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 2024-01-16.

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	18% 7% 3 5% 5% 3 4 4 5% 7% 38%	74586
	Night	19% 7% 3 5% 5% 3 4 4 5% 7% 38%	75204
Transparency	Day	45% 6% 11% 22% 14% 2	74292
	Night	46% 5% 11% 22% 14% 2	75033
Seeing	Day	23% 18% 31% 19% 7% 2	14277
	Night	23% 14% 31% 25% 7% 1	23018

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	15% 8% 4 6% 6% 3 4 3 4 6% 40%	1517
	11th to 20th	Day	21% 8% 3 5% 5% 3 4 4 5% 6% 37%	1486
	21st to end	Day	14% 9% 3 7% 5% 3 5% 4 5% 6% 40%	1584
February	1st to 10th	Day	19% 11% 2 6% 5% 3 4 4 6% 6% 36%	1615
	11th to 20th	Day	14% 9% 4 7% 6% 4 5% 5% 4 5% 37%	1716
	21st to end	Day	16% 10% 5% 7% 6% 3 5% 5% 5% 6% 35%	1480
March	1st to 10th	Day	17% 8% 3 7% 5% 3 5% 4 5% 7% 36%	1797
	11th to 20th	Day	18% 7% 2 6% 5% 4 4 4 3 6% 42%	1962
	21st to end	Day	18% 9% 3 6% 5% 2 4 4 5% 6% 40%	2200
April	1st to 10th	Day	21% 9% 4 6% 5% 3 5% 4 4 5% 35%	2160
	11th to 20th	Day	19% 6% 2 5% 5% 3 5% 4 4 7% 39%	2251
	21st to end	Day	24% 8% 3 6% 5% 3 5% 4 4 6% 32%	2340
May	1st to 10th	Day	24% 8% 3 6% 5% 2 4 5% 4 6% 33%	2340
	11th to 20th	Day	20% 7% 3 5% 5% 3 4 4 4 6% 38%	2306
	21st to end	Day	19% 6% 3 5% 5% 3 4 5% 6% 8% 38%	2725
June	1st to 10th	Day	23% 7% 2 7% 5% 4 4 4 5% 7% 32%	2632
	11th to 20th	Day	19% 6% 3 5% 5% 2 4 5% 5% 7% 38%	2700
	21st to end	Day	23% 7% 2 5% 5% 3 5% 6% 5% 7% 30%	2613
July	1st to 10th	Day	15% 7% 2 5% 5% 3 4 5% 5% 9% 39%	2520
	11th to 20th	Day	13% 6% 3 6% 6% 3 6% 6% 7% 8% 36%	2520
	21st to end	Day	15% 6% 2 5% 5% 4 5% 5% 8% 9% 37%	2736
August	1st to 10th	Day	14% 5% 3 5% 5% 4 5% 6% 6% 9% 39%	2327
	11th to 20th	Day	15% 5% 3 6% 5% 4 5% 5% 6% 9% 37%	2304
	21st to end	Day	14% 5% 2 4 4 3 4 5% 6% 8% 45%	2511
September	1st to 10th	Day	13% 4 2 5% 4 3 5% 4 5% 7% 47%	2098
	11th to 20th	Day	13% 5% 2 4 4 3 3 3 3 6% 54%	1980
	21st to end	Day	17% 6% 2 4 4 3 3 4 5% 7% 45%	1980
October	1st to 10th	Day	18% 6% 2 6% 5% 4 4 4 4 6% 40%	1966
	11th to 20th	Day	20% 6% 1 6% 5% 3 4 3 4 6% 41%	1751
	21st to end	Day	23% 8% 3 5% 4 3 3 4 4 5% 38%	1782
November	1st to 10th	Day	18% 6% 2 5% 4 3 4 3 4 5% 45%	1671
	11th to 20th	Day	21% 4 2 4 4 2 4 4 5% 7% 43%	1710
	21st to end	Day	21% 7% 3 4 3 2 3 4 4 5% 43%	1598
December	1st to 10th	Day	21% 11% 3 5% 5% 3 4 3 4 6% 35%	1383
	11th to 20th	Day	22% 9% 3 4 4 3 4 4 3 6% 39%	1668
	21st to end	Day	26% 10% 3 6% 5% 4 3 4 4 5% 31%	2657
January	1st to 10th	Night	16% 8% 3 6% 5% 3 4 4 5% 6% 40%	2649
	11th to 20th	Night	18% 11% 4 7% 5% 3 5% 5% 5% 6% 32%	2602
	21st to end	Night	12% 9% 3 6% 6% 3 4 5% 5% 6% 40%	2772
February	1st to 10th	Night	17% 9% 3 5% 7% 4 5% 4 5% 7% 33%	2344
	11th to 20th	Night	14% 9% 5% 6% 5% 4 5% 4 5% 6% 39%	2127
	21st to end	Night	15% 10% 4 6% 5% 3 4 4 5% 6% 40%	1776
March	1st to 10th	Night	15% 11% 3 6% 5% 3 4 4 5% 6% 37%	2150
	11th to 20th	Night	18% 6% 4 6% 6% 4 5% 4 5% 6% 36%	1997
	21st to end	Night	18% 8% 3 5% 5% 2 4 3 5% 5% 41%	2155
April	1st to 10th	Night	23% 9% 3 5% 5% 3 4 4 5% 6% 34%	1800
	11th to 20th	Night	19% 6% 2 6% 5% 4 4 5% 5% 7% 37%	1656
	21st to end	Night	24% 8% 3 6% 5% 3 4 4 4 6% 33%	1620
May	1st to 10th	Night	21% 9% 3 6% 5% 3 5% 5% 6% 7% 30%	1620
	11th to 20th	Night	21% 7% 3 6% 5% 3 5% 4 6% 8% 32%	1593
	21st to end	Night	20% 9% 3 6% 5% 2 4 4 5% 6% 34%	1559
June	1st to 10th	Night	24% 7% 2 6% 5% 3 5% 4 5% 7% 31%	1324
	11th to 20th	Night	17% 7% 2 5% 5% 3 4 4 5% 8% 38%	1260
	21st to end	Night	24% 8% 2 4 5% 3 5% 5% 6% 7% 31%	1338
July	1st to 10th	Night	17% 8% 3 4 5% 2 6% 5% 6% 9% 36%	1440
	11th to 20th	Night	16% 6% 3 6% 5% 3 5% 5% 5% 9% 36%	1440
	21st to end	Night	16% 5% 2 5% 6% 3 5% 5% 7% 10% 36%	1574
August	1st to 10th	Night	13% 5% 2 5% 5% 3 5% 5% 6% 8% 42%	1600
	11th to 20th	Night	14% 6% 2 6% 5% 4 6% 5% 6% 10% 36%	1596
	21st to end	Night	14% 6% 2 4 4 3 4 4 4 7% 48%	1842
September	1st to 10th	Night	13% 5% 2 5% 5% 3 4 4 5% 5% 49%	1862
	11th to 20th	Night	16% 6% 2 3 3 2 3 3 5% 6% 51%	1980
	21st to end	Night	16% 5% 3 5% 4 3 3 4 4 7% 47%	1980
October	1st to 10th	Night	19% 6% 3 5% 5% 3 4 5% 4 6% 40%	1993
	11th to 20th	Night	19% 5% 3 5% 5% 3 5% 4 5% 6% 41%	2204
	21st to end	Night	26% 7% 2 4 5% 3 3 4 4 6% 36%	2574
November	1st to 10th	Night	17% 6% 3 4 5% 2 4 4 4 7% 43%	2407
	11th to 20th	Night	19% 6% 2 4 4 3 5% 4 5% 6% 43%	2470
	21st to end	Night	17% 7% 2 5% 4 3 4 4 4 8% 41%	2578
December	1st to 10th	Night	22% 9% 3 5% 5% 4 4 4 4 6% 34%	2793
	11th to 20th	Night	23% 7% 3 4 4 2 4 4 4 6% 39%	3531
	21st to end	Night	25% 9% 2 5% 4 3 3 4 4 6% 34%	4998

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	45% 4 10% 24% 15% 1	1518
	11th to 20th	Day	48% 7% 11% 20% 12% 3	1486
	21st to end	Day	45% 4 11% 21% 17% 2	1584
February	1st to 10th	Day	50% 4 11% 19% 14% 2	1615
	11th to 20th	Day	49% 5% 11% 18% 13% 3	1725
	21st to end	Day	50% 5% 9% 19% 12% 5%	1480
March	1st to 10th	Day	48% 5% 12% 17% 14% 4	1800
	11th to 20th	Day	44% 4 9% 21% 17% 4	1962
	21st to end	Day	45% 5% 9% 17% 18% 6%	2200
April	1st to 10th	Day	52% 4 8% 17% 15% 4	2160
	11th to 20th	Day	45% 5% 10% 22% 15% 3	2251
	21st to end	Day	53% 5% 8% 17% 13% 4	2340
May	1st to 10th	Day	51% 5% 8% 19% 14% 2	2340
	11th to 20th	Day	46% 5% 8% 22% 15% 3	2310
	21st to end	Day	45% 7% 14% 21% 11% 2	2725
June	1st to 10th	Day	51% 7% 12% 22% 9%	2632
	11th to 20th	Day	45% 6% 11% 20% 15% 3	2700
	21st to end	Day	51% 8% 12% 18% 10% 1	2613
July	1st to 10th	Day	41% 7% 13% 24% 13%	2450
	11th to 20th	Day	43% 8% 13% 25% 10% 1	2445
	21st to end	Day	41% 7% 16% 23% 12% 1	2583
August	1st to 10th	Day	39% 9% 16% 26% 10%	2314
	11th to 20th	Day	41% 8% 14% 24% 12%	2304
	21st to end	Day	35% 8% 13% 25% 18% 1	2511
September	1st to 10th	Day	35% 7% 13% 27% 17% 1	2098
	11th to 20th	Day	33% 5% 9% 29% 22% 3	1980
	21st to end	Day	39% 5% 11% 27% 17% 1	1980
October	1st to 10th	Day	46% 5% 10% 24% 14% 2	1966
	11th to 20th	Day	44% 5% 12% 27% 11% 1	1751
	21st to end	Day	50% 4 9% 21% 14% 2	1782
November	1st to 10th	Day	41% 5% 12% 24% 16% 2	1671
	11th to 20th	Day	41% 6% 9% 22% 19% 3	1710
	21st to end	Day	43% 4 17% 22% 13% 1	1598
December	1st to 10th	Day	51% 5% 9% 21% 12% 2	1383
	11th to 20th	Day	48% 5% 9% 21% 14% 4	1668
	21st to end	Day	56% 5% 10% 18% 9% 3	2657
January	1st to 10th	Night	44% 5% 11% 25% 13% 2	2649
	11th to 20th	Night	53% 7% 12% 17% 10% 2	2602
	21st to end	Night	44% 4 10% 22% 17% 3	2772
February	1st to 10th	Night	51% 5% 11% 17% 13% 3	2344
	11th to 20th	Night	47% 4 9% 21% 15% 2	2129
	21st to end	Night	45% 5% 10% 20% 16% 4	1776
March	1st to 10th	Night	48% 5% 8% 21% 16% 3	2160
	11th to 20th	Night	49% 4 11% 17% 14% 4	1998
	21st to end	Night	45% 4 8% 19% 20% 4	2155
April	1st to 10th	Night	52% 4 8% 16% 15% 4	1800
	11th to 20th	Night	45% 5% 9% 23% 17% 1	1656
	21st to end	Night	52% 5% 10% 17% 11% 4	1620
May	1st to 10th	Night	53% 5% 9% 18% 14% 2	1620
	11th to 20th	Night	50% 6% 9% 19% 12% 3	1594
	21st to end	Night	51% 6% 11% 18% 13% 1	1560
June	1st to 10th	Night	53% 7% 10% 20% 9% 1	1324
	11th to 20th	Night	43% 6% 15% 20% 14% 2	1260
	21st to end	Night	51% 8% 12% 19% 8% 2	1338
July	1st to 10th	Night	42% 8% 16% 22% 12% 1	1400
	11th to 20th	Night	44% 6% 15% 25% 10%	1392
	21st to end	Night	41% 8% 16% 24% 9% 1	1486
August	1st to 10th	Night	39% 8% 16% 27% 10% 1	1589
	11th to 20th	Night	43% 8% 15% 22% 12%	1596
	21st to end	Night	36% 6% 12% 27% 18% 1	1842
September	1st to 10th	Night	37% 5% 14% 29% 16% 1	1862
	11th to 20th	Night	35% 4 12% 27% 20% 2	1980
	21st to end	Night	38% 6% 9% 30% 15% 2	1980
October	1st to 10th	Night	45% 6% 11% 24% 13% 2	1993
	11th to 20th	Night	43% 6% 11% 26% 12% 1	2204
	21st to end	Night	50% 5% 11% 19% 14% 1	2574
November	1st to 10th	Night	42% 5% 11% 25% 15% 1	2407
	11th to 20th	Night	42% 5% 10% 23% 18% 2	2470
	21st to end	Night	43% 5% 14% 20% 15% 2	2578
December	1st to 10th	Night	52% 5% 9% 21% 12% 1	2793
	11th to 20th	Night	47% 5% 10% 19% 18% 1	3532
	21st to end	Night	52% 5% 10% 19% 12% 1	4998

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% 26% 25% 14% 3 1	172
	21st to end	Day	20% 28% 32% 17% 2	176
February	1st to 10th	Day	27% 22% 31% 17% 3	335
	11th to 20th	Day	25% 29% 31% 13% 1	402
	21st to end	Day	26% 22% 33% 15% 4	346
March	1st to 10th	Day	26% 25% 34% 12% 3	420
	11th to 20th	Day	22% 30% 30% 13% 4 1	420
	21st to end	Day	26% 27% 27% 13% 6% 2	462
April	1st to 10th	Day	31% 25% 27% 11% 5% 1	420
	11th to 20th	Day	27% 27% 27% 14% 4 2	412
	21st to end	Day	29% 23% 30% 13% 4 2	420
May	1st to 10th	Day	31% 22% 29% 13% 4 1	423
	11th to 20th	Day	20% 23% 34% 16% 5% 2	447
	21st to end	Day	21% 19% 37% 18% 5% 1	485
June	1st to 10th	Day	24% 16% 32% 17% 9% 2	562
	11th to 20th	Day	21% 16% 29% 21% 12% 1	630
	21st to end	Day	24% 13% 30% 22% 9% 2	543
July	1st to 10th	Day	21% 9% 38% 22% 9% 2	445
	11th to 20th	Day	16% 9% 37% 25% 10% 3	444
	21st to end	Day	15% 11% 37% 25% 9% 3	488
August	1st to 10th	Day	13% 9% 35% 29% 12% 2	446
	11th to 20th	Day	17% 8% 31% 27% 12% 4	439
	21st to end	Day	13% 13% 33% 26% 12% 3	495
September	1st to 10th	Day	16% 10% 34% 26% 12% 3	450
	11th to 20th	Day	18% 13% 32% 19% 12% 6%	450
	21st to end	Day	18% 16% 30% 21% 10% 5%	450
October	1st to 10th	Day	22% 14% 28% 22% 10% 4	443
	11th to 20th	Day	23% 16% 32% 19% 7% 3	360
	21st to end	Day	31% 14% 27% 20% 6% 1	396
November	1st to 10th	Day	21% 19% 30% 21% 7% 2	365
	11th to 20th	Day	29% 17% 29% 20% 4 1	370
	21st to end	Day	26% 22% 25% 19% 7%	370
December	1st to 10th	Day	26% 20% 34% 16% 3	233
	11th to 20th	Day	32% 27% 30% 10% 1 1	180
	21st to end	Day	32% 25% 25% 12% 6%	198
January	1st to 10th	Night	21% 20% 34% 22% 3	760
	11th to 20th	Night	26% 21% 30% 19% 4	744
	21st to end	Night	19% 20% 38% 20% 4	792
February	1st to 10th	Night	23% 19% 34% 21% 3	720
	11th to 20th	Night	22% 21% 37% 17% 3	703
	21st to end	Night	23% 18% 36% 20% 4	592
March	1st to 10th	Night	25% 18% 37% 18% 2	720
	11th to 20th	Night	23% 17% 34% 19% 7%	720
	21st to end	Night	23% 18% 32% 21% 5% 1	792
April	1st to 10th	Night	29% 18% 31% 18% 4	720
	11th to 20th	Night	23% 16% 32% 24% 4	592
	21st to end	Night	30% 14% 30% 21% 4	540
May	1st to 10th	Night	25% 11% 32% 25% 6% 1	540
	11th to 20th	Night	24% 13% 35% 23% 4	532
	21st to end	Night	26% 7% 26% 32% 8%	398
June	1st to 10th	Night	27% 8% 25% 31% 9% 1	360
	11th to 20th	Night	18% 6% 29% 34% 13% 1	360
	21st to end	Night	26% 4 29% 34% 8%	360
July	1st to 10th	Night	21% 5% 23% 39% 12%	350
	11th to 20th	Night	20% 1 24% 39% 14% 1	348
	21st to end	Night	17% 4 24% 41% 13% 1	394
August	1st to 10th	Night	16% 5% 27% 39% 13%	526
	11th to 20th	Night	16% 5% 28% 36% 14% 1	532
	21st to end	Night	15% 7% 26% 35% 16% 1	654
September	1st to 10th	Night	14% 6% 32% 33% 14% 1	720
	11th to 20th	Night	19% 8% 27% 27% 16% 3	720
	21st to end	Night	18% 10% 29% 28% 14% 1	720
October	1st to 10th	Night	23% 9% 25% 28% 14% 1	720
	11th to 20th	Night	22% 10% 32% 25% 10% 2	720
	21st to end	Night	31% 13% 29% 20% 6%	792
November	1st to 10th	Night	22% 11% 31% 26% 8% 1	740
	11th to 20th	Night	23% 15% 33% 22% 7% 1	760
	21st to end	Night	22% 21% 31% 21% 5%	811
December	1st to 10th	Night	28% 14% 36% 16% 6%	850
	11th to 20th	Night	24% 21% 32% 20% 3	850
	21st to end	Night	27% 20% 31% 20% 3	866

Page generated at 2024-01-13 19:28:29. Version ac07

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