ITU-R P840-7 Cloud and Fog Model
This ITU cloud and fog model is based on the most recent revision of the ITU recommendation ITU-R P840. This model supersedes all previous versions of the cloud and fog model.
The ITU-R dataset is from 0° to 360° in longitude and from +90° to –90° in latitude, with a resolution of 1.125º in both latitude and longitude. The total columnar content of reduced cloud liquid water at any desired location on the surface of the Earth are derived by interpolating the data.
| Parameter | Description |
|---|---|
| Cloud Ceiling | Specifies the height of the cloud layer above the ground. |
| Cloud Layer Thickness | Specifies the thickness of the clouds and fog layer. |
| Cloud Temperature | Temperature of the clouds and fog layer |
| Liquid Water Content Density Value |
Specifies the amount of water content within the clouds and fog layer. For more information, see Relationship of liquid water content to Classification of Clouds. |
| Percent Time Liquid Water Exceeded Annual Average |
Specifies the percentage of time that the annual values of total columnar content of cloud liquid water are exceeded for an average year. For more information, see Notes for Percent Time Liquid Water Exceeded Annual/Monthly Average Parameters. |
| Percent Time Liquid Water Exceeded Monthly Average |
Specifies the percentage of time that the monthly values of total columnar content of cloud liquid water are exceeded for the average month specified below. For more information, see Notes for Percent Time Liquid Water Exceeded Annual/Monthly Average Parameters. |
| Liquid Water Average Data Month |
Specifies the month (1 for January through 12 for December) of the year used to get the average cloud liquid water content data. |
| Use Rain Height as Cloud Layer Thickness |
Uses rain height as the cloud thickness and sets the Cloud Ceiling to zero. When this option is selected, the Cloud Ceiling and Cloud Layer Thickness options are disabled. |
Notes for Percent Time Liquid Water Exceeded Annual/Monthly Average parameters
- The higher the outage percentage tolerated by your system, the less power required to compensate for it. So, as you increase the percentage, the loss decreases.
- The global averages for cloud water content are computed over a year and also over each month of the year, based on data gathered over several years.
- The instantaneous water content average values may spike above these average values. Sharp (steep) spikes happen for very short durations, e.g., 0.1% of the total time. Smaller "spikes" may last for longer durations, even 70% to 90% of the total time. You can choose a time value to analyze the impact of these spikes in the water content. You can then select from 0.1% to 99% for the percentage of time that the water content value exceeds a specified average water content of the time. For example, a lower percentage value (representing a sharper spike) will point to higher cloud liquid water content. The cloud water content value can spike far above the annual or monthly average for between 0.1% and 1.0% of the time in a year or a month. Compare this to the 90%-of-the-time value, which means that 90% of the time the water content average values will be slightly above the annual or the monthly average.
- Higher water content in clouds (lower percentage exceeded value) will, in general, lead to higher propagation loss through the clouds.
- Annual data exists for 0.1, 0.2, 0.3, 0.5, 1, 2, 3, 5, 10, 20, 30, 50, 60, 70, 80, 90, 95 and 99% of the time. Monthly data exists for 1, 2, 3, 5, 10, 20, 30, 50, 60, 70, 80, 90, 95, and 99 percent of the time. Values for other percentages are computed by interpolation.
- Cloud water content data is supplied with the Cloud and Fog Model ITU-R P840-7. This monthly or yearly average data is using the column of water content (kg/m^2), not the water density. STK uses cloud thickness to compute density. If you enter zero meters or small values as the cloud thickness, it will inflate cloud water density arbitrarily high, so you must carefully select these values.
You may select the rain height as the cloud thickness to compute cloud water content density. The cloud ceiling is taken as zero in this option. For example, fog may be extended from the ground to the cloud height altitude.
Relationship of liquid water content to classification of clouds
The following table shows typical liquid water content (LWC) values of various cloud types.
| Cloud Type | LWC (g/m3) |
|---|---|
| cirrus | .03 |
| fog | .05 |
| stratus | .25-.30 |
| cumulus | .25-.30 |
| stratocumulus | .45 |
| cumulonimbus | 1.0-3.0 |
"Simulating the Adiabatic Ascent of Atmospheric Air Parcels using the Cloud Chamber." Anne Thompson (2007), Department of Meteorology, Penn State.