1st to 10th	Day	50% 7% 12% 22% 9%	2779
	11th to 20th	Day	45% 6% 12% 20% 15% 3	2850
	21st to end	Day	51% 8% 11% 18% 10% 1	2758
July	1st to 10th	Day	45% 5% 12% 26% 12%	4370
	11th to 20th	Day	40% 5% 17% 26% 11% 1	4365
	21st to end	Day	47% 5% 13% 21% 14% 1	4640
August	1st to 10th	Day	47% 6% 15% 25% 7%	4092
	11th to 20th	Day	49% 5% 16% 22% 8%	4084
	21st to end	Day	35% 6% 13% 26% 19% 1	4413
September	1st to 10th	Day	34% 7% 13% 27% 19% 1	2214
	11th to 20th	Day	33% 4 9% 29% 22% 2	2090
	21st to end	Day	40% 5% 10% 27% 17% 1	2090
October	1st to 10th	Day	51% 4 9% 23% 13% 1	3452
	11th to 20th	Day	35% 4 9% 30% 22% 1	3051
	21st to end	Day	37% 3 12% 26% 20% 3	3081
November	1st to 10th	Day	41% 5% 13% 24% 16% 2	1761
	11th to 20th	Day	39% 6% 9% 23% 19% 4	1800
	21st to end	Day	44% 4 16% 23% 12% 1	1682
December	1st to 10th	Day	51% 5% 10% 22% 12% 1	1455
	11th to 20th	Day	47% 4 9% 20% 14% 5%	1738
	21st to end	Day	56% 5% 10% 18% 9% 3	2678
January	1st to 10th	Night	44% 5% 12% 27% 10% 2	4429
	11th to 20th	Night	48% 5% 14% 20% 12% 2	4440
	21st to end	Night	52% 4 10% 20% 12% 2	4884
February	1st to 10th	Night	49% 5% 12% 18% 13% 3	2475
	11th to 20th	Night	46% 4 9% 21% 16% 3	2251
	21st to end	Night	45% 5% 10% 20% 15% 5%	1884
March	1st to 10th	Night	57% 3 9% 16% 11% 4	3734
	11th to 20th	Night	49% 7% 13% 19% 10% 3	3512
	21st to end	Night	56% 3 6% 15% 17% 3	3767
April	1st to 10th	Night	51% 4 8% 17% 16% 4	1900
	11th to 20th	Night	46% 5% 9% 22% 16% 1	1749
	21st to end	Night	51% 5% 10% 17% 13% 4	1710
May	1st to 10th	Night	51% 7% 15% 18% 8% 1	2840
	11th to 20th	Night	50% 9% 13% 19% 7% 2	2814
	21st to end	Night	45% 8% 15% 19% 12% 1	2748
June	1st to 10th	Night	52% 8% 11% 20% 9% 1	1397
	11th to 20th	Night	42% 6% 17% 20% 13% 2	1330
	21st to end	Night	52% 8% 12% 18% 8% 2	1413
July	1st to 10th	Night	45% 5% 16% 21% 12%	2480
	11th to 20th	Night	46% 4 15% 25% 10%	2472
	21st to end	Night	41% 6% 15% 26% 12% 1	2651
August	1st to 10th	Night	54% 6% 14% 20% 7%	2802
	11th to 20th	Night	49% 5% 14% 21% 10%	2809
	21st to end	Night	36% 5% 14% 26% 18% 1	3240
September	1st to 10th	Night	36% 4 13% 29% 17% 1	1966
	11th to 20th	Night	34% 4 12% 28% 20% 2	2090
	21st to end	Night	39% 5% 9% 30% 16% 2	2090
October	1st to 10th	Night	44% 4 13% 23% 14% 1	3507
	11th to 20th	Night	35% 4 8% 28% 23% 1	3863
	21st to end	Night	39% 3 10% 28% 18% 3	4467
November	1st to 10th	Night	41% 5% 11% 26% 15% 2	2537
	11th to 20th	Night	42% 5% 10% 23% 18% 2	2600
	21st to end	Night	44% 5% 14% 21% 15% 2	2714
December	1st to 10th	Night	51% 5% 9% 22% 12% 1	2940
	11th to 20th	Night	47% 5% 10% 19% 18% 1	3682
	21st to end	Night	52% 5% 10% 19% 12% 1	5050

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	29% 24% 26% 18% 3	180
	11th to 20th	Day	30% 27% 26% 13% 3 1	180
	21st to end	Day	20% 27% 32% 18% 2	198
February	1st to 10th	Day	26% 21% 31% 19% 2	364
	11th to 20th	Day	26% 30% 30% 13% 1	440
	21st to end	Day	27% 21% 32% 15% 4	382
March	1st to 10th	Day	27% 24% 33% 13% 3	460
	11th to 20th	Day	22% 30% 30% 14% 3 1	460
	21st to end	Day	29% 26% 27% 12% 5% 2	506
April	1st to 10th	Day	31% 25% 27% 11% 5% 1	460
	11th to 20th	Day	27% 28% 26% 14% 4 2	452
	21st to end	Day	28% 23% 31% 13% 3 2	460
May	1st to 10th	Day	31% 22% 29% 13% 4 1	463
	11th to 20th	Day	21% 22% 34% 16% 6% 1	487
	21st to end	Day	21% 18% 37% 18% 5% 1	529
June	1st to 10th	Day	23% 17% 32% 17% 9% 2	609
	11th to 20th	Day	20% 16% 30% 21% 13% 1	680
	21st to end	Day	25% 14% 30% 21% 8% 2	588
July	1st to 10th	Day	20% 9% 39% 22% 8% 2	485
	11th to 20th	Day	16% 10% 37% 25% 10% 2	484
	21st to end	Day	17% 11% 37% 23% 9% 3	532
August	1st to 10th	Day	15% 9% 35% 28% 12% 2	486
	11th to 20th	Day	18% 8% 31% 27% 12% 4	479
	21st to end	Day	13% 12% 34% 26% 12% 3	539
September	1st to 10th	Day	15% 11% 34% 26% 12% 3	490
	11th to 20th	Day	18% 12% 30% 20% 13% 7%	490
	21st to end	Day	19% 15% 30% 21% 10% 5%	490
October	1st to 10th	Day	21% 13% 29% 22% 11% 4	482
	11th to 20th	Day	23% 16% 31% 19% 8% 3	390
	21st to end	Day	31% 15% 27% 20% 6% 1	429
November	1st to 10th	Day	22% 19% 29% 21% 7% 2	395
	11th to 20th	Day	29% 18% 30% 18% 4 1	400
	21st to end	Day	28% 23% 25% 18% 6%	400
December	1st to 10th	Day	25% 19% 35% 16% 4	255
	11th to 20th	Day	33% 26% 29% 11% 1 1	200
	21st to end	Day	31% 25% 26% 12% 6%	204
January	1st to 10th	Night	21% 20% 34% 22% 3	760
	11th to 20th	Night	27% 21% 31% 19% 4	760
	21st to end	Night	19% 19% 38% 20% 4	836
February	1st to 10th	Night	22% 19% 33% 22% 4	760
	11th to 20th	Night	21% 22% 38% 17% 2	743
	21st to end	Night	23% 18% 36% 20% 3	628
March	1st to 10th	Night	27% 17% 36% 18% 2	760
	11th to 20th	Night	22% 18% 34% 19% 7%	760
	21st to end	Night	24% 18% 33% 20% 4	836
April	1st to 10th	Night	28% 18% 31% 18% 4	760
	11th to 20th	Night	24% 16% 33% 24% 4	625
	21st to end	Night	29% 14% 31% 21% 4	570
May	1st to 10th	Night	25% 11% 32% 25% 6% 1	570
	11th to 20th	Night	23% 13% 35% 25% 4 1	562
	21st to end	Night	25% 7% 27% 33% 8%	420
June	1st to 10th	Night	26% 7% 26% 31% 9% 1	380
	11th to 20th	Night	17% 6% 29% 34% 13% 1	380
	21st to end	Night	26% 4 30% 32% 7%	380
July	1st to 10th	Night	21% 5% 23% 39% 12%	370
	11th to 20th	Night	21% 2 23% 40% 13% 1	368
	21st to end	Night	17% 4 25% 41% 13% 1	416
August	1st to 10th	Night	17% 5% 27% 38% 13%	556
	11th to 20th	Night	16% 5% 27% 36% 14% 1	562
	21st to end	Night	15% 6% 27% 35% 16% 1	691
September	1st to 10th	Night	14% 6% 32% 33% 15% 1	760
	11th to 20th	Night	18% 8% 26% 28% 17% 4	760
	21st to end	Night	18% 9% 28% 28% 14% 1	760
October	1st to 10th	Night	23% 9% 24% 28% 15% 1	760
	11th to 20th	Night	21% 11% 31% 24% 10% 2	760
	21st to end	Night	30% 13% 29% 21% 6%	836
November	1st to 10th	Night	22% 12% 31% 26% 7% 1	780
	11th to 20th	Night	22% 16% 34% 21% 7% 1	800
	21st to end	Night	23% 21% 30% 21% 5%	857
December	1st to 10th	Night	27% 14% 36% 16% 6%	900
	11th to 20th	Night	25% 21% 32% 19% 3	900
	21st to end	Night	27% 19% 31% 20% 3	884

Page generated at 2024-12-21 04:13:47. Version ac07

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