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36:
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a positive coupling between pressure and temperature. Such a coupling causes the slope of the isobars to increase with height, as illustrated in panel (b) of the figure to the left. Because isobars are steeper at higher elevations, the associated pressure gradient force is stronger there. However, the
Coriolis force is the same, so the resulting geostrophic wind at higher elevations must be greater in the direction of the pressure force.
938:
276:
the depth of the fluid in (b). The dotted lines enclose isobaric surfaces which remain at constant slope with increasing height in (a) and increase in slope with height in (b). Pink arrows illustrate the direction and amplitude of the horizontal wind. Only in the baroclinic atmosphere (b) do these vary with height. Such variation illustrates the thermal wind.
93:
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atmosphere, where density is a function only of pressure, a horizontal pressure gradient will drive a geostrophic wind that is constant with height. However, if a horizontal temperature gradient exists along isobars, the isobars will also vary with the temperature. In the mid-latitudes there often is
275:
The geostrophic wind on different isobaric levels in a barotropic atmosphere (a) and in a baroclinic atmosphere (b). The blue portion of the surface denotes a cold region while the orange portion denotes a warm region. This temperature structure is restricted to the surface in (a) but extends through
220:
seems appropriate. In the early years of meteorology, when data was scarce, the wind field could be estimated using the thermal wind relation and knowledge of a surface wind speed and direction as well as thermodynamic soundings aloft. In this way, the thermal wind relation acts to define the wind
256:
hemisphere). This is illustrated in panel (a) of the figure below. The balance that develops between these two forces results in a flow that parallels the horizontal pressure difference, or pressure gradient. In addition, when forces acting in the vertical dimension are dominated by the vertical
1041:
The strongest part of jet streams should be in proximity where temperature gradients are the largest. Due to land masses in the northern hemisphere, largest temperature contrasts are observed on the east coast of North
America (boundary between Canadian cold air mass and the Gulf Stream/warmer
255:
develops. Intuitively, a horizontal difference in pressure pushes air across that difference in a similar way that the horizontal difference in the height of a hill causes objects to roll downhill. However, the
Coriolis force intervenes and nudges the air towards the right (in the northern
522:
is the vertically-averaged temperature of the layer. This formula shows that the layer thickness is proportional to the temperature. When there is a horizontal temperature gradient, the thickness of the layer would be greatest where the temperature is greatest.
1042:
Atlantic) and
Eurasia (boundary between the boreal winter monsoon/Siberian cold air mass and the warm Pacific). Therefore, the strongest boreal winter jet streams are observed over east coast of North America and Eurasia. Since stronger vertical shear promotes
292:
atmosphere, where density is a function of both pressure and temperature, such horizontal temperature gradients can exist. The difference in horizontal wind speed with height that results is a vertical wind shear, traditionally called the thermal wind.
897:
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In (a), cold advection is occurring, so the thermal wind causes the geostrophic wind to rotate counterclockwise (for the northern hemisphere) with height. In (b), warm advection is occurring, so the geostrophic wind rotates clockwise with
587:
185:– a variation in wind speed or direction with height. The wind shear in this case is a function of a horizontal temperature gradient, which is a variation in temperature over some horizontal distance. Also called
170:, it follows that the thermal wind flows along thickness or temperature contours. For instance, the thermal wind associated with pole-to-equator temperature gradients is the primary physical explanation for the
946:
If a component of the geostrophic wind is parallel to the temperature gradient, the thermal wind will cause the geostrophic wind to rotate with height. If geostrophic wind blows from cold air to warm air (cold
246:
is an idealized wind that results from a balance of forces along a horizontal dimension. Whenever the Earth's rotation plays a dominant role in fluid dynamics, as in the mid-latitudes, a balance between the
402:
637:
is the vertical unit vector, and the subscript "p" on the gradient operator denotes gradient on a constant pressure surface) with respect to pressure, and integrate from pressure level
902:
Note that thermal wind is at right angles to the horizontal temperature gradient, counter clockwise in the northern hemisphere. In the southern hemisphere, the change in sign of
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is often considered a misnomer, since it really describes the change in wind with height, rather than the wind itself. However, one can view the thermal wind as a
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The lack of land masses in the
Southern Hemisphere leads to a more constant jet with longitude (i.e. a more zonally symmetric jet).
79:
189:, the thermal wind varies with height in proportion to the horizontal temperature gradient. The thermal wind relation results from
982:
component of the geostrophic wind, a sharpening of the temperature gradient results. Thermal wind causes a deformation field and
120:
99:(shown in pink) are well-known examples of thermal wind. They arise from the horizontal temperature gradients between the warm
271:
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known as wind backing. Otherwise, if geostrophic wind blows from warm air to cold air (warm advection) the wind will turn
50:
44:
178:, which is the atmospheric layer extending from the surface of the planet up to altitudes of about 12–15 km.
61:
257:
252:
163:
1047:
301:
The geopotential thickness of an atmospheric layer defined by two different pressures is described by the
1018:
geostrophic wind pattern to form in the mid-latitudes. Because thermal wind causes an increase in wind
966:
Wind backing and veering allow an estimation of the horizontal temperature gradient with data from an
225:
moniker, even though it describes a wind gradient, sometimes offering a clarification to that effect.
1003:
967:
892:{\displaystyle \mathbf {v} _{T}={\frac {R}{f}}\ln \left\mathbf {k} \times \nabla _{p}{\overline {T}}}
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779:{\displaystyle \mathbf {v} _{T}={\frac {1}{f}}\mathbf {k} \times \nabla _{p}(\Phi _{1}-\Phi _{0})}
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582:{\displaystyle \mathbf {v} _{g}={\frac {1}{f}}\mathbf {k} \times \nabla _{p}\Phi }
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Substituting the hypsometric equation, one gets a form based on temperature,
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17:
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Vector difference of geostrophic wind movement at high and low altitudes
1011:
143:
in the vertical. The combination of these two force balances is called
100:
92:
1054:) is also observed along the east coast of North America and Eurasia.
1022:
with height, the westerly pattern increases in intensity up until the
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270:
91:
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Since the geostrophic wind at a given pressure level flows along
397:{\displaystyle \Phi _{1}-\Phi _{0}=\ R{\overline {T}}\ln \left}
238:
The thermal wind is the change in the amplitude or sign of the
29:
181:
Mathematically, the thermal wind relation defines a vertical
221:
itself, rather than just its shear. Many authors retain the
978:
As in the case of advection turning, when there is a cross-
147:, a term generalizable also to more complicated horizontal
1038:
exhibit similar jet stream patterns in the mid-latitudes.
127:
at upper altitudes minus that at lower altitudes in the
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A horizontal temperature gradient exists while moving
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1006:because curvature of the Earth allows for more
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242:due to a horizontal temperature gradient. The
955:with height (for the northern hemisphere), a
8:
1187:Wallace, John M.; Hobbs, Peter V. (2006).
1079:Introduction to Geophysical Fluid Dynamics
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216:that varies with height, so that the term
963:with height, also known as wind veering.
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80:Learn how and when to remove this message
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162:contours on a map, and the geopotential
139:in the horizontal, while pressure obeys
43:This article includes a list of general
1066:
695:, we obtain the thermal wind equation:
166:of a pressure layer is proportional to
1168:Atmospheric and Oceanic Fluid Dynamics
1126:An Introduction to Dynamic Meteorology
1107:An Introduction to Dynamic Meteorology
526:Differentiating the geostrophic wind,
201:along constant pressure surfaces, or
7:
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135:that would exist if the winds obey
1014:than at the poles. This creates a
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131:. It is the hypothetical vertical
49:it lacks sufficient corresponding
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1149:. Weather Graphics Technologies.
951:) the geostrophic wind will turn
1046:, the most rapid development of
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34:
1077:Cushman-Roisin, Benoit (1994).
515:{\displaystyle {\overline {T}}}
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747:
1:
455:{\displaystyle \,\Phi _{n}\,}
1166:Vallis, Geoffrey K. (2006).
1147:Weather Forecasting Handbook
1128:. New York: Academic Press.
884:
630:{\displaystyle \mathbf {k} }
507:
351:
1191:. Elsevier Academic Press.
1232:
1124:Holton, James R. (2004).
688:{\displaystyle \,p_{1}\,}
659:{\displaystyle \,p_{0}\,}
488:{\displaystyle \,p_{n}\,}
174:in the upper half of the
258:pressure-gradient force
253:pressure-gradient force
123:difference between the
64:more precise citations.
1105:Holton, James (2004).
1081:. Prentice-Hall, Inc.
1048:extratropical cyclones
1044:baroclinic instability
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297:Mathematical formalism
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1145:Vasquez, Tim (2002).
940:
924:flips the direction.
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917:{\displaystyle \;f\;}
894:
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604:{\displaystyle \;f\;}
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424:
422:{\displaystyle \,R\,}
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197:in the presence of a
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1216:Atmospheric dynamics
1036:Southern Hemispheres
968:atmospheric sounding
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303:hypsometric equation
234:Physical explanation
199:temperature gradient
145:thermal wind balance
1189:Atmospheric Science
266:hydrostatic balance
262:gravitational force
195:geostrophic balance
191:hydrostatic balance
168:virtual temperature
160:geopotential height
141:hydrostatic balance
137:geostrophic balance
113:atmospheric science
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613:Coriolis parameter
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466:at pressure level
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933:Advection turning
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16:(Redirected from
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60:this article by
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70:February 2011
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223:thermal wind
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210:thermal wind
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117:thermal wind
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18:Backing wind
1109:. Elsevier.
1050:(so called
986:may occur.
229:Description
176:troposphere
97:Jet streams
62:introducing
1061:References
1028:jet stream
1024:tropopause
990:Jet stream
980:isothermal
957:phenomenon
290:baroclinic
282:barotropic
183:wind shear
172:jet stream
133:wind shear
129:atmosphere
45:references
961:clockwise
949:advection
885:¯
871:∇
867:×
827:
765:Φ
761:−
752:Φ
739:∇
735:×
577:Φ
568:∇
564:×
508:¯
443:Φ
433:for air,
360:
352:¯
329:Φ
325:−
316:Φ
208:The term
164:thickness
155:balance.
1210:Category
1032:Northern
1020:velocity
1016:westerly
1004:meridian
1002:along a
928:Examples
260:and the
251:and the
151:such as
1012:equator
1010:at the
942:height.
611:is the
589:(where
462:is the
268:occurs.
203:isobars
119:is the
101:tropics
58:improve
1195:
1174:
1153:
1132:
1085:
1030:. The
495:, and
407:where
341:
121:vector
115:, the
47:, but
1052:bombs
1000:South
996:North
288:In a
280:In a
1193:ISBN
1172:ISBN
1151:ISBN
1130:ISBN
1083:ISBN
1034:and
218:wind
193:and
666:to
111:In
1212::
1170:.
1097:^
1069:^
970:.
899:.
824:ln
786:.
615:,
404:,
357:ln
305::
264:,
205:.
1201:.
1180:.
1159:.
1138:.
1091:.
998:-
911:f
882:T
875:p
863:k
858:]
851:1
847:p
841:0
837:p
831:[
819:f
816:R
811:=
806:T
801:v
774:)
769:0
756:1
748:(
743:p
731:k
725:f
722:1
717:=
712:T
707:v
680:1
676:p
651:0
647:p
624:k
598:f
572:p
560:k
554:f
551:1
546:=
541:g
536:v
505:T
480:n
476:p
447:n
416:R
391:]
384:1
380:p
374:0
370:p
364:[
349:T
344:R
338:=
333:0
320:1
107:.
83:)
77:(
72:)
68:(
54:.
20:)
Text is available under the Creative Commons Attribution-ShareAlike License. Additional terms may apply.
