Bayt Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bayt Observatory clear sky chart from 2005-11-06 to 2024-04-25.

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	9% 5% 2 3 3 2 3 3 4 5% 61%	83469
	Night	10% 4 1 3 3 2 3 3 4 6% 61%	85715
Transparency	Day	26% 4 7% 20% 24% 19%	83181
	Night	26% 4 8% 22% 23% 17%	85525
Seeing	Day	12% 8% 28% 35% 13% 4	14483
	Night	11% 2 14% 35% 30% 7%	24108

By Month

Percentage Cloud Cover SW (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% 9% 3 4 4 2 3 4 5% 7% 45%	2539
	11th to 20th	Day	16% 7% 4 6% 6% 4 5% 5% 4 6% 37%	2714
	21st to end	Day	18% 7% 2 5% 5% 3 4 5% 6% 8% 36%	3146
February	1st to 10th	Day	17% 7% 2 4 2 1 3 4 4 6% 50%	1790
	11th to 20th	Day	13% 8% 2 4 3 3 4 4 5% 6% 49%	1850
	21st to end	Day	17% 9% 4 5% 4 2 3 3 4 4 43%	1570
March	1st to 10th	Day	15% 12% 3 6% 6% 3 5% 5% 6% 7% 32%	3138
	11th to 20th	Day	10% 5% 2 4 3 2 3 4 4 6% 57%	3422
	21st to end	Day	18% 10% 3 6% 4 3 4 5% 5% 6% 36%	4089
April	1st to 10th	Day	12% 6% 3 4 4 2 3 3 4 6% 53%	2280
	11th to 20th	Day	11% 6% 2 4 4 2 3 4 5% 7% 53%	2249
	21st to end	Day	9% 5% 2 3 3 2 4 4 4 7% 57%	2210
May	1st to 10th	Day	6% 4 2 3 3 2 3 4 3 5% 64%	2218
	11th to 20th	Day	6% 3 2 4 2 2 3 3 4 7% 65%	2319
	21st to end	Day	7% 3 1 3 3 3 3 3 4 6% 62%	2723
June	1st to 10th	Day	3 3 2 3 3 2 3 4 4 6% 68%	2520
	11th to 20th	Day	1 1 1 2 2 1 2 2 2 4 83%	2520
	21st to end	Day	2 2 1 1 2 1 1 2 2 3 84%	2520
July	1st to 10th	Day	1 1 1 1 2 2 2 4 85%	2520
	11th to 20th	Day	2 2 1 2 1 2 2 2 3 83%	2520
	21st to end	Day	1 1 1 2 2 1 2 3 4 5% 77%	2606
August	1st to 10th	Day	2 2 1 1 1 1 1 2 2 3 83%	2224
	11th to 20th	Day	1 1 1 1 1 1 2 3 3 85%	2122
	21st to end	Day	1 1 1 1 1 1 1 2 2 3 86%	2376
September	1st to 10th	Day	2 1 1 1 1 2 2 1 2 86%	2160
	11th to 20th	Day	2 1 2 2 1 2 2 3 3 81%	2114
	21st to end	Day	3 2 1 1 1 1 2 3 4 4 79%	1980
October	1st to 10th	Day	5% 4 1 2 2 1 2 4 4 4 70%	1870
	11th to 20th	Day	3 2 1 3 2 2 3 3 4 5% 73%	1800
	21st to end	Day	6% 2 1 2 2 1 2 2 4 5% 71%	1962
November	1st to 10th	Day	10% 4 2 3 4 3 3 4 4 6% 58%	1674
	11th to 20th	Day	8% 3 1 3 3 2 2 4 5% 6% 63%	1710
	21st to end	Day	10% 4 2 3 3 3 3 3 3 6% 60%	1710
December	1st to 10th	Day	14% 4 1 3 3 2 4 4 5% 7% 53%	1582
	11th to 20th	Day	25% 8% 2 6% 5% 3 3 3 3 6% 36%	1928
	21st to end	Day	13% 8% 4 6% 5% 3 4 5% 5% 7% 40%	2794
January	1st to 10th	Night	14% 7% 2 5% 3 2 4 3 6% 6% 48%	4406
	11th to 20th	Night	19% 8% 3 4 4 3 4 5% 6% 7% 39%	4234
	21st to end	Night	19% 7% 2 4 4 3 3 4 5% 8% 39%	4521
February	1st to 10th	Night	16% 7% 2 3 3 1 3 4 4 7% 51%	2387
	11th to 20th	Night	14% 5% 2 3 3 2 3 4 4 7% 54%	2218
	21st to end	Night	16% 8% 2 4 4 2 2 2 4 5% 51%	1884
March	1st to 10th	Night	19% 7% 2 4 4 3 4 5% 6% 6% 40%	3736
	11th to 20th	Night	12% 5% 1 3 3 1 2 3 3 6% 60%	3555
	21st to end	Night	21% 7% 2 3 4 2 3 4 4 6% 45%	3562
April	1st to 10th	Night	12% 5% 2 2 3 1 2 3 5% 6% 59%	1900
	11th to 20th	Night	8% 3 2 3 3 2 5% 3 4 4 62%	1879
	21st to end	Night	7% 4 1 2 3 1 3 3 5% 6% 66%	1850
May	1st to 10th	Night	5% 3 1 3 2 2 3 3 4 6% 68%	1740
	11th to 20th	Night	5% 1 2 3 2 1 2 3 4 6% 72%	1571
	21st to end	Night	5% 2 1 2 2 1 1 3 3 5% 73%	1552
June	1st to 10th	Night	4 3 1 2 2 1 3 3 3 6% 72%	1440
	11th to 20th	Night	1 1 1 1 1 2 2 2 4 85%	1440
	21st to end	Night	1 1 1 1 1 1 2 1 4 87%	1440
July	1st to 10th	Night	1 1 1 1 2 2 3 89%	1440
	11th to 20th	Night	2 2 2 1 1 2 2 3 3 81%	1440
	21st to end	Night	2 2 2 3 2 2 3 3 3 4 75%	1708
August	1st to 10th	Night	2 2 1 2 1 1 1 2 3 3 84%	1713
	11th to 20th	Night	1 1 1 2 1 2 2 2 3 85%	1780
	21st to end	Night	2 1 1 1 1 2 1 3 88%	1980
September	1st to 10th	Night	1 1 1 1 1 1 1 1 2 90%	1800
	11th to 20th	Night	2 1 1 1 1 1 2 2 3 3 83%	1844
	21st to end	Night	2 1 1 1 2 1 2 2 3 4 82%	1980
October	1st to 10th	Night	4 2 1 2 1 2 2 3 3 3 74%	2087
	11th to 20th	Night	4 2 1 2 1 1 2 3 4 5% 76%	2160
	21st to end	Night	5% 1 1 2 2 1 1 2 2 5% 78%	2392
November	1st to 10th	Night	9% 4 1 3 2 2 2 3 4 7% 64%	2405
	11th to 20th	Night	7% 3 1 3 3 1 2 3 5% 7% 65%	2470
	21st to end	Night	9% 4 2 3 3 2 3 4 5% 6% 60%	2470
December	1st to 10th	Night	13% 4 2 4 4 2 3 2 4 7% 54%	2593
	11th to 20th	Night	25% 6% 2 4 4 2 3 4 5% 6% 39%	3276
	21st to end	Night	14% 6% 2 6% 6% 4 5% 5% 6% 7% 39%	4862

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	37% 4 14% 27% 14% 3	2539
	11th to 20th	Day	47% 6% 9% 16% 12% 9%	2714
	21st to end	Day	44% 5% 12% 22% 11% 5%	3146
February	1st to 10th	Day	37% 4 7% 25% 18% 9%	1790
	11th to 20th	Day	37% 4 8% 22% 22% 8%	1860
	21st to end	Day	45% 5% 7% 23% 19% 2	1570
March	1st to 10th	Day	48% 8% 10% 16% 13% 5%	3138
	11th to 20th	Day	27% 7% 11% 34% 11% 11%	3422
	21st to end	Day	45% 9% 11% 20% 12% 2	4089
April	1st to 10th	Day	33% 4 12% 26% 20% 5%	2280
	11th to 20th	Day	31% 4 10% 27% 22% 6%	2252
	21st to end	Day	28% 5% 9% 31% 25% 3	2210
May	1st to 10th	Day	23% 5% 8% 28% 30% 6%	2218
	11th to 20th	Day	22% 3 8% 28% 32% 8%	2322
	21st to end	Day	24% 4 7% 34% 25% 5%	2724
June	1st to 10th	Day	18% 5% 9% 22% 32% 14%	2520
	11th to 20th	Day	9% 3 4 19% 41% 24%	2520
	21st to end	Day	9% 2 2 10% 37% 41%	2520
July	1st to 10th	Day	7% 2 1 8% 28% 55%	2451
	11th to 20th	Day	9% 2 3 10% 29% 48%	2445
	21st to end	Day	11% 3 3 11% 29% 43%	2461
August	1st to 10th	Day	9% 2 2 10% 27% 49%	2208
	11th to 20th	Day	7% 2 1 10% 35% 45%	2122
	21st to end	Day	7% 2 10% 38% 43%	2376
September	1st to 10th	Day	8% 2 2 8% 30% 50%	2160
	11th to 20th	Day	10% 3 2 21% 38% 27%	2114
	21st to end	Day	10% 3 3 18% 39% 28%	1980
October	1st to 10th	Day	18% 3 5% 17% 28% 29%	1870
	11th to 20th	Day	15% 3 5% 20% 29% 28%	1800
	21st to end	Day	17% 3 5% 21% 31% 24%	1962
November	1st to 10th	Day	28% 4 7% 20% 23% 18%	1674
	11th to 20th	Day	23% 4 8% 23% 28% 15%	1710
	21st to end	Day	29% 2 7% 17% 28% 17%	1710
December	1st to 10th	Day	32% 4 8% 22% 21% 13%	1582
	11th to 20th	Day	51% 5% 7% 12% 15% 10%	1928
	21st to end	Day	42% 5% 9% 26% 12% 6%	2794
January	1st to 10th	Night	36% 4 14% 25% 17% 4	4406
	11th to 20th	Night	43% 5% 9% 22% 12% 8%	4234
	21st to end	Night	43% 6% 10% 22% 13% 6%	4521
February	1st to 10th	Night	36% 3 8% 23% 20% 10%	2387
	11th to 20th	Night	31% 4 10% 24% 23% 8%	2219
	21st to end	Night	38% 2 11% 26% 17% 5%	1884
March	1st to 10th	Night	43% 6% 11% 21% 15% 5%	3736
	11th to 20th	Night	27% 4 9% 35% 14% 10%	3555
	21st to end	Night	40% 5% 15% 24% 13% 3	3562
April	1st to 10th	Night	28% 3 17% 27% 19% 6%	1900
	11th to 20th	Night	25% 5% 12% 30% 20% 8%	1879
	21st to end	Night	21% 5% 11% 35% 25% 4	1850
May	1st to 10th	Night	19% 3 10% 31% 29% 8%	1740
	11th to 20th	Night	16% 2 7% 33% 34% 9%	1572
	21st to end	Night	16% 3 11% 35% 29% 6%	1552
June	1st to 10th	Night	15% 4 10% 23% 33% 14%	1440
	11th to 20th	Night	7% 2 5% 24% 38% 26%	1440
	21st to end	Night	6% 1 2 13% 34% 43%	1440
July	1st to 10th	Night	4 2 1 6% 32% 55%	1400
	11th to 20th	Night	10% 2 2 8% 28% 49%	1392
	21st to end	Night	14% 3 2 11% 27% 44%	1613
August	1st to 10th	Night	9% 2 1 14% 25% 49%	1704
	11th to 20th	Night	7% 2 2 10% 33% 46%	1780
	21st to end	Night	7% 1 1 8% 38% 46%	1980
September	1st to 10th	Night	5% 2 1 9% 29% 54%	1800
	11th to 20th	Night	9% 2 3 16% 44% 26%	1844
	21st to end	Night	9% 2 2 20% 41% 27%	1980
October	1st to 10th	Night	16% 3 5% 18% 31% 27%	2087
	11th to 20th	Night	13% 3 6% 20% 31% 27%	2160
	21st to end	Night	12% 2 5% 24% 32% 26%	2392
November	1st to 10th	Night	23% 3 9% 22% 25% 19%	2405
	11th to 20th	Night	21% 3 8% 23% 29% 16%	2470
	21st to end	Night	27% 2 7% 21% 29% 15%	2470
December	1st to 10th	Night	33% 3 9% 19% 22% 13%	2593
	11th to 20th	Night	46% 4 10% 16% 17% 8%	3276
	21st to end	Night	42% 6% 10% 22% 13% 6%	4862

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	33% 6% 23% 22% 11% 5%	180
	11th to 20th	Day	22% 6% 25% 27% 15% 6%	289
	21st to end	Day	22% 6% 27% 26% 14% 5%	407
February	1st to 10th	Day	25% 11% 26% 23% 13% 3	408
	11th to 20th	Day	21% 12% 32% 23% 10% 2	455
	21st to end	Day	20% 13% 34% 24% 8% 1	382
March	1st to 10th	Day	27% 9% 32% 23% 8% 1	460
	11th to 20th	Day	24% 13% 27% 24% 10% 2	460
	21st to end	Day	24% 14% 30% 26% 5% 1	506
April	1st to 10th	Day	19% 15% 32% 25% 9%	460
	11th to 20th	Day	19% 17% 31% 26% 7% 1	452
	21st to end	Day	11% 15% 31% 28% 13% 2	437
May	1st to 10th	Day	12% 15% 34% 28% 8% 3	423
	11th to 20th	Day	13% 12% 31% 31% 10% 3	448
	21st to end	Day	13% 10% 27% 35% 11% 4	485
June	1st to 10th	Day	6% 12% 29% 33% 15% 5%	450
	11th to 20th	Day	4 10% 33% 35% 13% 5%	450
	21st to end	Day	3 7% 30% 40% 13% 7%	450
July	1st to 10th	Day	2 2 25% 50% 13% 7%	446
	11th to 20th	Day	2 1 18% 53% 14% 11%	444
	21st to end	Day	1 1 22% 52% 14% 8%	487
August	1st to 10th	Day	4 3 33% 41% 13% 5%	447
	11th to 20th	Day	2 4 24% 48% 16% 6%	437
	21st to end	Day	1 2 25% 49% 17% 5%	495
September	1st to 10th	Day	4 3 29% 45% 16% 3	450
	11th to 20th	Day	4 6% 33% 40% 15% 2	450
	21st to end	Day	4 6% 27% 46% 14% 2	450
October	1st to 10th	Day	4 9% 32% 37% 15% 2	450
	11th to 20th	Day	3 10% 31% 39% 14% 2	450
	21st to end	Day	7% 7% 26% 38% 18% 3	477
November	1st to 10th	Day	13% 4 26% 31% 21% 5%	270
	11th to 20th	Day	7% 6% 31% 31% 18% 7%	270
	21st to end	Day	14% 7% 23% 31% 19% 6%	270
December	1st to 10th	Day	17% 3 28% 29% 19% 4	210
	11th to 20th	Day	17% 6% 26% 31% 14% 7%	180
	21st to end	Day	22% 6% 27% 24% 13% 8%	198
January	1st to 10th	Night	20% 3 18% 33% 18% 8%	760
	11th to 20th	Night	14% 3 21% 36% 20% 6%	760
	21st to end	Night	20% 4 17% 34% 19% 6%	836
February	1st to 10th	Night	21% 3 15% 36% 19% 5%	760
	11th to 20th	Night	18% 6% 16% 31% 24% 7%	744
	21st to end	Night	21% 4 16% 39% 17% 3	628
March	1st to 10th	Night	23% 4 15% 37% 16% 4	760
	11th to 20th	Night	18% 5% 13% 32% 28% 5%	760
	21st to end	Night	19% 3 16% 38% 21% 3	836
April	1st to 10th	Night	15% 4 16% 39% 22% 3	760
	11th to 20th	Night	11% 6% 18% 36% 25% 4	751
	21st to end	Night	8% 3 16% 39% 27% 7%	740
May	1st to 10th	Night	7% 3 17% 38% 28% 6%	660
	11th to 20th	Night	6% 2 16% 36% 36% 3	509
	21st to end	Night	7% 1 14% 34% 37% 7%	389
June	1st to 10th	Night	6% 2 9% 34% 43% 7%	360
	11th to 20th	Night	1 1 13% 38% 44% 4	360
	21st to end	Night	1 11% 29% 53% 6%	360
July	1st to 10th	Night	8% 33% 51% 8%	350
	11th to 20th	Night	3 1 6% 33% 47% 10%	348
	21st to end	Night	2 8% 25% 50% 16%	526
August	1st to 10th	Night	2 1 11% 31% 46% 9%	639
	11th to 20th	Night	1 7% 32% 51% 9%	712
	21st to end	Night	2 6% 27% 54% 11%	792
September	1st to 10th	Night	1 1 7% 34% 45% 12%	720
	11th to 20th	Night	3 1 9% 35% 43% 9%	720
	21st to end	Night	3 1 10% 37% 43% 7%	720
October	1st to 10th	Night	6% 1 12% 44% 31% 7%	720
	11th to 20th	Night	6% 2 12% 40% 32% 9%	720
	21st to end	Night	5% 2 14% 31% 33% 15%	792
November	1st to 10th	Night	12% 1 11% 36% 28% 12%	740
	11th to 20th	Night	9% 2 17% 37% 28% 7%	760
	21st to end	Night	11% 2 18% 38% 24% 8%	760
December	1st to 10th	Night	17% 3 15% 38% 22% 5%	760
	11th to 20th	Night	21% 2 19% 35% 18% 5%	760
	21st to end	Night	22% 4 19% 35% 17% 5%	836

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