growing upward into the middle or high étage. Conversely, nimbostratus usually forms in the middle étage of the troposphere and becomes vertically developed by growing downward into the low étage. Although the SYNOP code has no separate formal group classification for vertical or multi-level clouds, the observer procedure for selecting numerical codes is designed to give high reporting priority to those genera or species that show significant vertical development. The symbols used for clouds emulate the cloud shape. Cirrus is indicated by a couple hooks, cumulus are indicated by a mound shape, with cumulonimbus indicated with an upside down trapezoid on top of the cumulus symbol to indicate its anvil. When there is more than one cloud type present per level, the cloud type with the highest priority is included. Knowing the cloud type at various locations can help determine whether or not a weather front has passed by a particular location. A low deck of stratus could indicate a station is still north of a warm front, while thunderstorms can indicate the approach of a squall line or cold front. Present weather and visibility To the left of the cloud shape in the center of the station model is the symbol depicting present weather. The present weather symbol depicts the current weather which normally is obstructing the visibility at the time of observation. The visibility itself is shown as a number, in statute miles in the United States and meters elsewhere, describing how far the observer can see at that time. This number is located to the left of the present weather symbol. For pilots, knowledge of the horizontal visibility helps determine if they are flying through instrument meteorological conditions (IMC), such as foggy or smoky conditions, as well as areas of intense precipitation. Present weather depicted with the station model can include: Dust Fog Haze Ocean spray Precipitation Sand Smoke Thunderstorms Volcanic ash Temperature and dew point To left of center in the station model, the temperature and dew point are plotted. Within the United States on surface weather maps, they are still plotted in degrees Fahrenheit. Otherwise, they will be in units of degrees Celsius. This knowledge is important to meteorologists because when this data is plotted on a map, isotherms and isodrosotherms (lines of equal dew point) are easily analyzed, either by man or machine, which can help determine location of weather fronts. Sea level pressure