Wind Rose, Geostrophic Wind, Mixing Ratio, Temperature Inversion

Wind Rose

The wind rose is defined as any one of the class of diagram designed to show the distribution of the wind direction experienced at a given location over a considerable period or it is a graphical representation of the wind data giving the percentage frequencies of the wind speed, wind direction for a given location. They may be drawn for a day, a month and season and for a long year. The data required for the wind rose diagram is wind speed period of blowing and wind direction. The wind data is usually recorded and maintained by the meteorological department of the government.

The most common form consists of the circle of which 8 or 16 lines are emerging one for each direction. The length of the each line is proportional to the frequency of the wind from that direction and the frequency of calm condition is entered at the center.

Geostrophic wind

When the wind, usually above the height of 600 meters, blows parallel to the isobars, it is called as the geostrophic wind. The geostrophic wind is the theoretical wind that would result from an exact balance between the Coriolis effects and the pressure gradient force. This condition is called the geostrophic balance. An air parcel initially at rest will have from high pressure to the low pressure because of the pressure gradient force (PGF). However, as that air parcel begins to move, it is deflected by the Coriolis force to the right in the northern hemisphere i.e to the left on the southern hemisphere. As the wind gains the speed, the deflection increases until the Coriolis force equal to the pressure gradient force. At this point, the wind will be blowing parallel to the isobars. When this happens, the wind is referred as geostrophic. The geostrophic wind is directed parallel to the isobars i.e lines of the constant pressure of a given height. This balance seldom holds exactly in nature. The true wind almost always differs from the geostrophic wind due to the other forces such as friction from the ground or the centrifugal force from the curved path or curved fluid flow. Thus, the actual wind would equal the geostrophic flow much of the time and it is a valuable first approximation.

Air naturally moves from the areas of the high pressure to the areas of the low pressure, due to the pressure gradient force. As soon as the air starts to move, however, the Coriolis force deflects it due to the rotation of the earth. The deflection is to right in the northern hemisphere and to the left in the southern hemisphere. As the air moves from the high-pressure area, its speed increases and so does the deflection from the Coriolis force. The deflection increases until the Coriolis and the pressure gradient forces are in the geostrophic balance, at which pint the air is no longer moving from the high to the low pressure, but instead moves along an isobar, a line of equal pressure. In practice, the flow is nearly always balanced. The geostrophic approximation has no predictive value since it does not contain any expression for the change it is purely diagnostic. The geostrophic balance helps to explain why low-pressure system spin counterclockwise and high-pressure systems spin clockwise in the northern hemisphere i.e and the opposite in the southern hemisphere.

As regards geographic wind speed, it should be remembered that it is

A: directly proportional to the pressure gradient or if the isobar pressure interval is taken to be constant, it is inversely proportional to the spacing of isobars.

B: inversely proportional to the sine of latitude.

C: inversely proportional to the air density.

The two forces are balancing to produce the geostrophic wind. The wind in nature are rarely exactly geostrophic but to a good approximation, the wind in the upper troposphere can be close. This is so because winds are only considered truly geostrophic when the isobars are straight and there are no longer forces acting on it and these conditions just not prevail too often in nature.

Mixing ratio

Mixing ratio is defined as the mass of water vapor per unit mass of dry air. The mixing ratio is essentially a recipe for a mixture of the water vapor and the dry air. A value of the 12 g per kg would entail a total of 1012g for the mixture. For most of the condition specific humidity and mixing ratio differs significantly.

Temperature inversion

In the meteorology, an inversion is defined as the deviation from the normal change of an atmospheric property with the altitude. It almost always refers to a temperature inversion i.e an increase in the temperature with the height or to the layer within which such an increase occurs.

An inversion can lead to the pollution such as smog being trapped close to the ground with the possible adverse effects on health. An investigation can also suppress the convection by acting as a cap. If this cap is broken for any of the several reasons, conviction of any moisture present can then erupt into the violent thunderstorm.

Under certain conditions, the normal vertical temperature gradient is inverted such that the air is colder near the surface of the earth. This can occur when, for the example, a warmer, less dense air mass moves over a cooler denser air mass. With sufficient humidity in the cooler layer, fog is typically present below the inversion cap. An inversion is also produced whenever radiation from the surface of the earth is less than the amount of the radiation received from the sun which commonly occurs at night or during the winter when an angle of the sun is very low in the sky. This effect is virtually confined to the land region the ocean retains heat far longer. In the polar regions during the wintertime, inversion is nearly always present over land.

A warmer air mass moving over a cooler one can shut off any conviction which may be present in the cooler air mass. This is known as a capping inversion. However, if this cap is broken either by the extreme convection overcoming the cap or by the lifting effect of a front or a mountain range the sudden release of the bottled up convective energy like the bursting of the balloon can result in the severe thunderstorms. Such capping inversion typically precedes the development of the tornadoes. In this instance, the cooler layer is actually quite warm but is still denser and usually cooler than the lower part of the inversion layer capping it.

Sometimes the inversion layer is higher so that the cumulus clouds can condense but they spread out under the inversion layer. This cuts out the ground and prevents new thermals forming. A period of cloudiness is followed by the sunny weather as the cloud disperse. This cycle can occur more than once in a day.

References:

.S.C., Santee. Environmental Science. India, New Center: New Center Book Agency (P) Ltd, 2004.

Lal, D.S. Climatology, Sharda Pustak Bhawan, Allahabad.(2010)