Strathmore
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Strathmore clear sky chart from 2005-11-06 to 2026-07-01.

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% 8% 3 8% 5% 5% 5% 4 6% 8% 39%	142092
	Night	6% 9% 4 9% 6% 5% 5% 4 6% 8% 37%	138960
Transparency	Day	41% 6% 15% 26% 11% 2	141792
	Night	43% 6% 15% 23% 11% 2	138844
Seeing	Day	12% 8% 26% 30% 19% 6%	17537
	Night	11% 6% 30% 36% 15% 2	26632

By Month

Percentage Cloud Cover NW (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	3 9% 6% 15% 8% 7% 6% 4 6% 8% 28%	4125
	11th to 20th	Day	2 9% 4 10% 5% 4 6% 6% 7% 8% 39%	4137
	21st to end	Day	3 6% 3 10% 6% 5% 5% 4 6% 8% 44%	5186
February	1st to 10th	Day	5% 12% 4 12% 8% 7% 6% 4 6% 7% 28%	1697
	11th to 20th	Day	6% 12% 4 11% 6% 5% 6% 6% 8% 7% 29%	1845
	21st to end	Day	4 12% 5% 11% 7% 6% 5% 4 8% 7% 31%	1657
March	1st to 10th	Day	4 10% 4 11% 6% 4 5% 4 5% 11% 36%	4618
	11th to 20th	Day	5% 9% 3 9% 5% 4 5% 4 6% 8% 40%	5103
	21st to end	Day	9% 19% 6% 14% 6% 5% 6% 4 5% 6% 21%	5623
April	1st to 10th	Day	9% 13% 5% 9% 6% 5% 5% 5% 7% 7% 30%	2599
	11th to 20th	Day	10% 13% 4 9% 5% 5% 5% 4 6% 8% 31%	2700
	21st to end	Day	12% 11% 4 8% 6% 5% 4 4 6% 8% 32%	2730
May	1st to 10th	Day	12% 9% 3 7% 4 5% 5% 4 6% 7% 40%	8024
	11th to 20th	Day	11% 10% 4 10% 6% 6% 7% 5% 8% 8% 26%	8755
	21st to end	Day	11% 6% 2 7% 5% 5% 6% 5% 7% 8% 37%	9649
June	1st to 10th	Day	16% 7% 2 7% 5% 5% 5% 5% 6% 9% 33%	3150
	11th to 20th	Day	16% 9% 3 6% 6% 6% 5% 5% 7% 8% 29%	3150
	21st to end	Day	8% 4 2 5% 5% 5% 6% 5% 8% 10% 42%	3150
July	1st to 10th	Day	7% 4 1 5% 4 4 4 4 7% 9% 51%	6781
	11th to 20th	Day	10% 3 2 5% 3 4 5% 4 7% 10% 47%	6780
	21st to end	Day	8% 4 2 5% 3 4 4 3 5% 8% 53%	7272
August	1st to 10th	Day	8% 6% 2 6% 5% 4 6% 5% 7% 9% 43%	4541
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 7% 10% 46%	4163
	21st to end	Day	10% 4 1 4 2 4 4 4 5% 7% 55%	4664
September	1st to 10th	Day	10% 5% 2 6% 3 4 3 2 5% 7% 52%	2489
	11th to 20th	Day	14% 7% 2 5% 4 4 4 3 5% 7% 46%	2200
	21st to end	Day	9% 7% 2 6% 4 4 4 3 6% 8% 46%	2200
October	1st to 10th	Day	8% 6% 2 7% 5% 4 6% 4 6% 7% 43%	3417
	11th to 20th	Day	7% 7% 2 6% 4 5% 5% 4 6% 9% 46%	3044
	21st to end	Day	7% 8% 2 7% 5% 5% 6% 5% 5% 5% 46%	3200
November	1st to 10th	Day	10% 15% 5% 9% 6% 4 4 4 6% 8% 30%	1695
	11th to 20th	Day	7% 12% 3 8% 7% 5% 5% 4 5% 6% 38%	1631
	21st to end	Day	4 12% 4 10% 6% 5% 5% 5% 8% 9% 31%	1470
December	1st to 10th	Day	4 13% 6% 11% 6% 6% 6% 5% 6% 7% 31%	2360
	11th to 20th	Day	5% 11% 5% 11% 7% 6% 6% 5% 6% 7% 31%	2712
	21st to end	Day	3 9% 4 8% 5% 5% 5% 5% 6% 8% 43%	3575
January	1st to 10th	Night	3 13% 5% 13% 8% 7% 6% 5% 6% 8% 27%	8768
	11th to 20th	Night	2 9% 4 11% 5% 6% 6% 5% 6% 8% 38%	8779
	21st to end	Night	3 8% 4 10% 7% 5% 5% 4 6% 8% 39%	9022
February	1st to 10th	Night	5% 10% 4 14% 7% 6% 6% 5% 6% 7% 30%	2922
	11th to 20th	Night	5% 13% 5% 10% 6% 5% 5% 6% 7% 7% 30%	2662
	21st to end	Night	4 11% 5% 11% 6% 5% 6% 4 7% 8% 33%	2146
March	1st to 10th	Night	3 8% 4 12% 7% 6% 5% 5% 8% 9% 33%	5496
	11th to 20th	Night	6% 8% 4 9% 6% 5% 5% 4 7% 8% 37%	5115
	21st to end	Night	9% 19% 6% 13% 7% 5% 5% 5% 6% 7% 20%	5578
April	1st to 10th	Night	9% 13% 4 9% 6% 6% 5% 4 7% 9% 30%	2021
	11th to 20th	Night	9% 12% 5% 9% 6% 6% 5% 4 6% 7% 31%	1871
	21st to end	Night	10% 12% 4 7% 6% 5% 4 4 8% 8% 31%	1890
May	1st to 10th	Night	13% 9% 3 8% 5% 5% 5% 4 5% 6% 37%	4912
	11th to 20th	Night	12% 8% 4 8% 5% 6% 5% 5% 7% 10% 29%	4105
	21st to end	Night	10% 7% 3 9% 6% 6% 6% 5% 7% 8% 35%	4522
June	1st to 10th	Night	14% 9% 3 8% 6% 4 6% 5% 8% 7% 31%	1470
	11th to 20th	Night	15% 8% 3 7% 5% 5% 6% 5% 7% 8% 31%	1470
	21st to end	Night	9% 6% 2 5% 5% 4 3 6% 8% 8% 44%	1470
July	1st to 10th	Night	7% 5% 2 6% 4 5% 5% 4 7% 7% 48%	3180
	11th to 20th	Night	7% 5% 2 6% 4 5% 5% 5% 8% 10% 44%	3180
	21st to end	Night	10% 4 2 6% 3 3 4 3 6% 8% 50%	3556
August	1st to 10th	Night	8% 7% 2 6% 5% 5% 5% 5% 7% 9% 43%	2609
	11th to 20th	Night	6% 5% 2 7% 5% 5% 5% 5% 8% 11% 43%	2896
	21st to end	Night	8% 3 1 4 4 4 5% 4 6% 9% 53%	3245
September	1st to 10th	Night	9% 6% 3 5% 4 4 5% 4 5% 7% 50%	1907
	11th to 20th	Night	13% 6% 2 5% 4 3 3 3 5% 7% 48%	2200
	21st to end	Night	9% 5% 2 5% 3 4 5% 5% 6% 8% 49%	2200
October	1st to 10th	Night	8% 6% 3 6% 4 4 4 4 6% 9% 47%	3777
	11th to 20th	Night	7% 7% 2 6% 4 4 5% 4 7% 8% 46%	4088
	21st to end	Night	5% 7% 2 7% 5% 5% 6% 5% 7% 7% 44%	4600
November	1st to 10th	Night	9% 13% 4 8% 5% 6% 5% 4 5% 8% 33%	2803
	11th to 20th	Night	5% 12% 3 8% 7% 6% 5% 5% 6% 8% 36%	2987
	21st to end	Night	5% 12% 3 9% 6% 5% 5% 5% 7% 9% 35%	3150
December	1st to 10th	Night	4 12% 5% 12% 7% 7% 7% 5% 7% 7% 29%	5040
	11th to 20th	Night	4 11% 6% 13% 7% 7% 6% 5% 7% 8% 27%	5700
	21st to end	Night	4 9% 4 8% 5% 5% 5% 4 6% 7% 42%	7623

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	53% 5% 10% 20% 11% 1	4125
	11th to 20th	Day	41% 5% 16% 18% 16% 4	4158
	21st to end	Day	38% 5% 15% 31% 12% 1	5187
February	1st to 10th	Day	54% 6% 13% 15% 10% 2	1697
	11th to 20th	Day	50% 6% 13% 20% 10% 1	1854
	21st to end	Day	49% 5% 12% 20% 12% 1	1657
March	1st to 10th	Day	42% 5% 13% 23% 16% 1	4618
	11th to 20th	Day	40% 6% 13% 24% 16% 1	5104
	21st to end	Day	62% 6% 13% 14% 5%	5623
April	1st to 10th	Day	50% 7% 15% 19% 9% 1	2599
	11th to 20th	Day	49% 6% 12% 22% 9% 1	2700
	21st to end	Day	49% 6% 12% 22% 10% 1	2730
May	1st to 10th	Day	42% 5% 10% 25% 14% 4	8024
	11th to 20th	Day	49% 7% 11% 25% 7% 1	8759
	21st to end	Day	41% 7% 14% 29% 8% 1	9650
June	1st to 10th	Day	45% 7% 14% 21% 10% 1	3150
	11th to 20th	Day	50% 8% 13% 21% 7% 2	3150
	21st to end	Day	34% 8% 21% 27% 9% 2	3150
July	1st to 10th	Day	28% 5% 18% 30% 16% 2	6707
	11th to 20th	Day	32% 5% 21% 30% 11% 1	6699
	21st to end	Day	30% 4 16% 27% 17% 6%	7067
August	1st to 10th	Day	36% 6% 15% 34% 8% 1	4524
	11th to 20th	Day	31% 5% 23% 32% 7% 1	4202
	21st to end	Day	28% 5% 18% 34% 14% 1	4663
September	1st to 10th	Day	32% 5% 15% 29% 14% 6%	2489
	11th to 20th	Day	38% 5% 14% 28% 13% 2	2200
	21st to end	Day	35% 6% 15% 29% 12% 3	2200
October	1st to 10th	Day	38% 5% 18% 30% 8% 1	3417
	11th to 20th	Day	34% 5% 18% 27% 13% 2	3044
	21st to end	Day	39% 5% 9% 32% 13% 1	3200
November	1st to 10th	Day	51% 6% 11% 20% 9% 2	1695
	11th to 20th	Day	47% 5% 14% 24% 9% 1	1631
	21st to end	Day	45% 8% 15% 22% 8% 2	1470
December	1st to 10th	Day	52% 5% 14% 21% 6% 2	2360
	11th to 20th	Day	51% 5% 16% 21% 6% 1	2714
	21st to end	Day	37% 6% 11% 26% 18% 3	3575
January	1st to 10th	Night	55% 5% 11% 19% 9% 2	8769
	11th to 20th	Night	43% 5% 16% 20% 13% 3	8820
	21st to end	Night	42% 4 17% 25% 11% 1	9022
February	1st to 10th	Night	50% 6% 12% 19% 12% 1	2922
	11th to 20th	Night	49% 6% 12% 20% 12% 1	2663
	21st to end	Night	45% 6% 12% 22% 12% 2	2146
March	1st to 10th	Night	43% 6% 17% 21% 11% 1	5496
	11th to 20th	Night	42% 7% 14% 25% 11% 1	5115
	21st to end	Night	62% 6% 12% 13% 7% 1	5578
April	1st to 10th	Night	49% 6% 13% 19% 12%	2021
	11th to 20th	Night	48% 7% 13% 21% 9% 1	1871
	21st to end	Night	46% 8% 12% 23% 10% 2	1890
May	1st to 10th	Night	47% 4 10% 24% 13% 1	4909
	11th to 20th	Night	46% 7% 13% 25% 9% 1	4105
	21st to end	Night	44% 6% 17% 26% 7% 1	4526
June	1st to 10th	Night	48% 8% 13% 20% 10% 1	1470
	11th to 20th	Night	48% 8% 16% 19% 7% 2	1470
	21st to end	Night	34% 8% 21% 27% 9% 1	1470
July	1st to 10th	Night	33% 5% 18% 24% 19% 1	3145
	11th to 20th	Night	34% 6% 19% 32% 7% 2	3138
	21st to end	Night	31% 6% 16% 28% 15% 4	3457
August	1st to 10th	Night	36% 6% 16% 29% 12% 1	2601
	11th to 20th	Night	33% 6% 23% 28% 9% 1	2923
	21st to end	Night	27% 5% 18% 34% 13% 2	3245
September	1st to 10th	Night	32% 6% 15% 27% 14% 6%	1907
	11th to 20th	Night	36% 5% 15% 27% 13% 4	2200
	21st to end	Night	31% 6% 16% 28% 15% 3	2200
October	1st to 10th	Night	33% 5% 15% 30% 15% 2	3777
	11th to 20th	Night	35% 5% 16% 27% 16% 2	4088
	21st to end	Night	37% 5% 12% 33% 13% 1	4600
November	1st to 10th	Night	48% 6% 12% 21% 11% 2	2803
	11th to 20th	Night	45% 6% 16% 21% 12% 1	2987
	21st to end	Night	45% 7% 17% 21% 8% 2	3150
December	1st to 10th	Night	53% 5% 17% 18% 6% 1	5040
	11th to 20th	Night	53% 5% 20% 17% 5%	5697
	21st to end	Night	40% 5% 10% 26% 16% 3	7623

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	5% 3 34% 33% 16% 8%	220
	11th to 20th	Day	9% 8% 32% 30% 17% 5%	220
	21st to end	Day	8% 8% 32% 34% 15% 3	242
February	1st to 10th	Day	12% 8% 25% 38% 13% 4	229
	11th to 20th	Day	8% 6% 35% 34% 15% 2	330
	21st to end	Day	8% 9% 32% 34% 14% 3	373
March	1st to 10th	Day	10% 11% 32% 29% 14% 5%	540
	11th to 20th	Day	8% 10% 34% 31% 13% 4	540
	21st to end	Day	14% 12% 33% 22% 14% 5%	594
April	1st to 10th	Day	13% 10% 31% 28% 14% 4	540
	11th to 20th	Day	13% 10% 30% 27% 17% 4	532
	21st to end	Day	15% 13% 26% 26% 16% 4	540
May	1st to 10th	Day	17% 9% 23% 26% 19% 6%	616
	11th to 20th	Day	15% 6% 21% 31% 22% 5%	775
	21st to end	Day	14% 6% 21% 28% 23% 7%	844
June	1st to 10th	Day	15% 8% 23% 32% 18% 4	780
	11th to 20th	Day	16% 8% 27% 31% 15% 2	780
	21st to end	Day	10% 5% 22% 32% 24% 7%	780
July	1st to 10th	Day	10% 4 17% 35% 25% 8%	722
	11th to 20th	Day	11% 4 18% 35% 25% 8%	718
	21st to end	Day	6% 4 20% 34% 26% 10%	752
August	1st to 10th	Day	8% 4 22% 33% 22% 10%	527
	11th to 20th	Day	7% 2 21% 34% 28% 7%	517
	21st to end	Day	9% 6% 19% 30% 23% 12%	583
September	1st to 10th	Day	12% 6% 23% 31% 22% 7%	530
	11th to 20th	Day	17% 6% 18% 29% 23% 8%	530
	21st to end	Day	11% 12% 32% 29% 13% 5%	530
October	1st to 10th	Day	16% 8% 20% 30% 19% 7%	425
	11th to 20th	Day	10% 7% 35% 25% 17% 5%	330
	21st to end	Day	13% 14% 32% 29% 9% 2	363
November	1st to 10th	Day	15% 11% 32% 29% 11% 3	330
	11th to 20th	Day	14% 10% 36% 26% 13% 1	303
	21st to end	Day	6% 7% 36% 31% 12% 9%	220
December	1st to 10th	Day	5% 9% 40% 31% 12% 3	220
	11th to 20th	Day	7% 13% 36% 31% 12% 1	220
	21st to end	Day	7% 6% 27% 33% 24% 3	242
January	1st to 10th	Night	8% 7% 35% 36% 12% 2	1050
	11th to 20th	Night	10% 7% 38% 33% 11% 1	1050
	21st to end	Night	11% 8% 35% 32% 12% 2	939
February	1st to 10th	Night	8% 7% 31% 35% 16% 3	840
	11th to 20th	Night	10% 5% 32% 38% 13% 2	824
	21st to end	Night	8% 4 32% 34% 18% 4	692
March	1st to 10th	Night	8% 5% 32% 35% 16% 3	840
	11th to 20th	Night	8% 7% 37% 35% 12% 2	840
	21st to end	Night	14% 6% 30% 33% 14% 2	924
April	1st to 10th	Night	13% 7% 29% 35% 15% 2	761
	11th to 20th	Night	14% 7% 30% 36% 12%	623
	21st to end	Night	15% 7% 30% 38% 11%	630
May	1st to 10th	Night	17% 7% 26% 35% 15% 1	562
	11th to 20th	Night	13% 4 25% 36% 20% 1	414
	21st to end	Night	17% 5% 23% 35% 19% 1	455
June	1st to 10th	Night	17% 8% 25% 36% 13% 2	420
	11th to 20th	Night	15% 6% 32% 33% 14% 1	420
	21st to end	Night	12% 3 21% 41% 22% 1	420
July	1st to 10th	Night	11% 4 13% 44% 26% 1	390
	11th to 20th	Night	9% 3 20% 43% 22% 2	388
	21st to end	Night	6% 3 21% 42% 26% 2	438
August	1st to 10th	Night	9% 4 16% 43% 26% 3	398
	11th to 20th	Night	8% 4 18% 47% 22% 1	589
	21st to end	Night	9% 5% 21% 40% 24% 1	660
September	1st to 10th	Night	11% 3 21% 40% 23% 2	672
	11th to 20th	Night	15% 4 17% 38% 24% 1	800
	21st to end	Night	10% 5% 30% 38% 15% 2	800
October	1st to 10th	Night	14% 5% 25% 37% 17% 2	800
	11th to 20th	Night	13% 6% 29% 37% 13% 3	800
	21st to end	Night	10% 9% 34% 34% 13% 1	880
November	1st to 10th	Night	15% 7% 37% 32% 9% 1	958
	11th to 20th	Night	9% 8% 41% 30% 10% 2	1050
	21st to end	Night	11% 4 36% 35% 12% 2	1050
December	1st to 10th	Night	11% 7% 33% 33% 14% 2	1050
	11th to 20th	Night	8% 7% 37% 36% 10% 1	1050
	21st to end	Night	12% 5% 32% 35% 14% 2	1155

Page generated at 2026-06-28 00:10:46. Version ac07

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