Gateway
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Gateway clear sky chart from 2005-11-06 to 2026-01-02.

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	3 4 2 4 3 4 4 4 6% 8% 58%	135760
	Night	3 5% 2 4 3 4 4 4 5% 7% 59%	133094
Transparency	Day	23% 4 9% 24% 26% 15%	130614
	Night	23% 4 9% 27% 26% 12%	129243
Seeing	Day	8% 12% 29% 30% 17% 4	16749
	Night	6% 4 19% 41% 25% 4	27035

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	1 4 2 6% 4 5% 5% 5% 7% 8% 52%	4107
	11th to 20th	Day	1 4 1 5% 4 5% 6% 5% 6% 8% 54%	4100
	21st to end	Day	3 8% 2 4 3 3 4 3 5% 6% 59%	4510
February	1st to 10th	Day	1 8% 3 6% 5% 3 4 4 6% 9% 50%	1800
	11th to 20th	Day	4 9% 3 6% 4 4 4 5% 5% 8% 49%	1755
	21st to end	Day	3 8% 3 6% 5% 4 4 3 6% 8% 51%	1685
March	1st to 10th	Day	1 8% 2 6% 4 5% 5% 4 6% 6% 53%	4954
	11th to 20th	Day	3 12% 3 7% 5% 5% 4 4 6% 7% 44%	5000
	21st to end	Day	3 9% 3 8% 6% 6% 5% 5% 7% 8% 41%	5486
April	1st to 10th	Day	3 6% 2 5% 5% 4 4 3 5% 7% 55%	2345
	11th to 20th	Day	3 5% 2 5% 4 4 6% 4 6% 8% 52%	2371
	21st to end	Day	2 5% 2 5% 4 5% 6% 4 6% 8% 54%	2472
May	1st to 10th	Day	3 3 1 5% 4 5% 5% 5% 8% 9% 51%	5880
	11th to 20th	Day	3 4 1 5% 4 4 4 4 6% 8% 56%	5841
	21st to end	Day	2 3 1 3 3 3 4 3 6% 8% 64%	6738
June	1st to 10th	Day	3 3 1 3 3 3 4 3 6% 8% 64%	2800
	11th to 20th	Day	2 3 1 3 2 3 3 3 4 5% 71%	2800
	21st to end	Day	1 2 1 3 3 3 3 3 5% 7% 69%	2800
July	1st to 10th	Day	2 2 1 2 3 3 3 3 4 5% 71%	6027
	11th to 20th	Day	2 2 1 3 3 4 5% 5% 8% 11% 58%	5880
	21st to end	Day	2 1 1 3 3 4 5% 5% 7% 9% 59%	6401
August	1st to 10th	Day	2 2 1 3 3 4 5% 5% 8% 11% 55%	5779
	11th to 20th	Day	2 2 1 4 4 4 5% 4 7% 7% 60%	5798
	21st to end	Day	5% 3 1 5% 4 5% 5% 4 7% 9% 53%	5994
September	1st to 10th	Day	5% 3 1 4 3 3 5% 4 5% 7% 62%	2400
	11th to 20th	Day	3 3 1 4 4 4 5% 4 6% 7% 59%	2278
	21st to end	Day	4 3 1 4 2 3 2 3 4 7% 67%	2170
October	1st to 10th	Day	4 2 1 3 2 2 3 2 3 4 74%	3270
	11th to 20th	Day	5% 4 2 4 3 4 4 4 6% 9% 57%	3250
	21st to end	Day	3 3 1 3 2 3 3 2 4 7% 69%	3548
November	1st to 10th	Day	3 3 2 3 3 3 4 3 4 6% 67%	2059
	11th to 20th	Day	3 4 1 3 3 3 3 2 4 6% 67%	1811
	21st to end	Day	3 5% 1 4 3 3 3 3 5% 6% 62%	1680
December	1st to 10th	Day	2 3 1 4 3 3 2 2 4 6% 69%	2700
	11th to 20th	Day	2 5% 2 4 3 3 4 4 6% 10% 58%	3106
	21st to end	Day	3 8% 2 5% 4 4 5% 4 6% 9% 51%	4165
January	1st to 10th	Night	1 4 3 7% 5% 5% 5% 4 5% 7% 54%	5880
	11th to 20th	Night	2 7% 2 5% 3 3 5% 5% 7% 8% 53%	5880
	21st to end	Night	3 8% 3 6% 4 4 5% 3 5% 6% 55%	6468
February	1st to 10th	Night	2 5% 2 5% 4 4 5% 4 6% 8% 56%	2600
	11th to 20th	Night	3 8% 2 6% 4 3 5% 3 6% 8% 52%	2532
	21st to end	Night	2 7% 3 7% 4 4 4 3 5% 7% 53%	1938
March	1st to 10th	Night	2 8% 2 5% 4 4 4 3 5% 7% 55%	4940
	11th to 20th	Night	3 8% 2 7% 4 4 5% 3 6% 8% 50%	5000
	21st to end	Night	3 6% 3 7% 5% 5% 5% 4 6% 7% 49%	5489
April	1st to 10th	Night	4 6% 2 4 4 4 5% 4 6% 7% 54%	2055
	11th to 20th	Night	4 6% 2 4 4 4 5% 4 5% 7% 55%	1977
	21st to end	Night	4 5% 2 4 4 4 5% 4 7% 9% 53%	1924
May	1st to 10th	Night	4 4 2 6% 4 5% 6% 4 6% 7% 54%	4100
	11th to 20th	Night	2 4 1 5% 3 4 4 4 6% 8% 58%	4074
	21st to end	Night	3 3 1 3 3 3 3 3 4 6% 68%	4154
June	1st to 10th	Night	2 2 1 3 3 4 4 4 5% 8% 64%	1600
	11th to 20th	Night	4 3 1 4 2 2 4 3 4 7% 66%	1600
	21st to end	Night	3 2 1 3 2 2 3 3 4 7% 70%	1600
July	1st to 10th	Night	2 1 1 2 3 3 4 3 5% 7% 70%	3933
	11th to 20th	Night	1 2 1 3 3 4 5% 5% 8% 10% 57%	4100
	21st to end	Night	2 2 1 4 3 3 4 4 7% 9% 60%	4472
August	1st to 10th	Night	2 2 1 4 4 5% 6% 5% 7% 9% 56%	4033
	11th to 20th	Night	2 2 1 3 3 3 4 4 6% 9% 63%	4051
	21st to end	Night	3 3 2 5% 3 4 5% 5% 8% 8% 53%	4981
September	1st to 10th	Night	4 3 1 3 3 3 4 3 6% 7% 63%	2000
	11th to 20th	Night	3 2 1 3 3 3 3 3 5% 8% 66%	2118
	21st to end	Night	4 3 1 3 2 2 3 2 4 5% 70%	2230
October	1st to 10th	Night	3 2 1 3 2 2 2 2 3 5% 75%	3920
	11th to 20th	Night	4 4 1 3 3 4 4 3 5% 8% 62%	3898
	21st to end	Night	3 4 2 3 2 3 3 2 4 6% 68%	4252
November	1st to 10th	Night	2 4 1 4 3 2 3 3 5% 6% 67%	2460
	11th to 20th	Night	2 3 1 3 3 3 3 3 4 6% 69%	2780
	21st to end	Night	3 6% 2 5% 3 3 3 4 5% 6% 59%	2940
December	1st to 10th	Night	2 4 1 2 2 2 3 2 4 7% 70%	4710
	11th to 20th	Night	3 5% 2 4 3 3 3 4 6% 9% 60%	5321
	21st to end	Night	4 7% 2 5% 3 4 4 4 5% 7% 54%	7084

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	27% 3 16% 37% 14% 2	4108
	11th to 20th	Day	26% 2 16% 26% 24% 6%	4100
	21st to end	Day	25% 5% 8% 32% 24% 7%	4510
February	1st to 10th	Day	28% 3 19% 25% 20% 4	1800
	11th to 20th	Day	31% 4 11% 30% 21% 3	1764
	21st to end	Day	30% 3 8% 28% 25% 7%	1685
March	1st to 10th	Day	28% 3 4 27% 31% 6%	4954
	11th to 20th	Day	35% 4 9% 31% 18% 4	5000
	21st to end	Day	38% 3 11% 30% 16% 2	5486
April	1st to 10th	Day	27% 3 7% 26% 26% 12%	2345
	11th to 20th	Day	27% 4 6% 25% 27% 10%	2374
	21st to end	Day	25% 3 6% 27% 32% 7%	2472
May	1st to 10th	Day	23% 5% 9% 21% 33% 9%	5880
	11th to 20th	Day	23% 4 4 22% 35% 13%	5844
	21st to end	Day	16% 3 2 19% 47% 11%	6739
June	1st to 10th	Day	17% 3 4 20% 34% 22%	2800
	11th to 20th	Day	16% 3 3 11% 27% 41%	2800
	21st to end	Day	15% 3 2 14% 24% 42%	2800
July	1st to 10th	Day	15% 4 5% 8% 16% 53%	5964
	11th to 20th	Day	17% 5% 7% 20% 28% 24%	5811
	21st to end	Day	18% 5% 9% 20% 24% 24%	6217
August	1st to 10th	Day	25% 6% 16% 22% 15% 16%	4208
	11th to 20th	Day	25% 6% 10% 19% 22% 18%	4197
	21st to end	Day	22% 7% 10% 20% 23% 18%	4321
September	1st to 10th	Day	22% 3 6% 15% 26% 28%	2400
	11th to 20th	Day	22% 4 7% 19% 24% 24%	2278
	21st to end	Day	19% 2 5% 15% 40% 19%	2170
October	1st to 10th	Day	16% 1 4 14% 33% 31%	3270
	11th to 20th	Day	23% 4 3 28% 34% 8%	3250
	21st to end	Day	16% 4 7% 34% 34% 5%	3548
November	1st to 10th	Day	18% 3 7% 32% 32% 8%	2059
	11th to 20th	Day	18% 3 9% 36% 30% 4	1811
	21st to end	Day	22% 2 12% 32% 26% 5%	1680
December	1st to 10th	Day	17% 2 15% 41% 22% 3	2700
	11th to 20th	Day	21% 4 19% 38% 18% 1	3104
	21st to end	Day	29% 4 18% 22% 23% 3	4165
January	1st to 10th	Night	26% 4 15% 34% 18% 3	5880
	11th to 20th	Night	26% 2 15% 26% 27% 4	5880
	21st to end	Night	30% 3 12% 27% 23% 5%	6468
February	1st to 10th	Night	24% 4 17% 34% 17% 4	2600
	11th to 20th	Night	28% 3 11% 31% 22% 4	2533
	21st to end	Night	30% 2 7% 29% 27% 6%	1938
March	1st to 10th	Night	28% 1 5% 27% 32% 7%	4940
	11th to 20th	Night	31% 3 7% 32% 24% 4	5000
	21st to end	Night	31% 4 10% 35% 19% 2	5489
April	1st to 10th	Night	27% 3 6% 25% 28% 11%	2055
	11th to 20th	Night	27% 3 6% 23% 29% 12%	1977
	21st to end	Night	25% 3 6% 26% 33% 8%	1924
May	1st to 10th	Night	26% 4 10% 19% 32% 9%	4097
	11th to 20th	Night	21% 4 4 26% 34% 11%	4075
	21st to end	Night	17% 3 3 18% 45% 15%	4154
June	1st to 10th	Night	17% 3 3 18% 34% 25%	1600
	11th to 20th	Night	18% 3 5% 13% 21% 41%	1600
	21st to end	Night	14% 2 3 12% 24% 45%	1600
July	1st to 10th	Night	13% 4 6% 9% 14% 54%	3888
	11th to 20th	Night	17% 5% 6% 24% 25% 22%	4046
	21st to end	Night	17% 4 9% 19% 27% 24%	4344
August	1st to 10th	Night	27% 6% 16% 19% 16% 16%	2928
	11th to 20th	Night	22% 7% 13% 16% 20% 21%	2928
	21st to end	Night	21% 6% 10% 18% 24% 20%	3585
September	1st to 10th	Night	20% 3 7% 16% 23% 32%	2000
	11th to 20th	Night	17% 3 10% 20% 25% 26%	2118
	21st to end	Night	17% 2 5% 15% 39% 22%	2230
October	1st to 10th	Night	14% 1 3 14% 31% 36%	3920
	11th to 20th	Night	19% 5% 6% 27% 32% 10%	3898
	21st to end	Night	17% 4 5% 38% 31% 5%	4252
November	1st to 10th	Night	18% 2 8% 29% 32% 10%	2460
	11th to 20th	Night	16% 3 8% 35% 33% 5%	2780
	21st to end	Night	25% 3 9% 29% 28% 7%	2940
December	1st to 10th	Night	15% 2 14% 46% 19% 4	4710
	11th to 20th	Night	19% 5% 15% 44% 16% 1	5322
	21st to end	Night	27% 6% 16% 29% 20% 3	7084

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	9% 10% 35% 28% 15% 3	303
	11th to 20th	Day	9% 8% 39% 29% 14% 1	300
	21st to end	Day	6% 9% 32% 37% 15% 1	330
February	1st to 10th	Day	10% 17% 37% 27% 9% 1	300
	11th to 20th	Day	11% 13% 42% 24% 9% 1	300
	21st to end	Day	11% 19% 35% 25% 9% 1	329
March	1st to 10th	Day	14% 22% 32% 23% 8% 2	500
	11th to 20th	Day	14% 18% 33% 24% 9% 2	500
	21st to end	Day	12% 32% 31% 19% 5% 1	550
April	1st to 10th	Day	12% 20% 35% 22% 10% 2	500
	11th to 20th	Day	15% 24% 36% 18% 5% 1	492
	21st to end	Day	12% 22% 33% 25% 7% 2	500
May	1st to 10th	Day	11% 23% 35% 23% 7% 1	503
	11th to 20th	Day	7% 16% 35% 27% 13% 2	528
	21st to end	Day	8% 13% 29% 27% 19% 4	706
June	1st to 10th	Day	4 8% 25% 37% 22% 4	730
	11th to 20th	Day	7% 10% 29% 31% 20% 3	730
	21st to end	Day	4 10% 26% 35% 22% 3	730
July	1st to 10th	Day	2 5% 24% 38% 25% 6%	596
	11th to 20th	Day	2 2 17% 43% 30% 6%	524
	21st to end	Day	4 2 16% 39% 33% 6%	575
August	1st to 10th	Day	5% 3 24% 36% 27% 5%	527
	11th to 20th	Day	4 3 21% 40% 27% 5%	517
	21st to end	Day	5% 2 23% 38% 28% 4	583
September	1st to 10th	Day	4 5% 24% 37% 23% 8%	530
	11th to 20th	Day	7% 11% 28% 35% 16% 4	530
	21st to end	Day	8% 11% 22% 32% 20% 6%	500
October	1st to 10th	Day	12% 12% 24% 21% 20% 11%	330
	11th to 20th	Day	9% 12% 28% 28% 19% 5%	330
	21st to end	Day	7% 15% 27% 31% 18% 2	363
November	1st to 10th	Day	8% 16% 28% 28% 16% 4	330
	11th to 20th	Day	5% 11% 33% 33% 13% 5%	330
	21st to end	Day	7% 12% 32% 28% 16% 4	330
December	1st to 10th	Day	8% 8% 35% 28% 16% 5%	330
	11th to 20th	Day	6% 12% 32% 30% 17% 3	330
	21st to end	Day	12% 15% 34% 28% 8% 3	363
January	1st to 10th	Night	5% 4 20% 44% 23% 4	804
	11th to 20th	Night	6% 4 21% 43% 24% 3	800
	21st to end	Night	7% 4 23% 43% 20% 3	880
February	1st to 10th	Night	4 4 28% 46% 16% 2	800
	11th to 20th	Night	7% 5% 28% 43% 14% 2	784
	21st to end	Night	6% 5% 28% 48% 11% 2	660
March	1st to 10th	Night	7% 6% 26% 44% 16% 2	800
	11th to 20th	Night	6% 6% 19% 44% 23% 2	800
	21st to end	Night	7% 7% 27% 43% 16% 1	880
April	1st to 10th	Night	6% 9% 28% 42% 14% 1	655
	11th to 20th	Night	10% 8% 27% 39% 14% 2	593
	21st to end	Night	7% 9% 27% 37% 17% 4	600
May	1st to 10th	Night	8% 8% 18% 45% 19% 2	600
	11th to 20th	Night	5% 5% 17% 41% 27% 5%	591
	21st to end	Night	7% 6% 22% 35% 26% 4	650
June	1st to 10th	Night	3 3 15% 41% 33% 4	600
	11th to 20th	Night	5% 4 19% 44% 24% 4	600
	21st to end	Night	4 3 11% 41% 37% 5%	600
July	1st to 10th	Night	3 2 9% 35% 44% 8%	585
	11th to 20th	Night	2 1 4 35% 48% 9%	582
	21st to end	Night	3 2 5% 28% 50% 11%	657
August	1st to 10th	Night	3 2 8% 32% 47% 8%	597
	11th to 20th	Night	5% 6% 42% 40% 7%	594
	21st to end	Night	3 1 7% 37% 42% 10%	660
September	1st to 10th	Night	5% 2 7% 40% 40% 7%	600
	11th to 20th	Night	4 3 13% 40% 33% 7%	718
	21st to end	Night	6% 2 14% 34% 34% 10%	800
October	1st to 10th	Night	8% 4 16% 40% 27% 6%	800
	11th to 20th	Night	5% 4 17% 42% 26% 7%	800
	21st to end	Night	7% 5% 18% 41% 25% 5%	880
November	1st to 10th	Night	5% 4 20% 41% 26% 5%	820
	11th to 20th	Night	3 3 22% 47% 20% 4	958
	21st to end	Night	7% 6% 23% 39% 22% 4	1050
December	1st to 10th	Night	6% 3 22% 47% 18% 3	1050
	11th to 20th	Night	9% 4 24% 40% 21% 2	1050
	21st to end	Night	10% 8% 24% 42% 14% 2	1137

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