Irma
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Irma clear sky chart from 2005-11-06 to 2024-11-05.

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	9% 9% 3 9% 6% 5% 5% 4 6% 8% 35%	105644
	Night	8% 9% 4 9% 6% 5% 5% 5% 7% 8% 35%	101985
Transparency	Day	45% 6% 17% 24% 8% 1	105385
	Night	45% 6% 16% 23% 9% 1	101890
Seeing	Day	12% 8% 27% 31% 17% 5%	15572
	Night	12% 6% 29% 36% 16% 2	24606

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	5% 11% 5% 15% 10% 8% 6% 4 6% 5% 24%	2219
	11th to 20th	Day	5% 8% 3 9% 8% 8% 8% 8% 12% 6% 24%	2199
	21st to end	Day	6% 14% 5% 12% 6% 5% 6% 4 6% 7% 31%	2510
February	1st to 10th	Day	9% 13% 5% 14% 8% 7% 6% 5% 5% 7% 22%	1568
	11th to 20th	Day	7% 13% 6% 12% 6% 5% 4 5% 8% 8% 26%	1665
	21st to end	Day	5% 13% 4 12% 8% 7% 7% 5% 8% 10% 21%	1413
March	1st to 10th	Day	5% 12% 6% 10% 7% 6% 8% 5% 7% 8% 25%	3188
	11th to 20th	Day	4 11% 5% 14% 5% 4 4 3 5% 10% 36%	3489
	21st to end	Day	6% 12% 6% 17% 7% 5% 4 4 5% 7% 26%	3825
April	1st to 10th	Day	8% 13% 4 10% 6% 6% 5% 5% 6% 8% 30%	2370
	11th to 20th	Day	12% 11% 5% 10% 5% 4 5% 5% 7% 7% 31%	2440
	21st to end	Day	13% 12% 3 6% 5% 5% 6% 5% 6% 8% 31%	2498
May	1st to 10th	Day	15% 12% 4 10% 7% 6% 5% 3 4 6% 29%	4490
	11th to 20th	Day	13% 11% 3 9% 6% 6% 5% 5% 7% 8% 28%	4694
	21st to end	Day	11% 8% 3 9% 6% 6% 7% 5% 7% 9% 28%	5161
June	1st to 10th	Day	14% 8% 3 7% 5% 5% 6% 5% 7% 8% 32%	2850
	11th to 20th	Day	19% 9% 3 8% 6% 5% 6% 5% 6% 7% 25%	2850
	21st to end	Day	11% 6% 2 6% 5% 5% 6% 5% 8% 9% 36%	2850
July	1st to 10th	Day	7% 5% 2 6% 4 4 5% 4 6% 9% 48%	4740
	11th to 20th	Day	9% 5% 2 6% 5% 5% 4 4 6% 9% 46%	4740
	21st to end	Day	6% 5% 2 5% 4 5% 4 4 7% 9% 48%	5138
August	1st to 10th	Day	13% 5% 2 6% 4 5% 5% 3 6% 7% 44%	4611
	11th to 20th	Day	7% 5% 2 6% 4 4 5% 4 7% 10% 45%	4028
	21st to end	Day	8% 6% 2 6% 4 4 5% 4 6% 9% 46%	4521
September	1st to 10th	Day	10% 6% 2 7% 4 5% 5% 4 5% 7% 44%	2382
	11th to 20th	Day	15% 8% 3 7% 4 4 3 3 5% 8% 41%	2141
	21st to end	Day	10% 6% 2 6% 4 4 4 4 5% 8% 48%	2090
October	1st to 10th	Day	8% 9% 3 8% 5% 6% 5% 4 5% 7% 42%	3179
	11th to 20th	Day	8% 9% 3 7% 4 4 4 4 6% 11% 39%	3059
	21st to end	Day	7% 9% 3 7% 5% 4 5% 4 6% 8% 41%	2945
November	1st to 10th	Day	11% 15% 4 10% 7% 6% 5% 5% 5% 7% 26%	1520
	11th to 20th	Day	7% 14% 6% 15% 6% 4 4 3 6% 7% 27%	1487
	21st to end	Day	6% 11% 5% 11% 8% 6% 5% 4 7% 9% 28%	1330
December	1st to 10th	Day	7% 14% 5% 11% 7% 8% 7% 6% 6% 6% 23%	1330
	11th to 20th	Day	7% 10% 4 13% 8% 7% 7% 5% 8% 8% 24%	1682
	21st to end	Day	4 9% 3 9% 6% 5% 4 3 6% 8% 43%	2442
January	1st to 10th	Night	4 10% 6% 14% 8% 7% 8% 6% 7% 8% 22%	4726
	11th to 20th	Night	3 8% 3 10% 9% 8% 7% 7% 7% 8% 29%	4699
	21st to end	Night	6% 12% 5% 11% 6% 5% 6% 3 6% 7% 32%	5178
February	1st to 10th	Night	6% 10% 4 13% 9% 6% 6% 5% 7% 7% 25%	2611
	11th to 20th	Night	6% 11% 4 10% 7% 7% 6% 6% 7% 7% 28%	2402
	21st to end	Night	4 10% 4 10% 8% 6% 6% 6% 7% 9% 30%	2041
March	1st to 10th	Night	5% 11% 5% 13% 7% 5% 5% 5% 8% 9% 26%	3685
	11th to 20th	Night	4 8% 3 11% 6% 5% 5% 6% 6% 9% 36%	3490
	21st to end	Night	6% 10% 5% 15% 7% 7% 6% 5% 7% 8% 24%	3828
April	1st to 10th	Night	7% 10% 3 8% 6% 5% 5% 4 6% 8% 36%	1808
	11th to 20th	Night	9% 10% 5% 11% 6% 6% 7% 5% 7% 7% 28%	1691
	21st to end	Night	12% 12% 3 8% 5% 4 3 4 7% 7% 35%	1682
May	1st to 10th	Night	21% 12% 3 9% 4 5% 5% 3 5% 6% 27%	2476
	11th to 20th	Night	11% 9% 3 9% 5% 5% 5% 5% 8% 8% 32%	2199
	21st to end	Night	14% 8% 3 9% 6% 6% 7% 4 7% 9% 27%	2414
June	1st to 10th	Night	12% 10% 3 8% 6% 6% 5% 5% 7% 7% 32%	1330
	11th to 20th	Night	17% 9% 3 8% 8% 5% 5% 5% 7% 7% 27%	1330
	21st to end	Night	8% 4 2 6% 5% 7% 7% 6% 7% 9% 39%	1330
July	1st to 10th	Night	9% 6% 2 6% 4 4 6% 6% 6% 7% 45%	2220
	11th to 20th	Night	6% 3 2 4 4 4 5% 5% 8% 11% 49%	2220
	21st to end	Night	7% 4 2 5% 4 4 4 4 6% 9% 51%	2414
August	1st to 10th	Night	12% 6% 2 5% 4 4 5% 4 6% 8% 44%	2317
	11th to 20th	Night	8% 5% 3 6% 5% 5% 5% 4 6% 9% 45%	2811
	21st to end	Night	9% 6% 2 7% 4 4 4 4 7% 8% 46%	3146
September	1st to 10th	Night	10% 4 3 6% 4 5% 5% 3 6% 8% 47%	1797
	11th to 20th	Night	12% 6% 2 6% 4 4 4 4 5% 7% 46%	2038
	21st to end	Night	7% 6% 2 6% 4 4 4 4 7% 7% 48%	2090
October	1st to 10th	Night	8% 7% 3 7% 5% 5% 5% 4 6% 9% 42%	3790
	11th to 20th	Night	7% 7% 3 6% 4 3 4 4 6% 9% 46%	3866
	21st to end	Night	7% 10% 4 9% 4 5% 5% 4 6% 8% 39%	4581
November	1st to 10th	Night	12% 14% 5% 9% 5% 4 5% 4 6% 6% 29%	2659
	11th to 20th	Night	8% 11% 4 13% 8% 6% 5% 4 6% 8% 26%	2692
	21st to end	Night	5% 12% 5% 12% 8% 6% 5% 4 6% 8% 31%	2850
December	1st to 10th	Night	7% 13% 5% 12% 8% 6% 6% 5% 6% 7% 25%	2850
	11th to 20th	Night	6% 12% 5% 14% 8% 7% 6% 5% 6% 8% 23%	3510
	21st to end	Night	3 7% 3 8% 6% 6% 6% 4 6% 7% 44%	5214

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	59% 5% 12% 19% 4	2219
	11th to 20th	Day	52% 8% 13% 17% 10% 1	2220
	21st to end	Day	53% 4 16% 21% 5%	2510
February	1st to 10th	Day	61% 5% 12% 17% 6%	1568
	11th to 20th	Day	55% 5% 14% 18% 8% 1	1674
	21st to end	Day	56% 5% 13% 18% 7%	1413
March	1st to 10th	Day	54% 5% 16% 17% 7%	3188
	11th to 20th	Day	45% 5% 23% 17% 9%	3490
	21st to end	Day	57% 6% 11% 20% 6%	3825
April	1st to 10th	Day	50% 6% 14% 20% 9% 1	2370
	11th to 20th	Day	51% 5% 14% 21% 9% 1	2440
	21st to end	Day	48% 6% 14% 21% 9% 1	2498
May	1st to 10th	Day	57% 5% 10% 22% 6%	4490
	11th to 20th	Day	51% 5% 11% 22% 9% 1	4698
	21st to end	Day	48% 7% 17% 18% 9%	5162
June	1st to 10th	Day	48% 7% 14% 22% 9% 1	2850
	11th to 20th	Day	55% 8% 15% 17% 4 1	2850
	21st to end	Day	41% 8% 23% 20% 8% 1	2850
July	1st to 10th	Day	32% 7% 20% 27% 13% 1	4666
	11th to 20th	Day	35% 6% 18% 30% 11%	4659
	21st to end	Day	30% 7% 21% 31% 10%	4976
August	1st to 10th	Day	37% 8% 17% 34% 3	4593
	11th to 20th	Day	31% 7% 24% 30% 7%	4067
	21st to end	Day	34% 6% 25% 30% 6% 1	4520
September	1st to 10th	Day	38% 5% 20% 25% 10% 3	2382
	11th to 20th	Day	43% 4 17% 28% 7%	2141
	21st to end	Day	34% 7% 18% 29% 10% 3	2090
October	1st to 10th	Day	41% 5% 23% 25% 6%	3179
	11th to 20th	Day	40% 5% 16% 31% 9%	3059
	21st to end	Day	40% 5% 22% 28% 5%	2945
November	1st to 10th	Day	57% 5% 13% 19% 5% 1	1520
	11th to 20th	Day	56% 6% 17% 19% 3	1487
	21st to end	Day	52% 6% 18% 18% 5% 1	1330
December	1st to 10th	Day	58% 7% 13% 15% 6% 1	1330
	11th to 20th	Day	54% 7% 17% 17% 5%	1684
	21st to end	Day	41% 4 12% 28% 15% 1	2442
January	1st to 10th	Night	57% 6% 14% 16% 6% 1	4726
	11th to 20th	Night	50% 6% 14% 18% 11% 1	4740
	21st to end	Night	52% 4 16% 22% 6%	5178
February	1st to 10th	Night	55% 6% 14% 17% 8% 1	2611
	11th to 20th	Night	51% 6% 14% 18% 11%	2403
	21st to end	Night	48% 6% 14% 23% 8% 1	2041
March	1st to 10th	Night	52% 6% 15% 20% 7%	3685
	11th to 20th	Night	42% 6% 20% 23% 8% 1	3490
	21st to end	Night	53% 9% 12% 20% 6%	3828
April	1st to 10th	Night	43% 6% 15% 22% 13% 1	1808
	11th to 20th	Night	53% 6% 11% 18% 11% 2	1691
	21st to end	Night	46% 5% 13% 22% 11% 2	1682
May	1st to 10th	Night	58% 4 8% 23% 7% 1	2476
	11th to 20th	Night	45% 6% 13% 23% 13% 1	2199
	21st to end	Night	50% 7% 14% 22% 6% 1	2414
June	1st to 10th	Night	48% 5% 14% 23% 9% 1	1330
	11th to 20th	Night	51% 10% 14% 18% 7%	1330
	21st to end	Night	37% 9% 23% 23% 7% 1	1330
July	1st to 10th	Night	34% 8% 21% 19% 17% 1	2185
	11th to 20th	Night	26% 8% 19% 32% 15%	2178
	21st to end	Night	27% 6% 23% 31% 12% 1	2337
August	1st to 10th	Night	36% 7% 17% 36% 4	2310
	11th to 20th	Night	34% 7% 21% 30% 7%	2838
	21st to end	Night	34% 6% 20% 31% 8%	3146
September	1st to 10th	Night	34% 6% 19% 24% 12% 5%	1797
	11th to 20th	Night	37% 5% 16% 31% 10% 2	2038
	21st to end	Night	33% 7% 14% 31% 13% 2	2090
October	1st to 10th	Night	38% 6% 19% 29% 7% 1	3790
	11th to 20th	Night	34% 4 16% 36% 9%	3866
	21st to end	Night	42% 6% 20% 28% 5%	4581
November	1st to 10th	Night	54% 6% 14% 18% 8% 1	2659
	11th to 20th	Night	56% 6% 16% 16% 6% 1	2692
	21st to end	Night	51% 6% 20% 18% 5%	2850
December	1st to 10th	Night	57% 5% 12% 17% 8% 2	2850
	11th to 20th	Night	59% 7% 16% 11% 6%	3507
	21st to end	Night	39% 4 15% 22% 18% 2	5214

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	10% 3 32% 33% 18% 4	180
	11th to 20th	Day	7% 8% 29% 33% 21% 2	180
	21st to end	Day	13% 7% 33% 29% 15% 3	198
February	1st to 10th	Day	11% 8% 37% 31% 9% 3	203
	11th to 20th	Day	12% 6% 30% 33% 16% 3	270
	21st to end	Day	8% 6% 29% 34% 16% 7%	225
March	1st to 10th	Day	12% 10% 30% 30% 16% 1	431
	11th to 20th	Day	10% 7% 36% 30% 13% 5%	460
	21st to end	Day	12% 9% 29% 30% 16% 3	506
April	1st to 10th	Day	10% 13% 37% 29% 8% 3	460
	11th to 20th	Day	14% 10% 30% 30% 13% 3	452
	21st to end	Day	17% 12% 28% 30% 11% 2	488
May	1st to 10th	Day	14% 9% 26% 29% 17% 5%	653
	11th to 20th	Day	10% 9% 24% 32% 19% 6%	675
	21st to end	Day	15% 8% 21% 28% 20% 8%	734
June	1st to 10th	Day	15% 8% 27% 34% 13% 3	680
	11th to 20th	Day	19% 9% 25% 28% 14% 5%	680
	21st to end	Day	11% 7% 22% 30% 21% 9%	680
July	1st to 10th	Day	10% 5% 16% 33% 26% 9%	671
	11th to 20th	Day	11% 6% 23% 32% 20% 7%	668
	21st to end	Day	9% 4 21% 30% 28% 9%	739
August	1st to 10th	Day	11% 5% 20% 31% 25% 9%	639
	11th to 20th	Day	8% 4 24% 34% 22% 7%	477
	21st to end	Day	12% 6% 22% 30% 21% 9%	539
September	1st to 10th	Day	12% 8% 31% 30% 15% 4	490
	11th to 20th	Day	20% 9% 25% 28% 14% 4	490
	21st to end	Day	15% 12% 28% 30% 12% 3	490
October	1st to 10th	Day	14% 8% 27% 33% 14% 4	309
	11th to 20th	Day	15% 8% 33% 29% 13% 1	300
	21st to end	Day	12% 11% 38% 26% 10% 3	330
November	1st to 10th	Day	16% 5% 33% 31% 12% 2	282
	11th to 20th	Day	11% 12% 38% 29% 9%	255
	21st to end	Day	8% 8% 32% 37% 10% 5%	180
December	1st to 10th	Day	11% 8% 31% 29% 16% 6%	180
	11th to 20th	Day	8% 12% 29% 29% 21% 1	180
	21st to end	Day	9% 5% 26% 37% 18% 5%	198
January	1st to 10th	Night	10% 7% 31% 37% 12% 2	950
	11th to 20th	Night	9% 8% 33% 35% 14% 2	950
	21st to end	Night	13% 10% 33% 32% 10% 1	1022
February	1st to 10th	Night	9% 8% 26% 38% 16% 3	760
	11th to 20th	Night	10% 6% 33% 35% 15% 2	744
	21st to end	Night	8% 3 24% 39% 23% 3	628
March	1st to 10th	Night	8% 6% 28% 41% 15% 2	760
	11th to 20th	Night	11% 4 32% 35% 16% 2	760
	21st to end	Night	11% 7% 29% 35% 15% 3	836
April	1st to 10th	Night	10% 8% 29% 34% 17% 2	599
	11th to 20th	Night	13% 8% 29% 37% 12% 2	563
	21st to end	Night	17% 8% 24% 37% 14%	570
May	1st to 10th	Night	17% 6% 24% 35% 16% 2	532
	11th to 20th	Night	12% 6% 24% 39% 18% 1	374
	21st to end	Night	19% 6% 23% 29% 21% 2	411
June	1st to 10th	Night	16% 7% 26% 36% 13% 2	380
	11th to 20th	Night	21% 5% 20% 34% 18% 3	380
	21st to end	Night	10% 4 23% 36% 24% 3	380
July	1st to 10th	Night	9% 5% 19% 41% 26%	370
	11th to 20th	Night	8% 4 21% 44% 21% 2	368
	21st to end	Night	8% 5% 18% 39% 25% 5%	416
August	1st to 10th	Night	9% 5% 17% 38% 25% 6%	398
	11th to 20th	Night	11% 5% 18% 41% 23% 2	564
	21st to end	Night	9% 4 25% 39% 22% 1	627
September	1st to 10th	Night	11% 4 26% 37% 21% 1	570
	11th to 20th	Night	15% 5% 19% 42% 17% 1	708
	21st to end	Night	10% 4 32% 38% 14% 1	760
October	1st to 10th	Night	13% 5% 30% 36% 15% 2	760
	11th to 20th	Night	13% 4 32% 36% 13% 2	760
	21st to end	Night	14% 8% 33% 35% 10% 1	911
November	1st to 10th	Night	18% 6% 31% 32% 11% 2	950
	11th to 20th	Night	13% 7% 34% 32% 11% 2	950
	21st to end	Night	8% 7% 37% 37% 10% 1	950
December	1st to 10th	Night	14% 7% 30% 30% 16% 4	950
	11th to 20th	Night	10% 10% 34% 32% 12% 1	950
	21st to end	Night	9% 4 33% 36% 16% 3	1045

Page generated at 2024-11-03 00:11:17. Version ac07

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