Robert and Elisabeth Moore Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Robert and Elisabeth Moore Observatory clear sky chart from 2005-11-06 to 2024-01-17.

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	8% 7% 3 6% 6% 3 5% 4 5% 7% 47%	74825
	Night	10% 8% 4 6% 6% 3 5% 5% 5% 7% 42%	75019
Transparency	Day	38% 3 11% 23% 18% 7%	74528
	Night	43% 4 11% 21% 15% 6%	74837
Seeing	Day	10% 8% 19% 26% 24% 14%	13283
	Night	12% 6% 19% 35% 24% 4	22493

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	16% 18% 7% 11% 9% 4 5% 4 4 7% 15%	1519
	11th to 20th	Day	17% 17% 8% 8% 10% 4 5% 5% 4 6% 17%	1488
	21st to end	Day	13% 14% 9% 12% 9% 5% 7% 5% 3 5% 18%	1584
February	1st to 10th	Day	15% 12% 6% 10% 9% 4 5% 5% 4 6% 24%	1562
	11th to 20th	Day	12% 11% 4 8% 6% 4 5% 6% 4 7% 32%	1639
	21st to end	Day	9% 8% 4 8% 8% 4 6% 6% 7% 9% 32%	1480
March	1st to 10th	Day	11% 9% 4 6% 6% 4 6% 5% 7% 8% 33%	1800
	11th to 20th	Day	9% 8% 4 8% 7% 4 5% 5% 7% 7% 35%	1867
	21st to end	Day	10% 7% 3 6% 7% 5% 7% 7% 7% 9% 30%	2362
April	1st to 10th	Day	8% 7% 3 6% 6% 4 6% 6% 7% 9% 39%	2160
	11th to 20th	Day	6% 7% 3 5% 5% 3 5% 6% 6% 9% 45%	2129
	21st to end	Day	7% 5% 2 5% 5% 3 5% 4 6% 8% 50%	2304
May	1st to 10th	Day	5% 4 2 3 5% 3 5% 5% 8% 8% 52%	2515
	11th to 20th	Day	7% 5% 3 5% 6% 3 6% 6% 7% 8% 44%	2477
	21st to end	Day	7% 6% 2 6% 6% 3 5% 6% 5% 9% 46%	2723
June	1st to 10th	Day	9% 5% 2 5% 5% 2 5% 5% 5% 9% 48%	2520
	11th to 20th	Day	7% 4 2 4 3 3 5% 5% 5% 7% 55%	2520
	21st to end	Day	3 2 1 3 3 2 3 3 4 6% 68%	2520
July	1st to 10th	Day	2 1 1 2 2 2 3 3 4 6% 76%	2520
	11th to 20th	Day	1 1 1 2 2 2 2 3 4 5% 76%	2520
	21st to end	Day	1 1 2 1 2 2 3 4 84%	2741
August	1st to 10th	Day	3 1 1 2 2 1 3 3 4 5% 76%	2507
	11th to 20th	Day	2 1 1 1 3 1 2 2 3 5% 78%	2325
	21st to end	Day	3 3 1 3 3 1 3 3 4 6% 71%	2376
September	1st to 10th	Day	2 1 2 3 2 3 3 5% 6% 75%	2160
	11th to 20th	Day	5% 3 2 3 4 2 4 5% 5% 7% 60%	2117
	21st to end	Day	7% 4 2 4 5% 3 4 4 5% 7% 56%	1980
October	1st to 10th	Day	7% 5% 2 4 6% 3 4 4 5% 8% 53%	1828
	11th to 20th	Day	7% 5% 2 5% 4 3 5% 4 7% 9% 48%	1782
	21st to end	Day	13% 8% 2 6% 6% 4 5% 5% 5% 7% 39%	1782
November	1st to 10th	Day	13% 9% 3 8% 8% 4 5% 6% 5% 7% 32%	1674
	11th to 20th	Day	11% 15% 5% 9% 8% 4 6% 5% 6% 6% 26%	1582
	21st to end	Day	12% 13% 7% 11% 9% 5% 7% 5% 5% 6% 20%	1520
December	1st to 10th	Day	10% 13% 7% 12% 10% 5% 7% 5% 5% 5% 21%	1520
	11th to 20th	Day	14% 19% 11% 13% 9% 4 5% 4 4 5% 12%	1928
	21st to end	Day	14% 15% 7% 15% 12% 5% 6% 4 4 4 15%	2794
January	1st to 10th	Night	18% 16% 5% 10% 8% 4 6% 5% 6% 6% 17%	2646
	11th to 20th	Night	15% 14% 7% 9% 8% 4 5% 5% 5% 7% 21%	2616
	21st to end	Night	14% 12% 6% 9% 8% 5% 6% 5% 6% 7% 23%	2772
February	1st to 10th	Night	13% 11% 4 7% 6% 4 6% 7% 5% 7% 31%	2395
	11th to 20th	Night	9% 11% 4 8% 7% 3 5% 5% 6% 7% 36%	2207
	21st to end	Night	9% 9% 5% 6% 8% 4 6% 5% 6% 6% 35%	1776
March	1st to 10th	Night	12% 8% 4 6% 6% 4 6% 6% 6% 8% 36%	2160
	11th to 20th	Night	9% 7% 4 7% 6% 4 5% 7% 7% 9% 36%	2089
	21st to end	Night	9% 9% 4 6% 6% 4 7% 7% 7% 8% 32%	1994
April	1st to 10th	Night	12% 9% 3 6% 6% 3 5% 5% 5% 8% 38%	1800
	11th to 20th	Night	9% 6% 3 6% 5% 3 4 4 5% 6% 48%	1779
	21st to end	Night	7% 5% 2 7% 6% 3 5% 5% 7% 7% 44%	1655
May	1st to 10th	Night	6% 4 2 4 4 3 5% 5% 5% 7% 53%	1443
	11th to 20th	Night	9% 5% 2 3 5% 4 6% 5% 7% 9% 45%	1416
	21st to end	Night	6% 5% 2 5% 5% 4 6% 7% 7% 9% 44%	1552
June	1st to 10th	Night	8% 4 2 4 4 3 5% 5% 7% 9% 48%	1440
	11th to 20th	Night	7% 6% 3 5% 5% 3 4 3 4 6% 54%	1440
	21st to end	Night	3 3 1 4 5% 3 4 5% 4 6% 63%	1440
July	1st to 10th	Night	2 2 1 2 3 1 3 4 5% 7% 72%	1440
	11th to 20th	Night	2 1 1 2 2 1 2 3 3 6% 77%	1440
	21st to end	Night	1 1 1 2 1 2 2 3 5% 81%	1576
August	1st to 10th	Night	4 2 1 2 2 2 1 2 4 6% 73%	1432
	11th to 20th	Night	2 2 1 2 3 2 3 3 3 4 77%	1571
	21st to end	Night	2 1 1 3 3 2 4 3 4 6% 71%	1980
September	1st to 10th	Night	3 2 1 2 2 1 3 4 3 4 75%	1800
	11th to 20th	Night	4 2 1 3 4 2 4 4 5% 7% 63%	1842
	21st to end	Night	6% 3 1 4 4 2 5% 3 5% 9% 59%	1980
October	1st to 10th	Night	9% 5% 2 4 5% 2 4 4 5% 5% 55%	2128
	11th to 20th	Night	10% 5% 2 6% 5% 3 4 4 5% 7% 51%	2178
	21st to end	Night	10% 5% 3 6% 6% 4 6% 4 5% 7% 45%	2574
November	1st to 10th	Night	14% 7% 3 7% 7% 4 5% 5% 5% 7% 36%	2405
	11th to 20th	Night	15% 10% 4 8% 7% 4 5% 6% 5% 6% 30%	2595
	21st to end	Night	12% 10% 4 8% 6% 5% 7% 5% 5% 8% 30%	2660
December	1st to 10th	Night	11% 14% 9% 10% 8% 4 4 5% 5% 6% 24%	2660
	11th to 20th	Night	16% 17% 9% 12% 8% 4 5% 5% 5% 5% 13%	3276
	21st to end	Night	14% 17% 5% 11% 8% 5% 5% 5% 4 6% 19%	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	71% 6% 10% 9% 3	1519
	11th to 20th	Day	69% 5% 14% 10% 2	1488
	21st to end	Day	71% 3 13% 10% 3	1584
February	1st to 10th	Day	61% 5% 18% 11% 4	1562
	11th to 20th	Day	52% 5% 15% 20% 7%	1648
	21st to end	Day	48% 6% 14% 20% 10% 1	1480
March	1st to 10th	Day	47% 5% 14% 24% 10% 1	1800
	11th to 20th	Day	47% 4 14% 26% 9%	1867
	21st to end	Day	48% 6% 16% 22% 8%	2362
April	1st to 10th	Day	41% 4 13% 25% 15% 2	2160
	11th to 20th	Day	35% 4 13% 28% 16% 5%	2132
	21st to end	Day	33% 3 12% 30% 19% 4	2304
May	1st to 10th	Day	28% 3 9% 34% 23% 4	2515
	11th to 20th	Day	36% 5% 10% 27% 18% 4	2480
	21st to end	Day	35% 4 11% 30% 17% 3	2724
June	1st to 10th	Day	35% 3 8% 29% 21% 5%	2520
	11th to 20th	Day	29% 2 9% 32% 24% 4	2520
	21st to end	Day	19% 2 6% 31% 29% 12%	2520
July	1st to 10th	Day	13% 1 4 22% 36% 23%	2451
	11th to 20th	Day	12% 2 5% 14% 36% 31%	2445
	21st to end	Day	8% 1 2 15% 42% 32%	2589
August	1st to 10th	Day	13% 2 5% 20% 38% 22%	2490
	11th to 20th	Day	11% 2 4 25% 39% 19%	2325
	21st to end	Day	17% 2 5% 28% 36% 12%	2376
September	1st to 10th	Day	12% 2 8% 30% 32% 16%	2160
	11th to 20th	Day	25% 3 10% 26% 28% 9%	2117
	21st to end	Day	29% 3 14% 34% 17% 4	1980
October	1st to 10th	Day	31% 3 13% 33% 18% 3	1828
	11th to 20th	Day	31% 3 21% 31% 12% 1	1782
	21st to end	Day	45% 4 17% 23% 9% 1	1782
November	1st to 10th	Day	51% 5% 15% 21% 8% 1	1674
	11th to 20th	Day	59% 5% 15% 16% 5% 1	1582
	21st to end	Day	65% 4 15% 12% 5%	1520
December	1st to 10th	Day	64% 5% 13% 13% 4	1520
	11th to 20th	Day	77% 4 10% 8% 2	1928
	21st to end	Day	75% 2 9% 11% 2	2794
January	1st to 10th	Night	67% 5% 14% 11% 3	2646
	11th to 20th	Night	62% 5% 16% 14% 3	2616
	21st to end	Night	60% 6% 17% 14% 3 1	2772
February	1st to 10th	Night	52% 5% 20% 17% 4 1	2395
	11th to 20th	Night	47% 6% 15% 24% 8%	2209
	21st to end	Night	49% 4 14% 22% 9% 2	1776
March	1st to 10th	Night	46% 4 12% 25% 11% 2	2160
	11th to 20th	Night	43% 4 15% 27% 10%	2089
	21st to end	Night	48% 6% 13% 24% 9% 1	1994
April	1st to 10th	Night	45% 3 13% 25% 13% 2	1800
	11th to 20th	Night	38% 3 8% 28% 18% 5%	1779
	21st to end	Night	37% 4 11% 25% 18% 5%	1655
May	1st to 10th	Night	30% 2 7% 31% 24% 5%	1443
	11th to 20th	Night	34% 3 10% 26% 22% 5%	1416
	21st to end	Night	35% 4 12% 28% 19% 3	1552
June	1st to 10th	Night	32% 4 10% 27% 23% 5%	1440
	11th to 20th	Night	33% 3 5% 30% 24% 5%	1440
	21st to end	Night	22% 3 5% 27% 29% 15%	1440
July	1st to 10th	Night	13% 2 4 21% 32% 28%	1400
	11th to 20th	Night	11% 3 4 18% 30% 34%	1392
	21st to end	Night	8% 1 1 12% 35% 42%	1488
August	1st to 10th	Night	15% 1 3 15% 38% 28%	1424
	11th to 20th	Night	13% 2 4 20% 32% 28%	1571
	21st to end	Night	16% 2 4 22% 37% 19%	1980
September	1st to 10th	Night	14% 2 7% 23% 33% 21%	1800
	11th to 20th	Night	22% 3 9% 25% 27% 15%	1842
	21st to end	Night	25% 3 8% 31% 25% 9%	1980
October	1st to 10th	Night	32% 3 9% 28% 27% 2	2128
	11th to 20th	Night	35% 2 11% 30% 18% 3	2178
	21st to end	Night	40% 3 12% 27% 15% 2	2574
November	1st to 10th	Night	48% 5% 13% 24% 9%	2405
	11th to 20th	Night	54% 4 15% 20% 6% 1	2595
	21st to end	Night	53% 4 18% 18% 6% 1	2660
December	1st to 10th	Night	62% 5% 14% 16% 3	2660
	11th to 20th	Night	74% 5% 12% 8% 2	3276
	21st to end	Night	68% 3 13% 11% 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	27% 7% 21% 25% 16% 3	180
	11th to 20th	Day	23% 9% 17% 28% 17% 5%	172
	21st to end	Day	19% 6% 22% 27% 23% 3	176
February	1st to 10th	Day	18% 13% 24% 27% 13% 5%	282
	11th to 20th	Day	19% 10% 21% 26% 18% 6%	364
	21st to end	Day	14% 11% 24% 28% 18% 4	346
March	1st to 10th	Day	17% 12% 28% 23% 14% 5%	420
	11th to 20th	Day	12% 10% 20% 29% 22% 7%	420
	21st to end	Day	16% 11% 25% 28% 16% 3	462
April	1st to 10th	Day	13% 15% 24% 24% 15% 9%	420
	11th to 20th	Day	10% 14% 30% 24% 14% 8%	412
	21st to end	Day	7% 11% 27% 26% 20% 8%	420
May	1st to 10th	Day	6% 9% 24% 29% 21% 12%	423
	11th to 20th	Day	11% 7% 21% 23% 23% 15%	448
	21st to end	Day	10% 5% 18% 26% 26% 16%	485
June	1st to 10th	Day	9% 8% 19% 23% 24% 17%	450
	11th to 20th	Day	7% 11% 25% 29% 19% 10%	450
	21st to end	Day	2 4 19% 25% 31% 19%	450
July	1st to 10th	Day	2 2 11% 28% 41% 15%	446
	11th to 20th	Day	1 3 14% 31% 32% 19%	444
	21st to end	Day	1 2 11% 23% 33% 30%	487
August	1st to 10th	Day	3 3 9% 22% 34% 28%	447
	11th to 20th	Day	1 2 10% 22% 31% 34%	437
	21st to end	Day	4 6% 15% 27% 28% 21%	495
September	1st to 10th	Day	4 3 12% 28% 32% 22%	450
	11th to 20th	Day	5% 5% 16% 24% 27% 23%	450
	21st to end	Day	7% 7% 17% 28% 28% 14%	450
October	1st to 10th	Day	11% 5% 20% 24% 24% 16%	450
	11th to 20th	Day	11% 9% 14% 25% 25% 15%	432
	21st to end	Day	19% 9% 22% 27% 15% 8%	297
November	1st to 10th	Day	19% 9% 22% 30% 17% 3	270
	11th to 20th	Day	21% 10% 20% 23% 21% 3	210
	21st to end	Day	9% 11% 19% 27% 27% 7%	180
December	1st to 10th	Day	10% 11% 26% 28% 14% 12%	180
	11th to 20th	Day	17% 9% 22% 33% 17% 2	180
	21st to end	Day	15% 6% 32% 30% 14% 3	198
January	1st to 10th	Night	26% 9% 18% 28% 16% 3	760
	11th to 20th	Night	20% 7% 16% 35% 19% 3	748
	21st to end	Night	18% 6% 14% 36% 23% 3	792
February	1st to 10th	Night	18% 9% 20% 32% 19% 2	720
	11th to 20th	Night	13% 10% 20% 33% 20% 3	704
	21st to end	Night	13% 13% 20% 35% 17% 2	592
March	1st to 10th	Night	14% 8% 23% 35% 18% 3	720
	11th to 20th	Night	10% 10% 19% 35% 24% 2	720
	21st to end	Night	13% 8% 30% 33% 14% 2	792
April	1st to 10th	Night	17% 7% 20% 37% 17% 2	720
	11th to 20th	Night	11% 9% 28% 29% 19% 4	711
	21st to end	Night	8% 7% 27% 34% 21% 2	575
May	1st to 10th	Night	6% 7% 24% 37% 23% 3	363
	11th to 20th	Night	10% 5% 15% 36% 31% 3	354
	21st to end	Night	8% 3 20% 38% 26% 4	389
June	1st to 10th	Night	9% 4 25% 36% 22% 4	360
	11th to 20th	Night	10% 6% 32% 33% 18% 2	360
	21st to end	Night	3 3 24% 38% 29% 3	360
July	1st to 10th	Night	4 3 18% 44% 29% 3	350
	11th to 20th	Night	4 2 21% 44% 26% 2	348
	21st to end	Night	1 2 16% 41% 36% 4	394
August	1st to 10th	Night	4 2 12% 41% 37% 4	358
	11th to 20th	Night	3 1 12% 34% 39% 11%	503
	21st to end	Night	3 2 14% 41% 34% 6%	792
September	1st to 10th	Night	3 3 15% 39% 36% 5%	720
	11th to 20th	Night	4 2 12% 33% 40% 9%	720
	21st to end	Night	7% 3 17% 33% 35% 6%	720
October	1st to 10th	Night	11% 3 15% 38% 29% 4	720
	11th to 20th	Night	11% 7% 16% 33% 26% 7%	720
	21st to end	Night	11% 7% 23% 32% 22% 5%	792
November	1st to 10th	Night	16% 7% 20% 36% 19% 1	740
	11th to 20th	Night	17% 10% 21% 33% 18% 3	760
	21st to end	Night	15% 6% 17% 38% 20% 4	760
December	1st to 10th	Night	15% 7% 19% 35% 21% 3	760
	11th to 20th	Night	21% 8% 18% 36% 17% 1	760
	21st to end	Night	24% 7% 19% 36% 13% 1	836

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