Sunday, 06 May 2018 18:35

Thunder and Lightning

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Sunday, 06 May 2018 18:33

A Standard Atmosphere

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Sunday, 06 May 2018 18:26

Temperature, Humidity and Dewpoint

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Temperature, Humidity, Dew Point
 

Temperature

A temperature is an objective comparative measurement of hot or cold. It is measured by a thermometer. Several scales and units exist for measuring temperature, the most common being Celsius (denoted °C; formerly called centigrade), Fahrenheit (denoted °F), and, especially in science, Kelvin (denoted K). The coldest theoretical temperature is absolute zero, at which the thermal motion of atoms and molecules reaches its minimum – classically, this would be a state of motionlessness, but quantum uncertainty dictates that the particles still possess a finite zero-point energy. Absolute zero is denoted as 0 K on the Kelvin scale, −273.15 °C on the Celsius scale, and −459.67 °F on the Fahrenheit scale. Source:Wikipedia

The BBC have an interesting infographic about temperature.

Humidity

Humidity is the amount of water vapor present in the air. Water vapor is the gaseous state of water and is invisible to the human eye. Humidity indicates the likelihood of precipitation, dew, or fog. Higher humidity reduces the effectiveness of sweating in cooling the body by reducing the rate of evaporation of moisture from the skin. This effect is calculated in a heat index table or humidex. The amount of water vapor that is needed to achieve saturation increases as the temperature increases. As the temperature of a parcel of water becomes lower it will eventually not reach the point of saturation without adding or losing water mass. The differences in the amount of water vapor in a parcel of air can be quite large. For example, a parcel of air that is near saturation may contain 28 grams of water per cubic meter of air at 30 °C, but only 8 grams of water per cubic meter of air at 8 °C. There are three main measurements of humidity: absolute, relative and specific. Absolute humidity is the water content of air expressed in gram per cubic meter. Relative humidity, expressed as a percent, measures the current absolute humidity relative to the maximum (highest point) for that temperature. Specific humidity is the ratio of the mass of water vapor to the total mass of the moist air parcel. Source:Wikipedia

Dew Point

Dew point is the temperature to which air must be cooled to become saturated with water vapor. When further cooled, the airborne water vapor will condense to form liquid dew. When air with a high dew point contacts surfaces at colder temperatures, then liquid dew will form on the surfaces as condensation. Dew point is sometimes called frost point when the temperature is below freezing. The measurement of dew point is related to humidity. A higher dew point means there will be more moisture in the air. Source:Wikipedia

 
Sunday, 06 May 2018 18:18

Introduction - What creates the weather

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 Basics

When considering the atmosphere as a whole, it can largely be considered as a fluid. It may not look like a fluid, but it behaves in very much the same way that a fluid would. The impacts of a number of natural phenomena has an effect on this fluid atmosphere in the following ways

  • Rotation which is in part responsible for creating air exchange between the equator and the mid latitudes, and the mid latitudes and the polar regions
  • The sun, which has a stronger heating effect at the equator, than it does at higher latitudes
  • Reflectivity which is responsible for reflecting back light and heat from the sun - stronger at higher latitudes than the equator
  • Angle of the sun to the earth, which differs at higher latitudes to lower latitudes and therefore receives less heating at higher latitudes.

The Seasons

In addition to this the Earth has a wobble which creates the seasons. In summer the Southern Hemisphere is more exposed to the sun, and therefore recieves more heating than it would do in Winter. The impact on the weather is not as noticeable in the tropics, where the difference between seasons is less pronounced (eg they tend to just have a "rainy season") as it is in mid to higher latitudes, where ther is a definite sense of Spring, Summer, Autumn and Winter

Localised effects

More localised effects can occur. For example the sun heating a large body of water such as the Pacific Ocean, has a different effect than it does heating large areas of land such as Europe. In the Nothern Hemisphere, New Zealands latitude is similar to that of Spains, which typically has warmer weather due to the warm ocean currents. But it's latitude neighbour of northern Italy often experiences snow during winter due to the fact that the land surrounding that area is cooler because it is not heated by the sun as much during winter and the land tends to hold its heat (or cold) more readily than the sea.

Sea Temperature and currents

Sea temperature and currents (again a fluid in the system, but just a thicker or more dense fluid) also plays a major part.
For example sea currents generated by many of the same processes outlined above that drive the atmospheric weather, bring warmer water from the Atlantic to the UK, which means that the climate in the UK is warmer than it would be at that latitude normally. The warmer sea currents around New Zealand lead to a more mild winter than we would normally experience if the country was surrounded by land.

At a more local level the Southern Alps of New Zealand have a significant impact on the amount of rain that falls on the Canterbury plains vs on the West Coast

The 3 Dimensional Atmosphere

Finally the atmosphere varies with height. The atmosphere can be considered as the meat in the sandwich between two heat sources, one being the earth from below, and the other the sun from above. So for example atmospheric temperature cools to a point as you go up the further it gets from the earths heat source, and then increases in temperature once through a boundary layer where the predominant heating force is the sun. Density or thickness of the air (fluid) also changes as height increases, which also has an impact.

Each of these effects happening at different places on the Earth and in the atmosphere at different heigts have different effects, which combined determine the type of weather we get.

The study of meteorology is essentially the study of these natural variables and the many ways they interact to form the weather that we experience.