Michael L. Britton Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Michael L. Britton Observatory clear sky chart from 2005-11-06 to 2024-11-08.

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	15% 7% 3 6% 5% 5% 5% 4 6% 8% 38%	101382
	Night	15% 7% 2 6% 4 4 4 4 6% 7% 39%	97669
Transparency	Day	43% 4 12% 24% 15% 2	105020
	Night	43% 4 12% 24% 15% 2	102170
Seeing	Day	22% 15% 27% 20% 12% 4	14879
	Night	20% 14% 28% 26% 11% 1	24293

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 7% 5% 4 4 4 5% 6% 40%	2517
	11th to 20th	Day	12% 12% 4 11% 6% 5% 6% 4 6% 6% 28%	2808
	21st to end	Day	17% 13% 4 8% 5% 4 5% 4 5% 6% 30%	3124
February	1st to 10th	Day	15% 11% 3 7% 5% 6% 5% 3 5% 6% 35%	1710
	11th to 20th	Day	15% 8% 2 7% 6% 6% 4 4 5% 8% 36%	1691
	21st to end	Day	18% 11% 3 7% 4 4 4 4 4 6% 35%	1570
March	1st to 10th	Day	22% 8% 3 6% 4 5% 4 4 5% 6% 34%	3357
	11th to 20th	Day	10% 7% 3 6% 5% 4 5% 4 6% 8% 42%	3480
	21st to end	Day	18% 9% 3 5% 4 3 4 3 4 5% 42%	3896
April	1st to 10th	Day	15% 7% 3 7% 5% 5% 5% 4 6% 7% 37%	2280
	11th to 20th	Day	14% 8% 2 7% 4 5% 4 4 6% 8% 40%	2254
	21st to end	Day	19% 6% 2 6% 4 4 4 4 5% 7% 39%	2281
May	1st to 10th	Day	28% 6% 2 5% 5% 4 4 3 5% 7% 30%	4082
	11th to 20th	Day	23% 7% 3 8% 6% 5% 5% 4 6% 6% 29%	4384
	21st to end	Day	11% 5% 2 6% 4 4 5% 5% 7% 9% 42%	4823
June	1st to 10th	Day	13% 5% 3 6% 4 4 5% 4 7% 9% 40%	2660
	11th to 20th	Day	13% 5% 2 6% 6% 5% 4 4 7% 9% 39%	2660
	21st to end	Day	10% 4 2 5% 4 5% 6% 4 7% 11% 43%	2660
July	1st to 10th	Day	5% 4 2 7% 6% 6% 7% 5% 8% 10% 39%	4440
	11th to 20th	Day	6% 6% 2 6% 5% 5% 6% 5% 8% 11% 40%	4440
	21st to end	Day	10% 7% 3 8% 5% 5% 5% 5% 6% 8% 39%	4467
August	1st to 10th	Day	16% 5% 3 6% 5% 5% 6% 5% 7% 9% 33%	4104
	11th to 20th	Day	10% 6% 3 7% 4 5% 5% 5% 7% 8% 40%	3921
	21st to end	Day	11% 5% 2 6% 4 4 4 3 6% 8% 47%	4180
September	1st to 10th	Day	16% 5% 3 5% 4 4 4 3 5% 6% 45%	2280
	11th to 20th	Day	13% 5% 2 4 3 4 4 4 5% 8% 49%	2280
	21st to end	Day	21% 7% 1 6% 4 4 3 3 5% 7% 39%	1996
October	1st to 10th	Day	17% 6% 1 6% 4 4 4 3 5% 6% 44%	1900
	11th to 20th	Day	16% 5% 3 6% 4 4 4 3 5% 6% 44%	1900
	21st to end	Day	14% 6% 3 6% 5% 4 4 4 5% 6% 43%	2090
November	1st to 10th	Day	12% 6% 2 4 3 3 4 3 6% 8% 49%	1786
	11th to 20th	Day	16% 6% 2 5% 5% 3 5% 3 5% 7% 43%	1639
	21st to end	Day	17% 6% 3 5% 5% 4 4 5% 6% 7% 40%	1520
December	1st to 10th	Day	18% 9% 3 8% 5% 5% 5% 3 7% 7% 32%	1520
	11th to 20th	Day	17% 9% 3 6% 4 6% 4 4 6% 6% 35%	1910
	21st to end	Day	19% 8% 3 9% 6% 5% 6% 5% 6% 7% 27%	2772
January	1st to 10th	Night	13% 10% 3 8% 5% 5% 5% 3 5% 7% 35%	4429
	11th to 20th	Night	16% 9% 3 8% 4 4 5% 4 6% 7% 34%	4150
	21st to end	Night	22% 11% 3 8% 5% 4 4 3 5% 5% 31%	4532
February	1st to 10th	Night	16% 8% 2 6% 5% 4 5% 4 5% 7% 37%	2470
	11th to 20th	Night	15% 10% 3 6% 5% 4 4 4 5% 7% 38%	2373
	21st to end	Night	16% 9% 3 8% 4 4 5% 4 5% 6% 37%	1884
March	1st to 10th	Night	21% 9% 2 7% 4 4 4 2 5% 5% 37%	3493
	11th to 20th	Night	11% 6% 3 7% 5% 4 4 3 5% 6% 46%	3480
	21st to end	Night	14% 9% 3 6% 4 4 4 3 4 5% 45%	3736
April	1st to 10th	Night	16% 9% 3 7% 6% 4 5% 3 6% 6% 35%	1900
	11th to 20th	Night	15% 6% 3 6% 4 4 4 4 5% 7% 43%	1875
	21st to end	Night	17% 6% 2 6% 3 3 4 4 6% 6% 43%	1900
May	1st to 10th	Night	26% 8% 2 7% 4 4 3 3 5% 6% 30%	2827
	11th to 20th	Night	24% 9% 3 6% 4 5% 5% 4 5% 6% 29%	2507
	21st to end	Night	11% 7% 2 6% 4 4 5% 4 7% 10% 40%	2732
June	1st to 10th	Night	13% 5% 2 5% 3 3 4 4 6% 9% 45%	1520
	11th to 20th	Night	12% 4 2 6% 7% 4 5% 5% 8% 9% 40%	1520
	21st to end	Night	10% 5% 1 6% 5% 5% 4 4 7% 10% 44%	1520
July	1st to 10th	Night	7% 5% 2 5% 6% 6% 6% 5% 7% 10% 42%	2520
	11th to 20th	Night	6% 5% 2 7% 5% 6% 5% 5% 8% 11% 40%	2520
	21st to end	Night	11% 7% 3 7% 5% 5% 5% 4 6% 7% 41%	3081
August	1st to 10th	Night	18% 5% 2 7% 4 5% 5% 5% 6% 8% 36%	2825
	11th to 20th	Night	11% 7% 3 7% 5% 4 4 3 6% 9% 41%	2876
	21st to end	Night	12% 7% 2 5% 4 3 5% 4 5% 7% 46%	3476
September	1st to 10th	Night	14% 6% 2 5% 4 4 4 3 4 6% 48%	1900
	11th to 20th	Night	12% 5% 2 5% 3 3 4 3 5% 7% 52%	1900
	21st to end	Night	20% 6% 2 5% 4 3 4 4 5% 6% 40%	2177
October	1st to 10th	Night	19% 7% 2 5% 4 3 4 3 4 5% 45%	2280
	11th to 20th	Night	14% 5% 2 5% 4 3 4 3 5% 7% 47%	2280
	21st to end	Night	16% 6% 2 5% 3 4 4 4 5% 7% 44%	2508
November	1st to 10th	Night	10% 6% 2 6% 3 4 4 3 5% 6% 52%	2453
	11th to 20th	Night	14% 8% 2 5% 4 4 4 4 5% 7% 44%	2536
	21st to end	Night	14% 6% 2 5% 5% 5% 5% 4 6% 8% 41%	2660
December	1st to 10th	Night	17% 9% 3 7% 4 5% 5% 4 5% 6% 35%	2660
	11th to 20th	Night	18% 7% 2 6% 4 4 5% 5% 6% 6% 37%	3285
	21st to end	Night	17% 8% 3 8% 6% 5% 6% 5% 6% 7% 29%	4884

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	46% 3 13% 25% 10% 3	2518
	11th to 20th	Day	56% 2 11% 15% 12% 2	2808
	21st to end	Day	57% 2 10% 22% 7% 1	3124
February	1st to 10th	Day	51% 3 12% 23% 10% 2	1710
	11th to 20th	Day	48% 4 10% 24% 12% 2	1700
	21st to end	Day	51% 3 10% 22% 10% 4	1570
March	1st to 10th	Day	51% 3 9% 25% 10% 2	3357
	11th to 20th	Day	40% 3 9% 30% 15% 3	3480
	21st to end	Day	47% 2 10% 22% 16% 3	3896
April	1st to 10th	Day	47% 3 12% 19% 15% 3	2280
	11th to 20th	Day	43% 4 11% 22% 17% 4	2254
	21st to end	Day	45% 4 8% 24% 16% 3	2281
May	1st to 10th	Day	54% 3 10% 20% 12% 1	4120
	11th to 20th	Day	56% 4 12% 18% 8% 1	4388
	21st to end	Day	37% 5% 13% 25% 18% 2	4837
June	1st to 10th	Day	39% 6% 13% 25% 16% 1	2660
	11th to 20th	Day	40% 5% 15% 24% 13% 2	2660
	21st to end	Day	35% 6% 13% 29% 16% 1	2660
July	1st to 10th	Day	37% 9% 13% 26% 13% 2	4370
	11th to 20th	Day	34% 7% 17% 26% 15% 1	4365
	21st to end	Day	42% 5% 11% 21% 19% 1	4324
August	1st to 10th	Day	46% 7% 13% 25% 9%	4091
	11th to 20th	Day	38% 7% 12% 27% 15% 1	3960
	21st to end	Day	34% 5% 12% 29% 18% 2	4180
September	1st to 10th	Day	39% 5% 12% 21% 20% 3	2280
	11th to 20th	Day	34% 4 10% 27% 21% 4	2280
	21st to end	Day	46% 3 9% 19% 20% 3	1996
October	1st to 10th	Day	37% 4 8% 25% 22% 5%	3160
	11th to 20th	Day	30% 4 13% 27% 22% 4	3140
	21st to end	Day	27% 3 12% 31% 25% 2	3428
November	1st to 10th	Day	34% 4 14% 25% 19% 4	1786
	11th to 20th	Day	42% 2 11% 24% 18% 3	1639
	21st to end	Day	43% 3 12% 29% 10% 2	1520
December	1st to 10th	Day	52% 4 15% 20% 9% 1	1520
	11th to 20th	Day	49% 3 13% 26% 7% 1	1906
	21st to end	Day	57% 3 15% 17% 8%	2772
January	1st to 10th	Night	50% 2 11% 26% 10% 2	4429
	11th to 20th	Night	48% 3 15% 23% 10% 2	4150
	21st to end	Night	56% 1 8% 24% 9% 1	4532
February	1st to 10th	Night	47% 4 14% 24% 10% 1	2470
	11th to 20th	Night	47% 4 9% 21% 17% 2	2375
	21st to end	Night	48% 3 10% 22% 13% 4	1884
March	1st to 10th	Night	50% 2 10% 26% 11% 1	3493
	11th to 20th	Night	39% 3 10% 32% 14% 2	3480
	21st to end	Night	43% 2 8% 22% 21% 4	3736
April	1st to 10th	Night	50% 4 9% 20% 15% 3	1900
	11th to 20th	Night	40% 5% 11% 22% 19% 3	1875
	21st to end	Night	41% 5% 10% 24% 17% 4	1900
May	1st to 10th	Night	55% 3 8% 19% 12% 3	2854
	11th to 20th	Night	55% 4 12% 17% 10% 1	2508
	21st to end	Night	39% 5% 15% 25% 13% 3	2739
June	1st to 10th	Night	35% 6% 13% 25% 18% 3	1520
	11th to 20th	Night	39% 6% 13% 23% 17% 2	1520
	21st to end	Night	35% 5% 14% 27% 17% 1	1520
July	1st to 10th	Night	37% 6% 18% 19% 15% 5%	2480
	11th to 20th	Night	36% 7% 16% 24% 16% 1	2472
	21st to end	Night	41% 6% 13% 18% 21% 2	2983
August	1st to 10th	Night	45% 6% 16% 24% 9% 1	2814
	11th to 20th	Night	40% 5% 11% 26% 17% 1	2903
	21st to end	Night	36% 5% 12% 30% 16% 2	3476
September	1st to 10th	Night	38% 3 12% 22% 21% 4	1900
	11th to 20th	Night	31% 4 9% 30% 23% 3	1900
	21st to end	Night	44% 6% 8% 21% 19% 2	2177
October	1st to 10th	Night	40% 4 10% 23% 21% 2	3800
	11th to 20th	Night	29% 4 10% 28% 25% 4	3779
	21st to end	Night	28% 3 11% 32% 24% 2	4120
November	1st to 10th	Night	34% 4 11% 26% 22% 4	2453
	11th to 20th	Night	40% 2 10% 26% 18% 4	2536
	21st to end	Night	41% 3 12% 28% 14% 2	2660
December	1st to 10th	Night	49% 4 13% 22% 12%	2660
	11th to 20th	Night	46% 3 13% 23% 14% 2	3288
	21st to end	Night	54% 3 13% 21% 8%	4884

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	36% 21% 29% 9% 4	180
	11th to 20th	Day	40% 20% 31% 8% 2	261
	21st to end	Day	32% 27% 30% 9% 2 1	297
February	1st to 10th	Day	31% 27% 30% 11% 2	270
	11th to 20th	Day	39% 23% 33% 4 1	298
	21st to end	Day	35% 23% 27% 13% 2	382
March	1st to 10th	Day	30% 21% 27% 18% 3	460
	11th to 20th	Day	32% 23% 27% 12% 5%	460
	21st to end	Day	29% 23% 30% 12% 4 1	506
April	1st to 10th	Day	28% 23% 30% 13% 5% 1	460
	11th to 20th	Day	21% 22% 29% 16% 9% 2	452
	21st to end	Day	27% 18% 31% 15% 7% 2	460
May	1st to 10th	Day	33% 15% 27% 15% 8% 2	463
	11th to 20th	Day	25% 16% 28% 18% 10% 2	487
	21st to end	Day	17% 10% 27% 25% 16% 6%	529
June	1st to 10th	Day	18% 15% 29% 19% 16% 4	490
	11th to 20th	Day	18% 11% 30% 23% 15% 5%	490
	21st to end	Day	11% 11% 33% 27% 15% 4	490
July	1st to 10th	Day	11% 5% 24% 32% 20% 8%	485
	11th to 20th	Day	7% 7% 26% 29% 24% 7%	484
	21st to end	Day	12% 7% 26% 32% 18% 5%	532
August	1st to 10th	Day	11% 5% 24% 29% 24% 7%	486
	11th to 20th	Day	13% 6% 22% 32% 19% 8%	479
	21st to end	Day	12% 5% 22% 32% 19% 9%	539
September	1st to 10th	Day	18% 7% 22% 25% 19% 9%	490
	11th to 20th	Day	15% 6% 20% 27% 23% 9%	490
	21st to end	Day	29% 10% 24% 20% 13% 4	490
October	1st to 10th	Day	22% 9% 24% 24% 16% 6%	490
	11th to 20th	Day	20% 17% 29% 21% 12% 2	490
	21st to end	Day	19% 17% 36% 17% 10% 1	539
November	1st to 10th	Day	22% 14% 27% 22% 13% 2	413
	11th to 20th	Day	25% 21% 26% 21% 6%	299
	21st to end	Day	28% 19% 33% 14% 5%	180
December	1st to 10th	Day	30% 24% 27% 13% 4 1	180
	11th to 20th	Day	31% 23% 30% 12% 3	180
	21st to end	Day	30% 19% 29% 16% 5% 1	198
January	1st to 10th	Night	23% 23% 33% 18% 3	760
	11th to 20th	Night	26% 18% 33% 21% 3	760
	21st to end	Night	21% 24% 33% 21% 1	836
February	1st to 10th	Night	23% 20% 30% 25% 2	760
	11th to 20th	Night	27% 25% 33% 15% 1	743
	21st to end	Night	27% 24% 31% 15% 2	628
March	1st to 10th	Night	23% 19% 36% 19% 3	760
	11th to 20th	Night	27% 20% 29% 18% 5%	760
	21st to end	Night	23% 18% 36% 18% 5%	836
April	1st to 10th	Night	26% 21% 32% 17% 4	760
	11th to 20th	Night	20% 17% 32% 24% 7% 1	751
	21st to end	Night	23% 16% 29% 26% 7%	760
May	1st to 10th	Night	23% 12% 28% 28% 9%	584
	11th to 20th	Night	20% 8% 33% 30% 9%	386
	21st to end	Night	15% 8% 23% 36% 16% 1	411
June	1st to 10th	Night	14% 4 28% 35% 18%	380
	11th to 20th	Night	15% 6% 26% 34% 18%	380
	21st to end	Night	11% 4 28% 45% 13%	380
July	1st to 10th	Night	12% 4 20% 38% 23% 2	370
	11th to 20th	Night	8% 2 20% 46% 22% 2	368
	21st to end	Night	11% 3 21% 39% 24% 1	618
August	1st to 10th	Night	12% 2 18% 41% 25% 2	567
	11th to 20th	Night	12% 5% 16% 39% 25% 3	616
	21st to end	Night	14% 4 18% 34% 27% 3	836
September	1st to 10th	Night	16% 3 22% 34% 22% 3	760
	11th to 20th	Night	13% 6% 18% 32% 28% 3	760
	21st to end	Night	23% 9% 20% 27% 18% 2	760
October	1st to 10th	Night	21% 9% 22% 27% 18% 3	760
	11th to 20th	Night	17% 14% 29% 29% 10% 1	760
	21st to end	Night	20% 15% 27% 26% 10%	836
November	1st to 10th	Night	15% 13% 32% 29% 10% 1	771
	11th to 20th	Night	22% 19% 29% 22% 7% 1	760
	21st to end	Night	20% 19% 30% 25% 6%	760
December	1st to 10th	Night	23% 20% 32% 19% 5% 1	760
	11th to 20th	Night	24% 17% 34% 22% 3	760
	21st to end	Night	20% 20% 33% 24% 3	836

Page generated at 2024-11-05 08:15:50. Version ac07

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