Saint John's University Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Saint John's University Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	12% 8% 3 8% 5% 4 4 4 6% 7% 39%	127198
	Night	11% 9% 3 9% 5% 4 4 4 5% 7% 38%	122388
Transparency	Day	44% 4 13% 27% 11% 1	130768
	Night	44% 4 13% 25% 12% 2	126884
Seeing	Day	16% 10% 31% 27% 13% 3	17359
	Night	15% 9% 32% 32% 11% 1	27156

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	8% 12% 3 12% 10% 6% 5% 4 5% 8% 26%	3178
	11th to 20th	Day	5% 8% 5% 14% 8% 7% 7% 7% 10% 7% 21%	3716
	21st to end	Day	4 13% 8% 14% 7% 5% 5% 4 4 6% 31%	4488
February	1st to 10th	Day	8% 12% 6% 9% 8% 6% 5% 4 4 6% 33%	1800
	11th to 20th	Day	7% 10% 4 11% 8% 5% 5% 4 6% 7% 34%	1754
	21st to end	Day	8% 12% 4 11% 7% 5% 4 4 5% 7% 34%	1491
March	1st to 10th	Day	11% 9% 3 6% 5% 4 4 3 5% 7% 43%	4936
	11th to 20th	Day	10% 8% 3 8% 4 5% 4 4 5% 8% 40%	4980
	21st to end	Day	20% 13% 4 9% 6% 4 4 3 5% 6% 26%	5462
April	1st to 10th	Day	17% 10% 3 8% 4 4 3 3 5% 7% 35%	2380
	11th to 20th	Day	17% 13% 4 8% 4 4 4 3 4 6% 31%	2470
	21st to end	Day	22% 10% 3 7% 5% 4 4 3 5% 5% 33%	2600
May	1st to 10th	Day	13% 6% 2 6% 3 4 4 3 4 6% 49%	6018
	11th to 20th	Day	17% 8% 2 6% 3 3 4 3 5% 6% 42%	6319
	21st to end	Day	16% 7% 2 6% 4 4 5% 4 6% 8% 37%	6935
June	1st to 10th	Day	12% 6% 2 6% 3 4 5% 4 6% 8% 45%	2846
	11th to 20th	Day	14% 6% 3 7% 6% 5% 4 4 6% 8% 37%	3000
	21st to end	Day	11% 5% 2 6% 5% 5% 5% 4 8% 11% 40%	3000
July	1st to 10th	Day	7% 4 2 6% 4 5% 5% 5% 7% 10% 45%	6800
	11th to 20th	Day	9% 4 2 4 3 3 4 4 6% 9% 54%	6548
	21st to end	Day	8% 5% 2 6% 4 4 4 5% 7% 10% 47%	6729
August	1st to 10th	Day	12% 6% 2 6% 4 4 4 4 6% 8% 44%	4234
	11th to 20th	Day	11% 5% 2 6% 4 5% 5% 5% 6% 10% 40%	4171
	21st to end	Day	10% 7% 2 5% 4 4 4 4 6% 8% 46%	4562
September	1st to 10th	Day	9% 5% 2 6% 4 4 4 3 5% 8% 50%	2400
	11th to 20th	Day	11% 5% 2 6% 5% 4 5% 4 6% 8% 45%	2300
	21st to end	Day	14% 6% 2 5% 3 3 4 3 5% 7% 47%	2150
October	1st to 10th	Day	17% 8% 2 6% 3 3 3 4 5% 6% 42%	1930
	11th to 20th	Day	13% 11% 3 7% 4 3 4 3 5% 7% 39%	1900
	21st to end	Day	17% 10% 3 7% 4 3 4 3 4 7% 37%	2085
November	1st to 10th	Day	14% 12% 2 7% 6% 4 4 3 6% 6% 36%	1585
	11th to 20th	Day	9% 13% 7% 12% 5% 5% 4 3 5% 6% 30%	1600
	21st to end	Day	13% 11% 5% 8% 5% 3 5% 4 6% 7% 34%	1600
December	1st to 10th	Day	7% 10% 4 11% 9% 9% 7% 5% 6% 6% 25%	2610
	11th to 20th	Day	9% 17% 7% 11% 5% 4 4 3 5% 6% 30%	3008
	21st to end	Day	19% 17% 6% 20% 5% 4 4 2 3 4 16%	3613
January	1st to 10th	Night	7% 9% 5% 15% 9% 6% 5% 3 5% 6% 30%	6787
	11th to 20th	Night	6% 11% 4 10% 6% 8% 10% 8% 8% 6% 22%	6243
	21st to end	Night	5% 17% 8% 10% 5% 5% 5% 4 5% 6% 31%	6490
February	1st to 10th	Night	6% 11% 5% 9% 6% 5% 4 4 6% 8% 35%	2600
	11th to 20th	Night	5% 9% 4 10% 5% 4 5% 4 6% 8% 39%	2533
	21st to end	Night	11% 11% 3 8% 6% 5% 5% 4 5% 6% 37%	2144
March	1st to 10th	Night	10% 7% 2 6% 4 4 3 3 6% 8% 47%	4929
	11th to 20th	Night	9% 8% 2 8% 5% 4 5% 4 7% 9% 40%	4980
	21st to end	Night	22% 12% 4 11% 5% 4 4 3 4 4 28%	5467
April	1st to 10th	Night	14% 10% 3 7% 5% 4 4 3 5% 6% 39%	2020
	11th to 20th	Night	17% 10% 4 7% 4 4 3 3 5% 6% 37%	1878
	21st to end	Night	19% 10% 3 7% 4 3 3 4 5% 6% 35%	1800
May	1st to 10th	Night	10% 6% 2 5% 3 3 3 3 4 6% 54%	3897
	11th to 20th	Night	14% 8% 2 5% 4 4 4 3 4 6% 48%	3596
	21st to end	Night	16% 8% 3 6% 4 3 4 3 5% 7% 40%	3942
June	1st to 10th	Night	10% 5% 1 5% 4 4 5% 5% 6% 8% 47%	1549
	11th to 20th	Night	9% 5% 2 5% 4 4 5% 5% 6% 8% 47%	1400
	21st to end	Night	10% 4 3 5% 4 3 4 3 6% 7% 51%	1400
July	1st to 10th	Night	8% 3 1 4 4 4 5% 4 7% 10% 49%	3178
	11th to 20th	Night	7% 4 1 4 3 3 3 3 5% 9% 57%	3414
	21st to end	Night	8% 6% 2 6% 5% 3 5% 4 7% 8% 47%	4125
August	1st to 10th	Night	11% 6% 2 6% 4 3 4 3 5% 7% 50%	2926
	11th to 20th	Night	13% 5% 2 5% 3 4 5% 4 5% 7% 47%	2888
	21st to end	Night	9% 6% 2 6% 3 3 4 3 6% 9% 48%	3330
September	1st to 10th	Night	8% 3 2 6% 3 3 4 4 6% 9% 54%	2000
	11th to 20th	Night	9% 6% 1 5% 3 3 4 4 4 7% 54%	2098
	21st to end	Night	12% 5% 2 5% 3 3 3 3 5% 6% 53%	2230
October	1st to 10th	Night	14% 6% 2 5% 4 4 3 3 5% 6% 48%	2317
	11th to 20th	Night	14% 9% 2 6% 4 3 4 3 5% 6% 44%	2280
	21st to end	Night	16% 10% 3 8% 4 4 3 3 5% 7% 37%	2508
November	1st to 10th	Night	12% 9% 3 7% 4 4 4 4 6% 7% 40%	2712
	11th to 20th	Night	13% 10% 5% 11% 5% 4 4 4 4 6% 34%	2800
	21st to end	Night	12% 9% 4 9% 5% 4 4 3 5% 7% 39%	2800
December	1st to 10th	Night	6% 7% 4 12% 9% 7% 6% 4 7% 8% 30%	4580
	11th to 20th	Night	10% 14% 6% 14% 7% 5% 5% 4 5% 5% 25%	5199
	21st to end	Night	19% 17% 6% 16% 6% 4 3 3 3 4 17%	7348

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	57% 4 10% 18% 11%	3179
	11th to 20th	Day	57% 3 11% 20% 9%	3716
	21st to end	Day	55% 4 15% 18% 9%	4488
February	1st to 10th	Day	53% 3 12% 23% 9% 1	1800
	11th to 20th	Day	49% 4 12% 26% 8% 1	1763
	21st to end	Day	51% 4 12% 21% 11% 1	1491
March	1st to 10th	Day	41% 3 16% 26% 10% 3	4936
	11th to 20th	Day	42% 3 13% 30% 10% 1	4980
	21st to end	Day	61% 3 10% 18% 7%	5464
April	1st to 10th	Day	49% 3 11% 22% 14% 1	2380
	11th to 20th	Day	55% 4 9% 19% 12% 1	2470
	21st to end	Day	54% 3 11% 21% 10% 1	2600
May	1st to 10th	Day	36% 4 12% 28% 18% 1	6056
	11th to 20th	Day	42% 3 9% 25% 14% 7%	6323
	21st to end	Day	44% 4 10% 32% 10% 1	6949
June	1st to 10th	Day	37% 4 15% 31% 12% 1	2846
	11th to 20th	Day	44% 6% 15% 21% 14% 1	3000
	21st to end	Day	36% 5% 18% 27% 12% 2	3000
July	1st to 10th	Day	32% 7% 18% 32% 11%	6726
	11th to 20th	Day	28% 5% 16% 38% 13% 1	6468
	21st to end	Day	32% 6% 17% 32% 13% 1	6537
August	1st to 10th	Day	38% 5% 12% 31% 12% 2	4220
	11th to 20th	Day	38% 5% 15% 32% 9%	4210
	21st to end	Day	35% 5% 16% 30% 12% 1	4561
September	1st to 10th	Day	32% 4 15% 37% 12%	2400
	11th to 20th	Day	36% 5% 18% 31% 9% 1	2300
	21st to end	Day	37% 5% 11% 35% 12% 1	2150
October	1st to 10th	Day	31% 3 10% 34% 21% 1	3190
	11th to 20th	Day	38% 4 12% 23% 21% 2	3140
	21st to end	Day	47% 2 14% 28% 9%	3412
November	1st to 10th	Day	48% 4 12% 21% 12% 2	1585
	11th to 20th	Day	55% 3 12% 19% 10% 1	1600
	21st to end	Day	50% 4 15% 22% 7% 2	1600
December	1st to 10th	Day	58% 5% 15% 17% 4 1	2610
	11th to 20th	Day	57% 2 15% 19% 7%	3005
	21st to end	Day	73% 2 8% 11% 5%	3613
January	1st to 10th	Night	56% 3 10% 20% 10%	6787
	11th to 20th	Night	58% 3 12% 16% 11% 1	6243
	21st to end	Night	54% 3 12% 24% 6%	6490
February	1st to 10th	Night	47% 3 13% 23% 11% 2	2600
	11th to 20th	Night	42% 5% 15% 25% 12% 1	2534
	21st to end	Night	46% 4 13% 22% 13% 2	2144
March	1st to 10th	Night	35% 3 14% 28% 17% 2	4929
	11th to 20th	Night	39% 3 13% 29% 15%	4980
	21st to end	Night	61% 3 8% 19% 9% 1	5467
April	1st to 10th	Night	47% 3 9% 26% 14% 1	2020
	11th to 20th	Night	48% 3 8% 23% 16% 3	1878
	21st to end	Night	50% 4 9% 22% 13% 2	1800
May	1st to 10th	Night	32% 3 12% 31% 19% 3	3924
	11th to 20th	Night	39% 3 9% 21% 21% 8%	3597
	21st to end	Night	45% 3 11% 27% 13% 1	3950
June	1st to 10th	Night	33% 5% 16% 30% 14% 2	1549
	11th to 20th	Night	33% 5% 18% 26% 15% 3	1400
	21st to end	Night	31% 4 16% 28% 17% 3	1400
July	1st to 10th	Night	29% 6% 20% 28% 17%	3145
	11th to 20th	Night	24% 5% 16% 39% 15% 2	3374
	21st to end	Night	33% 7% 16% 31% 12% 2	4007
August	1st to 10th	Night	34% 4 14% 30% 15% 5%	2916
	11th to 20th	Night	36% 5% 15% 32% 12% 1	2915
	21st to end	Night	32% 5% 18% 31% 12% 2	3330
September	1st to 10th	Night	27% 5% 17% 34% 16% 1	2000
	11th to 20th	Night	31% 4 17% 32% 16%	2098
	21st to end	Night	32% 3 12% 36% 17% 1	2230
October	1st to 10th	Night	24% 2 6% 37% 25% 4	3837
	11th to 20th	Night	34% 3 11% 28% 21% 2	3779
	21st to end	Night	42% 3 15% 27% 12%	4120
November	1st to 10th	Night	43% 3 14% 26% 12% 2	2712
	11th to 20th	Night	52% 4 13% 20% 11% 1	2800
	21st to end	Night	46% 4 17% 24% 7% 2	2800
December	1st to 10th	Night	51% 5% 18% 20% 6%	4580
	11th to 20th	Night	60% 4 11% 19% 6% 1	5201
	21st to end	Night	72% 3 7% 14% 4	7348

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	14% 13% 29% 30% 12% 2	300
	11th to 20th	Day	14% 11% 38% 32% 5%	300
	21st to end	Day	13% 8% 40% 24% 12% 2	330
February	1st to 10th	Day	16% 16% 41% 24% 3	300
	11th to 20th	Day	13% 18% 40% 21% 8%	300
	21st to end	Day	17% 12% 44% 25% 2	249
March	1st to 10th	Day	18% 15% 39% 22% 5% 1	300
	11th to 20th	Day	20% 9% 37% 23% 9% 2	300
	21st to end	Day	26% 13% 28% 21% 10% 2	330
April	1st to 10th	Day	26% 15% 29% 23% 6%	480
	11th to 20th	Day	26% 16% 27% 21% 9% 1	588
	21st to end	Day	25% 12% 33% 20% 9% 2	700
May	1st to 10th	Day	20% 12% 33% 23% 10% 3	703
	11th to 20th	Day	20% 10% 30% 26% 12% 2	725
	21st to end	Day	19% 7% 29% 30% 12% 3	789
June	1st to 10th	Day	11% 9% 27% 30% 20% 3	730
	11th to 20th	Day	17% 8% 33% 29% 12% 1	730
	21st to end	Day	14% 7% 33% 27% 15% 4	730
July	1st to 10th	Day	8% 4 24% 35% 21% 9%	721
	11th to 20th	Day	9% 6% 28% 31% 20% 7%	718
	21st to end	Day	8% 4 26% 34% 23% 6%	794
August	1st to 10th	Day	11% 5% 24% 33% 23% 5%	726
	11th to 20th	Day	14% 6% 22% 29% 24% 5%	715
	21st to end	Day	11% 6% 24% 33% 22% 4	742
September	1st to 10th	Day	12% 6% 30% 32% 15% 4	530
	11th to 20th	Day	10% 5% 28% 35% 18% 4	430
	21st to end	Day	18% 11% 29% 23% 14% 6%	330
October	1st to 10th	Day	19% 10% 32% 23% 12% 5%	309
	11th to 20th	Day	15% 18% 36% 23% 7% 1	300
	21st to end	Day	22% 18% 37% 17% 6% 1	330
November	1st to 10th	Day	23% 13% 29% 26% 6% 2	300
	11th to 20th	Day	16% 19% 42% 20% 2	300
	21st to end	Day	20% 14% 40% 21% 5%	300
December	1st to 10th	Day	15% 11% 42% 23% 7% 1	300
	11th to 20th	Day	20% 16% 38% 23% 3	300
	21st to end	Day	29% 12% 34% 20% 3 1	330
January	1st to 10th	Night	10% 12% 37% 31% 9% 2	1000
	11th to 20th	Night	17% 10% 37% 28% 8% 1	1000
	21st to end	Night	16% 12% 37% 28% 7%	1100
February	1st to 10th	Night	12% 12% 39% 29% 8% 1	1000
	11th to 20th	Night	11% 13% 40% 28% 7% 1	979
	21st to end	Night	16% 11% 33% 31% 8% 1	824
March	1st to 10th	Night	15% 10% 38% 27% 9% 1	610
	11th to 20th	Night	17% 11% 40% 26% 6% 1	600
	21st to end	Night	22% 9% 32% 26% 10% 2	660
April	1st to 10th	Night	20% 11% 36% 27% 6%	600
	11th to 20th	Night	21% 14% 29% 28% 7%	593
	21st to end	Night	23% 8% 32% 29% 8% 2	600
May	1st to 10th	Night	17% 10% 35% 29% 9% 1	600
	11th to 20th	Night	16% 9% 29% 34% 11% 1	591
	21st to end	Night	16% 7% 25% 39% 13%	650
June	1st to 10th	Night	13% 6% 27% 39% 16% 1	600
	11th to 20th	Night	12% 4 33% 42% 9%	600
	21st to end	Night	10% 6% 27% 42% 14% 1	600
July	1st to 10th	Night	4 3 22% 44% 25% 1	585
	11th to 20th	Night	7% 4 25% 46% 17% 2	582
	21st to end	Night	5% 3 23% 47% 20% 2	657
August	1st to 10th	Night	10% 3 18% 46% 22% 1	597
	11th to 20th	Night	12% 4 18% 41% 23% 2	594
	21st to end	Night	8% 4 24% 38% 24% 1	660
September	1st to 10th	Night	10% 5% 26% 38% 18% 3	600
	11th to 20th	Night	12% 5% 30% 33% 19% 1	600
	21st to end	Night	13% 9% 31% 30% 15% 1	630
October	1st to 10th	Night	16% 9% 34% 29% 11% 1	773
	11th to 20th	Night	19% 10% 34% 30% 7% 1	760
	21st to end	Night	22% 13% 32% 27% 7%	836
November	1st to 10th	Night	17% 11% 32% 31% 8% 1	975
	11th to 20th	Night	18% 14% 35% 25% 7% 1	1000
	21st to end	Night	17% 14% 38% 25% 7%	1000
December	1st to 10th	Night	14% 12% 36% 30% 8% 1	1000
	11th to 20th	Night	19% 10% 37% 28% 6% 1	1000
	21st to end	Night	24% 10% 33% 26% 6% 1	1100

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