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 2024-01-12.

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	14% 6% 3 5% 5% 3 5% 5% 6% 8% 39%	74683
	Night	15% 6% 3 5% 5% 3 5% 5% 6% 7% 41%	75042
Transparency	Day	44% 5% 12% 23% 14% 2	74392
	Night	43% 4 12% 24% 15% 2	74852
Seeing	Day	22% 15% 28% 20% 12% 4	13721
	Night	20% 14% 28% 26% 10% 1	23270

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	12% 9% 3 6% 6% 4 4 6% 6% 7% 38%	1517
	11th to 20th	Day	14% 9% 4 8% 6% 4 6% 4 7% 7% 32%	1616
	21st to end	Day	13% 12% 5% 6% 6% 3 5% 5% 5% 6% 34%	1782
February	1st to 10th	Day	16% 11% 3 6% 6% 4 6% 4 5% 7% 33%	1620
	11th to 20th	Day	15% 7% 3 6% 7% 4 5% 5% 5% 8% 35%	1600
	21st to end	Day	18% 10% 4 7% 5% 2 4 4 4 7% 35%	1480
March	1st to 10th	Day	12% 7% 3 5% 6% 4 4 5% 5% 6% 45%	1927
	11th to 20th	Day	16% 8% 3 4 7% 2 5% 5% 7% 8% 34%	1980
	21st to end	Day	19% 8% 2 5% 5% 3 5% 5% 4 6% 37%	2218
April	1st to 10th	Day	15% 7% 3 6% 6% 4 5% 5% 5% 7% 38%	2160
	11th to 20th	Day	14% 6% 2 6% 4 3 5% 5% 5% 8% 41%	2134
	21st to end	Day	19% 6% 2 5% 5% 3 4 5% 5% 7% 38%	2160
May	1st to 10th	Day	18% 6% 2 5% 6% 3 5% 5% 6% 8% 35%	2340
	11th to 20th	Day	20% 6% 3 6% 6% 3 5% 5% 4 7% 34%	2464
	21st to end	Day	13% 6% 3 6% 4 3 5% 6% 6% 8% 39%	2725
June	1st to 10th	Day	13% 5% 3 6% 5% 3 5% 5% 6% 9% 40%	2520
	11th to 20th	Day	14% 5% 2 5% 6% 4 5% 5% 7% 9% 38%	2520
	21st to end	Day	11% 3 2 4 5% 3 6% 5% 7% 12% 43%	2520
July	1st to 10th	Day	9% 5% 2 6% 6% 4 6% 7% 7% 9% 40%	2520
	11th to 20th	Day	6% 5% 2 5% 5% 4 5% 6% 8% 11% 43%	2520
	21st to end	Day	9% 5% 3 6% 6% 4 6% 7% 7% 8% 38%	2548
August	1st to 10th	Day	8% 5% 2 6% 6% 4 7% 7% 7% 10% 37%	2327
	11th to 20th	Day	11% 6% 2 6% 5% 3 5% 6% 6% 9% 40%	2236
	21st to end	Day	13% 5% 2 5% 5% 3 5% 5% 7% 9% 41%	2376
September	1st to 10th	Day	17% 5% 3 4 4 3 4 4 6% 7% 44%	2160
	11th to 20th	Day	14% 4 2 4 3 3 4 5% 5% 8% 49%	2160
	21st to end	Day	18% 7% 2 5% 5% 3 4 4 5% 7% 41%	1891
October	1st to 10th	Day	17% 5% 2 4 6% 2 5% 4 5% 7% 44%	1800
	11th to 20th	Day	17% 5% 3 5% 4 2 5% 4 5% 7% 43%	1800
	21st to end	Day	15% 6% 3 6% 6% 3 4 5% 5% 6% 41%	1980
November	1st to 10th	Day	12% 6% 2 4 3 2 3 4 6% 8% 49%	1721
	11th to 20th	Day	17% 5% 2 4 5% 3 5% 4 5% 7% 43%	1639
	21st to end	Day	17% 5% 3 4 5% 3 4 6% 6% 7% 40%	1520
December	1st to 10th	Day	18% 8% 3 7% 6% 4 5% 4 6% 7% 32%	1520
	11th to 20th	Day	17% 8% 3 5% 5% 4 5% 5% 6% 6% 35%	1910
	21st to end	Day	19% 8% 3 8% 7% 4 6% 6% 6% 7% 27%	2772
January	1st to 10th	Night	14% 9% 3 5% 6% 3 5% 5% 5% 7% 38%	2649
	11th to 20th	Night	17% 8% 3 6% 5% 3 4 5% 7% 7% 34%	2382
	21st to end	Night	14% 7% 3 6% 5% 3 5% 4 6% 8% 39%	2574
February	1st to 10th	Night	17% 8% 3 5% 5% 3 5% 5% 5% 7% 36%	2340
	11th to 20th	Night	15% 9% 4 5% 5% 3 4 5% 6% 7% 37%	2244
	21st to end	Night	17% 8% 3 7% 5% 3 5% 5% 4 6% 37%	1776
March	1st to 10th	Night	14% 5% 2 5% 5% 3 4 4 5% 7% 46%	2030
	11th to 20th	Night	18% 6% 4 4 5% 3 3 4 5% 5% 42%	1980
	21st to end	Night	17% 7% 3 6% 5% 3 5% 4 5% 7% 38%	2138
April	1st to 10th	Night	16% 8% 4 6% 6% 3 5% 4 5% 7% 35%	1800
	11th to 20th	Night	15% 5% 3 5% 5% 3 4 5% 5% 8% 43%	1775
	21st to end	Night	18% 6% 2 5% 4 2 4 5% 5% 7% 43%	1800
May	1st to 10th	Night	18% 7% 2 6% 5% 3 4 4 6% 7% 38%	1634
	11th to 20th	Night	18% 6% 3 4 5% 3 4 6% 5% 7% 38%	1427
	21st to end	Night	12% 7% 3 5% 4 3 5% 5% 6% 9% 41%	1550
June	1st to 10th	Night	14% 5% 2 5% 3 3 4 5% 5% 9% 47%	1440
	11th to 20th	Night	13% 4 2 5% 7% 4 5% 6% 7% 9% 38%	1440
	21st to end	Night	10% 4 2 4 5% 3 5% 5% 7% 10% 45%	1440
July	1st to 10th	Night	10% 5% 3 4 7% 4 6% 6% 6% 9% 40%	1440
	11th to 20th	Night	6% 4 2 4 5% 3 5% 6% 8% 10% 47%	1440
	21st to end	Night	9% 4 2 5% 6% 3 5% 5% 7% 10% 45%	1765
August	1st to 10th	Night	9% 4 2 6% 5% 3 5% 6% 7% 10% 44%	1611
	11th to 20th	Night	11% 6% 3 6% 6% 3 4 5% 6% 9% 41%	1647
	21st to end	Night	12% 6% 2 4 5% 3 5% 5% 6% 8% 44%	1980
September	1st to 10th	Night	15% 6% 3 5% 5% 3 4 3 4 6% 47%	1800
	11th to 20th	Night	12% 4 2 5% 4 2 3 3 5% 8% 52%	1800
	21st to end	Night	17% 6% 2 4 5% 3 4 5% 5% 7% 42%	2062
October	1st to 10th	Night	18% 6% 3 5% 4 2 4 4 4 5% 45%	2160
	11th to 20th	Night	15% 5% 2 4 5% 3 4 4 5% 7% 46%	2160
	21st to end	Night	17% 6% 2 4 4 3 5% 4 5% 6% 43%	2376
November	1st to 10th	Night	10% 6% 2 4 4 3 3 4 5% 6% 52%	2357
	11th to 20th	Night	15% 7% 2 4 4 2 4 4 5% 7% 44%	2536
	21st to end	Night	14% 6% 2 5% 5% 3 5% 5% 6% 8% 41%	2660
December	1st to 10th	Night	17% 8% 3 6% 5% 3 5% 5% 5% 6% 36%	2660
	11th to 20th	Night	18% 7% 2 5% 5% 3 4 6% 6% 6% 37%	3285
	21st to end	Night	17% 8% 3 7% 7% 4 5% 6% 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	45% 5% 12% 24% 13% 2	1518
	11th to 20th	Day	52% 4 13% 20% 10% 2	1616
	21st to end	Day	52% 3 11% 21% 11% 2	1782
February	1st to 10th	Day	52% 3 12% 22% 10% 2	1620
	11th to 20th	Day	49% 4 11% 24% 11% 2	1609
	21st to end	Day	51% 3 10% 22% 11% 4	1480
March	1st to 10th	Day	41% 4 12% 27% 14% 2	1927
	11th to 20th	Day	46% 3 12% 20% 14% 4	1980
	21st to end	Day	50% 3 9% 19% 16% 3	2218
April	1st to 10th	Day	46% 3 12% 19% 16% 4	2160
	11th to 20th	Day	43% 4 11% 21% 17% 4	2134
	21st to end	Day	46% 4 8% 23% 15% 3	2160
May	1st to 10th	Day	47% 5% 10% 21% 15% 2	2340
	11th to 20th	Day	51% 5% 10% 18% 13% 3	2468
	21st to end	Day	42% 6% 16% 22% 12% 2	2725
June	1st to 10th	Day	40% 6% 14% 24% 15% 1	2520
	11th to 20th	Day	41% 5% 15% 23% 13% 2	2520
	21st to end	Day	35% 6% 14% 30% 15% 1	2520
July	1st to 10th	Day	40% 8% 11% 23% 16% 3	2450
	11th to 20th	Day	33% 6% 15% 26% 18% 2	2445
	21st to end	Day	41% 6% 12% 22% 16% 2	2405
August	1st to 10th	Day	40% 7% 14% 26% 12% 1	2314
	11th to 20th	Day	40% 6% 13% 24% 16% 1	2236
	21st to end	Day	39% 5% 13% 24% 16% 3	2376
September	1st to 10th	Day	40% 5% 12% 21% 18% 3	2160
	11th to 20th	Day	34% 4 10% 27% 21% 4	2160
	21st to end	Day	44% 3 9% 20% 21% 3	1891
October	1st to 10th	Day	41% 5% 12% 25% 15% 2	1800
	11th to 20th	Day	42% 3 12% 24% 18% 1	1800
	21st to end	Day	43% 4 9% 26% 16% 2	1980
November	1st to 10th	Day	34% 4 14% 25% 19% 4	1721
	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	46% 3 11% 26% 12% 2	2649
	11th to 20th	Night	47% 4 16% 22% 9% 1	2382
	21st to end	Night	45% 2 11% 25% 14% 2	2574
February	1st to 10th	Night	48% 4 14% 23% 10% 1	2340
	11th to 20th	Night	47% 4 10% 21% 16% 2	2246
	21st to end	Night	49% 3 10% 22% 14% 3	1776
March	1st to 10th	Night	39% 3 12% 27% 17% 2	2030
	11th to 20th	Night	45% 3 10% 23% 16% 3	1980
	21st to end	Night	46% 4 9% 21% 17% 4	2138
April	1st to 10th	Night	50% 4 9% 19% 15% 3	1800
	11th to 20th	Night	39% 5% 11% 22% 19% 4	1775
	21st to end	Night	42% 5% 10% 23% 17% 5%	1800
May	1st to 10th	Night	46% 4 10% 22% 13% 4	1634
	11th to 20th	Night	44% 6% 11% 19% 18% 2	1428
	21st to end	Night	39% 6% 14% 24% 14% 2	1551
June	1st to 10th	Night	35% 6% 13% 25% 18% 3	1440
	11th to 20th	Night	41% 7% 13% 21% 16% 2	1440
	21st to end	Night	35% 5% 14% 28% 17% 1	1440
July	1st to 10th	Night	40% 6% 15% 21% 15% 4	1400
	11th to 20th	Night	32% 7% 16% 25% 18% 2	1392
	21st to end	Night	34% 6% 17% 23% 18% 2	1667
August	1st to 10th	Night	35% 6% 17% 27% 15% 1	1600
	11th to 20th	Night	41% 5% 13% 22% 18% 1	1647
	21st to end	Night	36% 6% 14% 24% 16% 3	1980
September	1st to 10th	Night	40% 3 12% 22% 19% 4	1800
	11th to 20th	Night	32% 4 9% 30% 23% 3	1800
	21st to end	Night	41% 6% 8% 22% 20% 2	2062
October	1st to 10th	Night	42% 4 11% 25% 16% 2	2160
	11th to 20th	Night	39% 3 11% 29% 16% 2	2160
	21st to end	Night	42% 4 8% 27% 18% 2	2376
November	1st to 10th	Night	34% 4 11% 26% 21% 4	2357
	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	36% 21% 31% 9% 2	236
	21st to end	Day	33% 29% 27% 7% 3 1	264
February	1st to 10th	Day	34% 28% 29% 8% 2	240
	11th to 20th	Day	38% 24% 34% 3	267
	21st to end	Day	35% 24% 28% 11% 2	346
March	1st to 10th	Day	29% 22% 28% 19% 2	420
	11th to 20th	Day	32% 23% 28% 12% 5%	420
	21st to end	Day	30% 23% 29% 12% 4 2	462
April	1st to 10th	Day	27% 23% 30% 13% 6% 1	420
	11th to 20th	Day	21% 21% 30% 16% 9% 3	412
	21st to end	Day	28% 18% 31% 14% 6% 2	420
May	1st to 10th	Day	31% 16% 28% 15% 8% 2	423
	11th to 20th	Day	26% 17% 29% 17% 10% 2	447
	21st to end	Day	18% 9% 26% 25% 15% 6%	485
June	1st to 10th	Day	18% 14% 29% 20% 15% 3	450
	11th to 20th	Day	19% 11% 31% 22% 12% 5%	450
	21st to end	Day	11% 10% 33% 27% 16% 4	450
July	1st to 10th	Day	12% 6% 23% 30% 20% 9%	445
	11th to 20th	Day	7% 7% 27% 29% 25% 7%	444
	21st to end	Day	12% 6% 27% 32% 19% 5%	488
August	1st to 10th	Day	10% 5% 24% 29% 25% 7%	446
	11th to 20th	Day	13% 6% 21% 32% 19% 8%	439
	21st to end	Day	12% 5% 23% 33% 18% 9%	495
September	1st to 10th	Day	20% 6% 22% 24% 18% 10%	450
	11th to 20th	Day	16% 6% 21% 28% 22% 8%	450
	21st to end	Day	25% 11% 25% 21% 14% 5%	450
October	1st to 10th	Day	23% 8% 24% 24% 16% 6%	450
	11th to 20th	Day	22% 17% 28% 21% 11% 1	450
	21st to end	Day	20% 18% 36% 16% 9% 1	495
November	1st to 10th	Day	23% 14% 26% 22% 13% 3	390
	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	25% 17% 33% 22% 3	728
	21st to end	Night	20% 23% 33% 22% 1	792
February	1st to 10th	Night	24% 21% 29% 24% 1	720
	11th to 20th	Night	26% 25% 34% 14% 1	703
	21st to end	Night	28% 24% 32% 15% 2	592
March	1st to 10th	Night	23% 20% 36% 18% 3	720
	11th to 20th	Night	28% 19% 29% 18% 6%	720
	21st to end	Night	22% 17% 37% 18% 6%	792
April	1st to 10th	Night	26% 22% 31% 17% 4	720
	11th to 20th	Night	20% 17% 32% 23% 7% 1	711
	21st to end	Night	24% 15% 29% 26% 6%	720
May	1st to 10th	Night	22% 12% 29% 28% 10%	554
	11th to 20th	Night	19% 9% 33% 30% 8%	366
	21st to end	Night	15% 8% 23% 37% 16% 1	389
June	1st to 10th	Night	14% 4 28% 35% 19%	360
	11th to 20th	Night	16% 7% 28% 32% 18%	360
	21st to end	Night	11% 4 27% 45% 13%	360
July	1st to 10th	Night	13% 4 21% 37% 23% 3	350
	11th to 20th	Night	8% 3 20% 45% 22% 2	348
	21st to end	Night	11% 3 21% 40% 24% 1	585
August	1st to 10th	Night	11% 2 19% 41% 25% 3	537
	11th to 20th	Night	12% 5% 16% 38% 26% 4	583
	21st to end	Night	15% 4 19% 34% 26% 3	792
September	1st to 10th	Night	17% 3 22% 33% 22% 3	720
	11th to 20th	Night	13% 6% 19% 32% 27% 3	720
	21st to end	Night	21% 8% 21% 28% 19% 2	720
October	1st to 10th	Night	22% 8% 22% 27% 18% 3	720
	11th to 20th	Night	18% 15% 28% 29% 9% 1	720
	21st to end	Night	21% 16% 28% 24% 9%	792
November	1st to 10th	Night	15% 14% 32% 28% 10% 1	740
	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

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