GEM 'N I Dobservatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the GEM 'N I Dobservatory clear sky chart from 2005-11-06 to 2024-01-27.

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% 9% 4 7% 6% 4 5% 5% 6% 7% 33%	74773
	Night	15% 9% 4 7% 6% 3 5% 5% 5% 7% 33%	75263
Transparency	Day	48% 7% 14% 20% 10% 1	74490
	Night	50% 7% 12% 19% 11% 1	75080
Seeing	Day	18% 11% 26% 25% 15% 6%	13384
	Night	19% 10% 28% 28% 13% 2	22973

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	14% 15% 4 10% 9% 5% 6% 6% 5% 7% 19%	1517
	11th to 20th	Day	14% 16% 5% 11% 9% 5% 6% 5% 6% 5% 18%	1695
	21st to end	Day	14% 13% 6% 13% 10% 6% 8% 5% 5% 5% 14%	1836
February	1st to 10th	Day	18% 15% 5% 10% 9% 5% 6% 4 6% 5% 18%	1620
	11th to 20th	Day	12% 15% 7% 12% 8% 5% 5% 5% 5% 6% 21%	1573
	21st to end	Day	18% 14% 4 10% 9% 3 5% 4 5% 6% 22%	1399
March	1st to 10th	Day	14% 10% 5% 9% 8% 4 5% 5% 5% 6% 28%	1977
	11th to 20th	Day	12% 12% 5% 7% 7% 3 6% 4 4 6% 32%	1980
	21st to end	Day	18% 9% 4 8% 5% 3 5% 4 5% 6% 32%	2178
April	1st to 10th	Day	17% 10% 4 7% 6% 4 5% 5% 4 6% 32%	2117
	11th to 20th	Day	17% 7% 4 6% 5% 4 4 4 6% 7% 36%	2194
	21st to end	Day	19% 8% 3 7% 6% 4 5% 4 5% 6% 35%	2340
May	1st to 10th	Day	13% 9% 3 7% 7% 4 5% 5% 6% 7% 35%	2340
	11th to 20th	Day	14% 7% 3 6% 6% 4 6% 5% 6% 6% 38%	2432
	21st to end	Day	10% 5% 2 4 5% 3 5% 6% 6% 9% 44%	2725
June	1st to 10th	Day	13% 5% 3 5% 5% 3 5% 6% 6% 8% 42%	2520
	11th to 20th	Day	11% 5% 2 6% 5% 3 6% 6% 7% 8% 41%	2520
	21st to end	Day	11% 4 2 5% 6% 2 5% 5% 7% 9% 43%	2520
July	1st to 10th	Day	6% 4 2 4 5% 3 6% 6% 7% 9% 49%	2520
	11th to 20th	Day	11% 5% 3 4 5% 4 5% 5% 8% 10% 40%	2506
	21st to end	Day	8% 5% 2 6% 6% 4 6% 6% 7% 10% 40%	2547
August	1st to 10th	Day	7% 5% 3 5% 5% 4 6% 6% 7% 11% 43%	2327
	11th to 20th	Day	8% 5% 2 6% 6% 4 6% 7% 8% 10% 39%	2303
	21st to end	Day	10% 6% 2 5% 5% 3 4 5% 7% 9% 44%	2475
September	1st to 10th	Day	13% 6% 3 5% 5% 3 5% 6% 6% 9% 40%	2160
	11th to 20th	Day	10% 6% 2 4 5% 3 4 4 5% 9% 48%	2044
	21st to end	Day	13% 7% 2 5% 8% 3 5% 5% 7% 7% 38%	1949
October	1st to 10th	Day	18% 9% 3 6% 6% 3 5% 4 5% 7% 34%	1800
	11th to 20th	Day	17% 10% 3 9% 6% 3 5% 5% 6% 6% 31%	1800
	21st to end	Day	24% 11% 4 8% 6% 3 5% 5% 4 6% 24%	1980
November	1st to 10th	Day	13% 8% 4 8% 7% 4 4 5% 5% 8% 34%	1641
	11th to 20th	Day	13% 12% 4 8% 7% 4 5% 5% 6% 7% 30%	1520
	21st to end	Day	19% 13% 4 7% 6% 4 4 4 5% 8% 25%	1520
December	1st to 10th	Day	17% 13% 5% 9% 8% 4 6% 5% 5% 6% 21%	1520
	11th to 20th	Day	17% 14% 6% 9% 7% 5% 5% 5% 6% 6% 20%	1906
	21st to end	Day	27% 17% 8% 11% 7% 3 4 4 4 4 11%	2772
January	1st to 10th	Night	15% 15% 6% 10% 8% 5% 5% 5% 5% 5% 21%	2649
	11th to 20th	Night	18% 14% 5% 11% 9% 5% 6% 5% 5% 6% 17%	2480
	21st to end	Night	14% 13% 6% 10% 8% 4 6% 5% 6% 7% 21%	2664
February	1st to 10th	Night	16% 14% 6% 9% 8% 4 6% 4 5% 6% 22%	2340
	11th to 20th	Night	14% 13% 5% 10% 6% 4 4 5% 5% 7% 26%	2273
	21st to end	Night	17% 13% 5% 9% 7% 4 5% 5% 5% 6% 24%	1853
March	1st to 10th	Night	14% 9% 3 8% 8% 4 5% 5% 6% 7% 31%	1970
	11th to 20th	Night	16% 11% 6% 7% 5% 2 5% 5% 4 5% 33%	1979
	21st to end	Night	15% 10% 4 7% 6% 3 5% 4 4 5% 36%	2178
April	1st to 10th	Night	16% 10% 5% 7% 6% 4 4 4 5% 7% 33%	1842
	11th to 20th	Night	17% 8% 3 5% 5% 3 5% 5% 5% 6% 38%	1729
	21st to end	Night	16% 8% 4 5% 5% 3 6% 4 6% 6% 36%	1620
May	1st to 10th	Night	15% 8% 4 6% 5% 3 3 4 5% 8% 37%	1620
	11th to 20th	Night	14% 5% 3 6% 5% 3 4 6% 4 8% 42%	1461
	21st to end	Night	10% 6% 3 4 5% 3 3 4 5% 7% 49%	1550
June	1st to 10th	Night	11% 5% 2 5% 6% 4 6% 5% 7% 9% 39%	1440
	11th to 20th	Night	9% 5% 3 5% 6% 4 5% 4 6% 8% 44%	1440
	21st to end	Night	10% 5% 2 5% 4 4 4 5% 6% 8% 45%	1440
July	1st to 10th	Night	6% 4 2 5% 4 3 3 5% 5% 8% 57%	1440
	11th to 20th	Night	9% 3 3 5% 5% 3 4 6% 6% 10% 47%	1450
	21st to end	Night	9% 4 3 5% 4 3 4 5% 7% 9% 48%	1771
August	1st to 10th	Night	8% 3 2 5% 5% 4 5% 5% 8% 8% 47%	1611
	11th to 20th	Night	7% 4 2 5% 4 4 5% 5% 6% 9% 49%	1598
	21st to end	Night	8% 5% 2 5% 5% 3 5% 4 6% 10% 48%	1862
September	1st to 10th	Night	12% 6% 2 5% 5% 4 4 5% 5% 7% 44%	1800
	11th to 20th	Night	11% 5% 2 5% 4 2 4 4 4 8% 52%	1912
	21st to end	Night	14% 7% 3 7% 5% 3 4 5% 5% 7% 39%	2011
October	1st to 10th	Night	17% 8% 3 6% 5% 3 4 4 5% 6% 39%	2160
	11th to 20th	Night	20% 9% 3 6% 6% 3 4 5% 5% 7% 32%	2160
	21st to end	Night	24% 9% 3 6% 6% 3 5% 5% 5% 6% 27%	2376
November	1st to 10th	Night	16% 7% 3 7% 5% 3 5% 5% 4 7% 39%	2432
	11th to 20th	Night	15% 9% 3 7% 6% 4 5% 6% 6% 7% 32%	2660
	21st to end	Night	19% 10% 5% 6% 7% 4 5% 5% 6% 7% 28%	2660
December	1st to 10th	Night	18% 14% 6% 9% 7% 4 4 5% 5% 7% 22%	2660
	11th to 20th	Night	14% 16% 6% 9% 7% 4 5% 5% 5% 6% 23%	3288
	21st to end	Night	28% 16% 6% 10% 7% 3 5% 4 5% 5% 12%	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	61% 12% 10% 12% 5%	1518
	11th to 20th	Day	68% 7% 11% 10% 3	1695
	21st to end	Day	70% 7% 10% 9% 4	1836
February	1st to 10th	Day	67% 7% 10% 12% 4 1	1620
	11th to 20th	Day	64% 6% 11% 13% 5% 1	1582
	21st to end	Day	62% 7% 10% 13% 7% 1	1399
March	1st to 10th	Day	56% 6% 14% 16% 7% 1	1980
	11th to 20th	Day	53% 6% 11% 18% 10% 2	1980
	21st to end	Day	52% 6% 9% 20% 11% 2	2178
April	1st to 10th	Day	52% 5% 12% 18% 12% 1	2117
	11th to 20th	Day	48% 5% 12% 19% 14% 2	2194
	21st to end	Day	51% 5% 13% 18% 12% 2	2340
May	1st to 10th	Day	47% 5% 12% 22% 12% 2	2340
	11th to 20th	Day	45% 6% 12% 22% 13% 2	2436
	21st to end	Day	34% 8% 17% 25% 14% 2	2725
June	1st to 10th	Day	38% 8% 13% 23% 16% 2	2520
	11th to 20th	Day	38% 6% 15% 24% 15% 1	2520
	21st to end	Day	36% 7% 15% 26% 15% 1	2520
July	1st to 10th	Day	29% 7% 15% 26% 19% 3	2450
	11th to 20th	Day	36% 7% 18% 26% 12% 1	2431
	21st to end	Day	38% 9% 16% 23% 14% 1	2405
August	1st to 10th	Day	34% 7% 18% 26% 13% 2	2314
	11th to 20th	Day	36% 9% 19% 24% 11% 1	2303
	21st to end	Day	36% 5% 16% 28% 14% 1	2475
September	1st to 10th	Day	40% 7% 17% 24% 12% 1	2160
	11th to 20th	Day	34% 6% 15% 30% 15%	2044
	21st to end	Day	42% 7% 15% 25% 10% 1	1949
October	1st to 10th	Day	49% 6% 13% 18% 12% 1	1800
	11th to 20th	Day	53% 6% 14% 19% 8% 1	1800
	21st to end	Day	61% 6% 12% 14% 6% 1	1980
November	1st to 10th	Day	47% 7% 15% 22% 8% 1	1641
	11th to 20th	Day	52% 8% 16% 14% 10% 1	1520
	21st to end	Day	57% 7% 17% 14% 5%	1520
December	1st to 10th	Day	61% 9% 15% 11% 3	1520
	11th to 20th	Day	62% 9% 15% 10% 5%	1906
	21st to end	Day	75% 8% 10% 6% 1	2772
January	1st to 10th	Night	63% 9% 10% 13% 6%	2649
	11th to 20th	Night	66% 10% 11% 9% 3	2480
	21st to end	Night	61% 9% 11% 14% 5% 1	2664
February	1st to 10th	Night	63% 6% 11% 14% 5% 1	2340
	11th to 20th	Night	59% 7% 12% 16% 7%	2275
	21st to end	Night	59% 8% 10% 15% 7%	1853
March	1st to 10th	Night	51% 8% 12% 20% 8% 1	1980
	11th to 20th	Night	52% 6% 10% 19% 11% 2	1980
	21st to end	Night	50% 5% 7% 21% 14% 2	2178
April	1st to 10th	Night	51% 5% 11% 20% 12% 1	1842
	11th to 20th	Night	47% 5% 9% 20% 16% 3	1729
	21st to end	Night	47% 6% 11% 19% 15% 2	1620
May	1st to 10th	Night	45% 6% 9% 22% 15% 4	1620
	11th to 20th	Night	40% 7% 11% 24% 14% 3	1462
	21st to end	Night	35% 6% 13% 25% 18% 3	1551
June	1st to 10th	Night	39% 9% 11% 23% 17% 2	1440
	11th to 20th	Night	37% 7% 11% 24% 19% 3	1440
	21st to end	Night	36% 7% 14% 25% 18% 2	1440
July	1st to 10th	Night	27% 7% 11% 26% 26% 4	1400
	11th to 20th	Night	31% 6% 13% 29% 18% 2	1402
	21st to end	Night	32% 7% 17% 24% 18% 3	1672
August	1st to 10th	Night	31% 9% 15% 29% 14% 2	1600
	11th to 20th	Night	31% 8% 17% 30% 13% 1	1598
	21st to end	Night	32% 6% 14% 30% 16% 2	1862
September	1st to 10th	Night	39% 5% 15% 25% 14% 2	1800
	11th to 20th	Night	31% 6% 11% 33% 17% 1	1912
	21st to end	Night	43% 7% 12% 25% 13% 1	2011
October	1st to 10th	Night	44% 8% 10% 22% 15% 1	2160
	11th to 20th	Night	51% 6% 13% 21% 9% 1	2160
	21st to end	Night	56% 7% 11% 16% 10% 1	2376
November	1st to 10th	Night	44% 7% 12% 24% 12% 1	2432
	11th to 20th	Night	49% 8% 14% 18% 10% 1	2660
	21st to end	Night	55% 10% 15% 16% 5%	2660
December	1st to 10th	Night	60% 11% 11% 13% 4	2660
	11th to 20th	Night	60% 9% 13% 14% 4	3288
	21st to end	Night	72% 8% 12% 6% 2	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	18% 20% 35% 17% 8% 2	180
	11th to 20th	Day	23% 19% 37% 13% 5% 2	263
	21st to end	Day	22% 18% 33% 23% 4 1	282
February	1st to 10th	Day	25% 19% 41% 14% 1	240
	11th to 20th	Day	24% 21% 39% 14% 2	240
	21st to end	Day	27% 23% 34% 15% 1	216
March	1st to 10th	Day	21% 13% 37% 23% 7%	420
	11th to 20th	Day	19% 14% 35% 24% 7% 1	420
	21st to end	Day	26% 12% 29% 22% 9% 2	462
April	1st to 10th	Day	17% 14% 32% 26% 9% 2	420
	11th to 20th	Day	22% 16% 23% 24% 11% 4	412
	21st to end	Day	24% 7% 31% 25% 10% 2	420
May	1st to 10th	Day	21% 9% 29% 25% 12% 5%	423
	11th to 20th	Day	18% 8% 20% 28% 20% 6%	447
	21st to end	Day	14% 4 20% 32% 20% 9%	485
June	1st to 10th	Day	12% 7% 26% 35% 16% 3	450
	11th to 20th	Day	13% 7% 24% 29% 23% 5%	450
	21st to end	Day	14% 5% 21% 32% 21% 6%	450
July	1st to 10th	Day	9% 1 14% 32% 31% 13%	445
	11th to 20th	Day	15% 3 19% 29% 24% 10%	444
	21st to end	Day	9% 3 22% 36% 20% 10%	488
August	1st to 10th	Day	9% 3 16% 33% 26% 14%	446
	11th to 20th	Day	10% 5% 18% 29% 26% 12%	439
	21st to end	Day	13% 5% 20% 28% 22% 12%	495
September	1st to 10th	Day	13% 7% 19% 27% 23% 12%	450
	11th to 20th	Day	11% 6% 18% 23% 27% 15%	450
	21st to end	Day	16% 9% 25% 24% 15% 10%	450
October	1st to 10th	Day	21% 11% 24% 20% 17% 8%	450
	11th to 20th	Day	18% 17% 31% 23% 8% 2	450
	21st to end	Day	31% 14% 25% 21% 9% 1	495
November	1st to 10th	Day	14% 24% 31% 19% 8% 4	284
	11th to 20th	Day	23% 21% 28% 19% 7% 2	180
	21st to end	Day	23% 19% 36% 17% 6%	180
December	1st to 10th	Day	22% 20% 38% 13% 6% 1	180
	11th to 20th	Day	23% 27% 33% 14% 3	180
	21st to end	Day	24% 16% 39% 17% 3	198
January	1st to 10th	Night	24% 19% 34% 18% 6% 1	760
	11th to 20th	Night	28% 16% 32% 20% 5%	760
	21st to end	Night	19% 14% 38% 24% 6%	820
February	1st to 10th	Night	24% 19% 36% 18% 3	720
	11th to 20th	Night	20% 15% 36% 25% 4	703
	21st to end	Night	24% 14% 35% 23% 3	592
March	1st to 10th	Night	18% 14% 33% 27% 7%	720
	11th to 20th	Night	23% 14% 31% 24% 8% 1	720
	21st to end	Night	20% 9% 30% 30% 11% 1	792
April	1st to 10th	Night	21% 11% 31% 30% 6% 1	720
	11th to 20th	Night	20% 10% 32% 25% 12% 2	665
	21st to end	Night	19% 5% 29% 36% 11%	540
May	1st to 10th	Night	19% 5% 24% 31% 19% 2	540
	11th to 20th	Night	16% 5% 24% 38% 17% 1	400
	21st to end	Night	13% 4 22% 35% 23% 3	389
June	1st to 10th	Night	11% 3 31% 35% 19% 1	360
	11th to 20th	Night	14% 4 24% 37% 21% 1	360
	21st to end	Night	11% 4 21% 44% 19% 1	360
July	1st to 10th	Night	7% 3 17% 45% 26% 2	350
	11th to 20th	Night	10% 4 20% 40% 22% 4	358
	21st to end	Night	10% 3 14% 45% 25% 3	591
August	1st to 10th	Night	9% 3 12% 40% 32% 4	537
	11th to 20th	Night	8% 2 16% 32% 37% 4	534
	21st to end	Night	9% 3 19% 35% 29% 4	674
September	1st to 10th	Night	13% 6% 15% 35% 23% 7%	720
	11th to 20th	Night	13% 4 18% 30% 28% 7%	720
	21st to end	Night	16% 6% 22% 28% 22% 6%	720
October	1st to 10th	Night	19% 7% 24% 27% 20% 3	720
	11th to 20th	Night	23% 12% 28% 27% 9% 1	720
	21st to end	Night	28% 11% 28% 25% 7%	792
November	1st to 10th	Night	21% 10% 27% 29% 11% 2	740
	11th to 20th	Night	20% 16% 33% 21% 9% 2	760
	21st to end	Night	24% 13% 35% 21% 7% 1	760
December	1st to 10th	Night	28% 17% 32% 17% 6%	760
	11th to 20th	Night	23% 18% 36% 19% 5%	760
	21st to end	Night	29% 14% 33% 21% 3	836

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