# Weather map

Weather map of the southeastern United States on October 9, 1843 by James Pollard Espy .

A weather map in the general sense is a map that makes the weather conditions over a geographic area at a specific point in time visually detectable. The point in time can be current, in the past or in the future ( weather forecast ).

From a meteorological point of view, it is understood to mean the momentary state of the air pressure conditions over a larger area. Such a weather map provides information about the wind speed and wind direction or allows possible weather hazards to be identified at an early stage. It also enables a forecast of the coming weather. It can also be used sensibly in everyday life.

## history

James Pollard Espy created the first series of weather maps of the United States for the US Army in 1843. The French scientist Urbain Le Verrier , who first created one for France on February 19, 1855, is considered to be the inventor of the map with weather forecast .

## Ground weather maps

A weather map (ground weather map) for Europe and the North Atlantic with station data

### Creation of a weather map

The traditional and well-known form of the weather map is the ground weather map , sometimes also called the ground map for short . For such a map can be created, must first air pressure readings of all the weather stations , that is all on the ground monitoring stations located, are present in the catchment area of the map. The data are entered in the map, which is still empty, according to the geographic position of the respective weather station.

In the next step on the map of the highest and the lowest air pressure value to be searched and with a H for high pressure area or with a T for depression (international also L for engl. Low highlighted). The isobars , i.e. the lines along which the same air pressure prevailed at the time of measurement, are then drawn around the high and low pressure centers . The isobars, like other isolines , each have discrete values. On German weather maps, these are usually values ​​that can be easily divided by five. The real measured values ​​of the weather stations, however, are distributed within a continuum . Therefore, many measuring stations are not exactly on one isobar: If an air pressure of 1002 hectopascals (hPa) was measured at weather station A  and a value of 1008 hPa at weather station B, then the isobar for 1005 hPa runs roughly in the middle between A and B.

#### Simple weather map features at a glance

In addition to the isobars, there are further details on a weather map, here the most important:

Fronts
symbol description
Warm fronts are drawn in with a line made up of red semicircles. They canmeanprolonged rain and lead to an increase in cloud cover and a slow rise in temperature.
Cold fronts are drawn in with a line of blue triangles. The weather remains inconsistent when a cold front passes through, and showers and thunderstorms occur frequently. The air temperature drops.
Occlusions are drawn as a purple semicircle next to a triangle. It arises when the faster cold front reaches the slower warm front and it combines with it. The weather remains fickle and rainy within an occlusion.
stationary fronts are drawn in with a line of alternating blue triangles and red semicircles.
Wedges of the relative topography are drawn with an orange line with alternating lines.
Trough lines are drawn in with a blue dashed line
Convergences are drawn in with a blue line with alternating lines
Pressure areas
Art Letter
High pressure area German : H och English : H igh French : A nticyclone (old: h aute pression) Russian : W erchnij
Low pressure area German : T ief English : L ow, C yclone French : D épression (old: b asse pression) Russian : N ischnij
High altitude core German : HTK English : Low Pressure Center french : Russian :
Precipitation
Art presentation
rain Area light green hatched or dotted.
snow Dark green hatched area or with asterisks.
fog Area hatched or dashed in yellow.

### More accurate weather maps and weather map symbols

Arrangement of the weather data

More detailed weather maps have further information on current weather events. They contain data on temperature , wind strength , wind direction , air pressure change, precipitation , cloud type , cloud cover , cloud base , dew point ( humidity ) and the current weather.

For this purpose, this data is inserted into the weather map in a special pattern. The position of the data on the weather map marks the geographic position of the weather station. Only the most important stations are inserted with their data. Otherwise the map would be too overloaded with weather data and no details would be recognizable.

#### Wind speed, cloud cover and air pressure tendency

Cloud cover
symbol code description
0/8 No clouds
1/8 Sunny
2/8 Bright
3/8 Slightly cloudy
4/8 Cloudy
5/8 Cloudy
6/8 Heavily cloudy
7/8 Almost covered
8/8 Covered
9/8 Sky not recognizable

Air pressure trend (3 hours)
symbol description
Increasing
Rising, then falling
Rising, then stable
Rising or stable, then falling
Falling or stable, then rising
Stable
Falling
Falling then rising
Falling then stable

Wind speed
symbol Knots, km / h
0 kn, 0 km / h
5 kn, 9 km / h
10 kn, 19 km / h
15 kn, 28 km / h
20 kn, 37 km / h
25 kn, 46 km / h
30 kn, 56 km / h
35 kn, 65 km / h
40 kn, 74 km / h
45 kn, 83 km / h
50 kn, 93 km / h
55 kn, 102 km / h
60 kn, 111 km / h
65 kn, 120 km / h
100 kn, 185 km / h
105 kn, 194 km / h
Wind direction of the symbols above:${\ displaystyle \ rightarrow}$

#### Weather

symbol number description
00 Cloud development in the last hour not observed or not observable
01 Cloud dissolution or regression during the last hour
02 Consistent cloud cover for the last hour
03 Increasing cloud cover for the last hour
04 Reduced visibility due to smoke
05 Dry haze
06 Dust vapor (dust particles partially visible)
07 Sweeping or drifting dust or sand , but not well-developed dust devils or sandstorms
08 Small bombs (dust devils) in the last hour or currently observable, but no sandstorm
09 Dust or sand storm in sight at the time of observation or during the last hour at the weather station
10 Moist haze
11 Flat fog with a height of less than two meters in individual swaths or banks at the weather station
12 Coherent, flat layer of fog with an elevation of less than two meters at the weather station
13 Weather lights , but no thunder audible
14th Virga (fall strip) in sight
15th Precipitation or praecipitatio in sight, but more than 5 km away
16 Precipitation or praecipitatio in sight, less than 5 km away but not directly at the weather station
17th Thunderstorm or distant thunderstorm at the station with audible thunder , but no precipitation
18th Gust collars visible in the last hour or striking gusts at the station
19th Funnel clouds or tornadoes during the last hour on or near the weather station
20th Snow drizzle or drizzle (not freezing and not as a shower ) stopped in the last hour
21st Rain (not freezing and not as a shower ) stopped in the last hour
22nd Snowfall (not as a shower) stopped in the last hour
23 Sleet or ice grains (not as showers) stopped in the last hour
24 Freezing rain or drizzle stopped in the last hour
25th The rain shower stopped in the last hour
26th Snow or snow showers stopped in the last hour
27 Hail or sleet has stopped in the last hour
28 Fog has cleared in the last hour
29 Thunderstorm stopped in the last hour
30th Light or moderate sand or dust storm, weaker in the last hour
31 Light or moderate sand or dust storm , unchanged during the last hour
32 Light or moderate sand or dust storm , got stronger during the last hour
33 Strong sand or dust storm , weaker during the last hour
34 Strong or moderate sand or dust storm , unchanged during the last hour
35 Strong or moderate sand or dust storm , got stronger during the last hour
36 Light or moderate snow sweeping (below eye level)
37 Heavy snow sweeping (below eye level)
38 Light or moderate snow drift (above eye level)
39 Heavy snow drift (above eye level)
40 Fog or ice fog at some distance, but has not reached the observer in the last hour. The visibility is over a kilometer and the fog itself extends vertically up to the height of the observer.
41 Fog or ice fog in swaths, therefore strongly fluctuating visibility.
42 Sky visible in spite of fog or ice fog , although it got thinner in the last hour.
43 The sky is obscured by fog or ice fog , which has become thinner in the last hour.
44 Sky visible despite fog or ice fog , with no changes in the last hour.
45 Sky obscured by fog or ice fog , with no changes in the last hour.
46 Sky visible in spite of fog or ice fog , which became thicker in the last hour.
47 Sky obscured by fog or ice fog , which has become thicker in the last hour.
48 Fog or ice fog with frost or clear ice formation with a visible sky
49 Fog or ice fog with frost or clear ice formation when the sky is overcast
50 Light, intermittent , non-freezing drizzle
51 Light, non-freezing drizzle without interruptions
52 Moderate, not freezing drizzle intermittently
53 Moderate non-freezing drizzle without interruptions
54 Strong, not freezing drizzle intermittently
55 Heavy, non-freezing drizzle without interruptions
56 Light freezing drizzle
57 Moderate or heavy freezing drizzle
58 Light rain mixed with drizzle
59 Moderate or heavy rain mixed with drizzle
60 Light, non-freezing rain intermittently
61 Light, non-freezing rain without interruptions
62 Moderate, non-freezing rain intermittently
63 Moderate, non-freezing rain without interruptions
64 Heavy, non-freezing rain intermittently
65 Heavy, non-freezing rain without interruptions
66 Light freezing rain
67 Moderate or heavy freezing rain
68 Light rain or drizzle mixed with snow
69 Moderate or heavy rain or drizzle mixed with snow
70 Light snowfall with interruptions
71 Light snowfall without interruptions
72 Moderate snowfall with interruptions
73 Moderate snowfall without interruptions
74 Heavy snowfall with interruptions
75 Heavy snowfall without interruptions
76 Ice needles , with or without fog
77 Snow pebbles , with or without fog
78 Single snowflakes , with or without fog
79 Ice grains
80 Light rain shower
81 Moderate or heavy rain shower
82 Very heavy rain shower
83 Light snow shower
84 Moderate or heavy snow showers
85 Light snow shower
86 Moderate or heavy snow showers
87 Slight sleet showers , with or without rain or sleet
88 Moderate or heavy sleet showers with or without rain or sleet
89 Light hailstorm with or without rain or sleet , but without thunder
90 Moderate or heavy hailstorm with or without rain or sleet , but without thunder
91 Thunderstorms during the last hour, but now only light rain at the time of observation
92 Thunderstorm during the last hour, but now only moderate or heavy rain at the time of observation
93 Thunderstorms during the last hour, but now only light precipitation as snowfall , sleet or hail at the time of observation
94 Thunderstorms during the last hour, but now only moderate or heavy precipitation as snowfall , sleet or hail at the time of observation
95 Light or moderate thunderstorm without hail but with rain , snowfall or sleet at the time of observation
96 Light or moderate thunderstorm with sleet or hail at the time of observation
97 Strong thunderstorm without hail but with rain or snowfall at the time of observation
98 Thunderstorm with dust or sand storm at the time of observation
99 Strong thunderstorm with sleet or hail at the time of observation

### Wind and pressure field

The wind is the movement of air and thus a mass flow . It results from the imbalance of two air masses of different pressure and temperature.

The gradient force controls the air flow from high pressure to low pressure. Due to the Coriolis force, their direction is subject to a deviation in the direction of movement to the right in the northern hemisphere and to the left in the southern hemisphere . The geostrophic winds run almost parallel to the isobars, but the ground wind (10 m wind) deviates by 25 to 40 degrees in the direction of the low due to the ground friction. This also enables the approximate wind direction to be determined.

The wind speed depends on the distance between the isobars. If the isobars are very close to one another, a storm or even a hurricane can occur. One speaks of a convergence when the lines run close together. If the isobars are far apart, as is the case with some high pressure weather conditions, then there will be weak winds or even no wind. Here one speaks of a divergence .

The following table gives an approximate information about the wind speeds depending on the distance between the isobars, but there is a significant dependence on the geographical latitude:

100 years of meteorological cooperation: The stamp from 1973 shows a weather map of the German weather service
Distance between the 5 hPa isobars Wind force
600 km Light breeze ( Beaufort scale 2)
500 km Moderate breeze (Beaufort scale 4)
400 km Fresh breeze (Beaufort scale 5)
300 km Strong wind (Beaufort scale 6)
200 km Stiff wind (Beaufort scale 7)
100 km Storm (Beaufort scale 9)

## Altitude weather maps

All previous sections dealt with ground weather maps , which are based on the air pressure from measurements on the ground. In contrast, altitude weather maps are determined with the help of data from radiosondes . Altitude weather maps provide the indispensable supplement with regard to what is happening in the free atmosphere , especially with regard to the pressure and flow fields at these altitudes. Disturbing influences from mountains, cities and other obstacles are absent here, and so the isohypses (contour lines of a certain air pressure surface ) usually have a smoother course than the isobars on the ground weather map.

The weather on the ground affects higher altitudes and vice versa.

The image of the altitude weather map can be very different from that of the ground weather map. The development of low pressure areas on the ground shows that the development at altitude only gradually reacts to the weather on the ground.

When assessing altitude weather maps, altitude lows with often extreme cold air inclusions ( cold air drops ) play a major role. Low temperatures at high altitudes trigger unstable weather conditions (showers, heavy rain ) , especially in summer with warm air close to the ground .

The following altitude weather maps are most commonly used:

Altitude weather maps for main printing areas :
Pressure level height
850 hPa 1457 meters
700 hPa 3013 meters
500 hPa 5574 meters
300 hPa 9164 meters
200 hPa 12000 meters

## literature

• Wolfgang Weischet: General climate geography . 3. Edition. de Gruyter, Berlin / New York 1980.
• Rolf F. Nohr: Maps on TV: the production of positioning . LIT, Münster 2002, p. 196–208 ( ruhr-uni-bochum.de [PDF]).