Daily Rainfall – It is the total amount of rainfall in centimeter from 8.30 in the morning of the previous day to the 8.30 morning of the day for which the forecast is issued.
Hourly rainfall – This is the forecast of the amount of rainfall which a station may receive every hour.
Minimum Temperature – It is the expected lowest atmospheric temperature during a period of 24 hours. Minimum temperature occurs in the early morning hours a few minutes after the Sun rise not at midnight.
Maximum Temperature – It is the expected highest atmospheric temperature during a period of 24 hours. Maximum temperature occurs in around 2.30 in the afternoon not during the mid day.
Relative vorticity – Vorticity is a measure of rotation in the atmosphere, or “spin,” in the air. If the spin, as viewed from above, is counterclockwise in the Northern Hemisphere, it is called a cyclonic vorticity and given a positive value. If the spin is clockwise, it is called an anticyclonic vorticity and given a negative value. The opposite conventions apply in the Southern Hemisphere. In forecasting large-scale weather events, one is concerned primarily with “relative vorticity” and focuses on the rotation of air around a vertical.
There is a good correlation between large positive values of vorticity and centers of low pressure in the atmosphere. The association of low pressure and high vorticity with foul weather is a generalization that is quite often true.
Divergence – It is a measure of outward flux or outflow of winds from a place. On the contrary the inward flux or inflow is known as convergence. Thus convergence is negative of divergence. Negative divergence or convergence near the ground can cause development of clouds with possibility of bad weather.
Temperature – It is the measure of the degree of hotness of a particular place. Both high and low temperatures cause discomfort to human beings.
Dew point Temperature – Atmospheric air contains moisture. The moisture content depends on the environmental temperature – at higher temperature air can hold more moisture than at lower temperature. At a particular temperature if air contains maximum amount of moisture it can hold at that temperature then the air is called saturated and the relative humidity is 100%. The Dew point temperature is that temperature to which air has to be cooled so that the air becomes just saturated. Of course there should be no change in atmospheric pressure. At 100% relative humidity, the dew point temperature and real temperature are the same, and clouds or fog can begin to form.
Equivalent Potential Temperature – Equivalent Potential Temperature is commonly referred to as a quantity related to the stability of a column of air in the atmosphere.
When a column of air is colder than its environment it is generally denser and cannot go up. Such a condition is called Stable condition. On the contrary when the temperature of a column of air is warmer than its environment it is lighter than its environment and can go up. Such a condition is called Instability. When the column of air and its environment are having the same temperature then the atmosphere is said to be in Neutral condition.
Fog occurs in Stable atmospheric condition. In the stable atmospheric condition near the ground Pollutants get trapped near the ground and can cause health problem. Normally during the early morning hours before the Sun rise and after the Sun set particularly in the Winter season stability prevails. For stability the equivalent potential temperature must increase with height.
Instability causes development of thunder clouds. Most of the bad weathers conditions are due to Instability of the atmosphere. The Equivalent Potential decreases with height in unstable atmosphere.
Wind Speed/Direction: Wind speed is the measure motion of the air with respect to the surface of the earth covering a unit distance over a unit time. Wind direction is an indicator of the direction that the wind is coming from. For example, a northerly wind is coming from the north and blowing toward the south.
Wind is bulk movement of air. Winds are often referred to according to their strength, and the direction from which it is blowing. Very strong wind is associated with Thunderstorms, Cyclones, and Depressions etc. Very light winds and stable atmospheric conditions are conducive for the formation of Fog.
Winds blow in both horizontal and vertical direction. The speed of the horizontal component of wind is normally more than 100 times than that of the vertical component of wind. The horizontal wind speed is normally a few meters per second whereas the vertical wind speed is only a few centimeter per second. But the vertical component of wind is very important for the development of weather systems like thunderstorms, cyclones etc. Strong upward winds cause the development of such systems.
Relative humidity – The amount of water vapor in the air at any given time is usually less than that required to saturate the air. Relative humidity is a term used to describe the amount of water vapor that exists in a gaseous mixture of air and water vapor.
Relative humidity is the amount of moisture in the air compared to what the air can "hold" at that temperature. When the air can't "hold" all the moisture, then it condenses as dew. There is another term called Absolute Humidity.
Absolute humidity is the mass of water vapor divided by the mass of dry air in a volume of air at a given temperature. The hotter the air is, the more water vapor it can contain.
Relative humidity is the ratio of the Absolute humidity to the highest possible absolute humidity (which depends on the air temperature). A reading of 100 percent relative humidity means that the air is totally saturated with water vapor and cannot hold any more, creating the possibility of rain. This doesn't mean that the relative humidity must be 100 percent in order for it to rain -- it must be 100 percent where the clouds are forming, but the relative humidity near the ground could be much less.
Humans are very sensitive to humidity, as the skin relies on the air to get rid of moisture. The process of sweating is our body's attempt to keep cool and maintain its current temperature. If the air is at 100-percent relative humidity, sweat will not evaporate into the air. As a result, we feel much hotter than the actual temperature when the relative humidity is high. If the relative humidity is low, we can feel much cooler than the actual temperature because our sweat evaporates easily, cooling us off. People feel comfortable if the Relative Humidity is around 45%.
Stability Index: Meteorologists use stability indices to quickly assess the susceptibility of the atmosphere to severe weather. Stability indices are a measure of the atmospheric static stability. The table below lists some of the commonly used stability indices.
The lifting condensation level (LCL) is formally defined as the height at which the relative humidity (RH) of an air parcel will reach 100% when it is cooled by dry adiabatic lifting.
The SWEAT index assess low level moisture, convective environment and changes in wind speed and direction with height low level and middle level jet, horizontal vorticity. When all these factors occur together, the severe weather threat and tornado threat is enhanced.