Sunday, July 10, 2011

Weather and Climate

Weather is basically the way the atmosphere is behaving, mainly with respect to its effects upon life and human activities. The difference between weather and climate is that weather consists of the short-term (minutes to months) changes in the atmosphere. Most people think of weather in terms of temperature, humidity, precipitation, cloudiness, brightness, visibility, wind, and atmospheric pressure, as in high and low pressure.

In most places, weather can change from minute-to-minute, hour-to-hour, day-to-day, and season-to-season. Climate, however, is the average of weather over time and space. An easy way to remember the difference is that climate is what you expect, like a very hot summer, and weather is what you get, like a hot day with pop-up thunderstorms.

The difference between weather and climate is a measure of time. Weather is what conditions of the atmosphere are over a short period of time, and climate is how the atmosphere "behaves" over relatively long periods of time.

When we talk about climate change, we talk about changes in long-term averages of daily weather. Today, children always hear stories from their parents and grandparents about how snow was always piled up to their waists as they trudged off to school. Children today in most areas of the country haven't experienced those kinds of dreadful snow-packed winters, except for the Northeastern U.S. in January 2005. The change in recent winter snows indicate that the climate has changed since their parents were young.

If summers seem hotter lately, then the recent climate may have changed. In various parts of the world, some people have even noticed that springtime comes earlier now than it did 30 years ago. An earlier springtime is indicative of a possible change in the climate.
The weather is made up of different elements, which are measured either by special instruments or are observed by a meteorologist. These measurements are then recorded and used in the making of climate graphs and weather forecasts. The table below shows the weather element details:

ElementDescriptionHow it is measuredUnits of measurement
PrecipitationMoisture from the sky e.g. rain, snow etc.By a Rain GaugeMillimetres (mm.)
TemperatureHow hot or cold it isBy Thermometers, found inside a Stevenson ScreenDegrees celsius ( c)
Wind SpeedHow fast the wind is blowingBy an AnemometerKnots, or by the Beaufort Scale
Wind DirectionWhere the wind is blowing fromBy a Wind VanePoints of the compass (north, north-west etc), or bearing in degrees
ElementDescriptionHow it is measuredUnits of measurement
HumidityThe amount of water vapour in the airBy a Hygrometer (wet and Dry Bulb Thermometers)Relative Humidity (% of water vapour that can be held by the air at the actual temperature)
Air PressureThe "weight" of the air pushing on the surface of the EarthBy a BarometerHectopascals (although most people know it as millibars)
Cloud CoverThe amount of cloud in the skyIt is observed by a meteorologistOktas - eighths of the sky
VisibilityHow far you can seeIt is observed by a meteorologistKilometres
SunshineThe hours of sunshineBy a Sunshine RecorderHours and minutes
ElementDescriptionHow it is measuredUnits of measurement


The Elements of Climate

Climatology is the study of the long-term state of the atmosphere, or climate. The long-term state of the atmosphere is a function of a variety of interacting elements. They are:
  • Solar radiation
  • Air masses
  • Pressure systems (and cyclone belts)
  • Ocean Currents 
  • Topography

Solar radiation

Solar radiation is probably the most important element of climate. Solar radiation first and foremost heats the Earth's surface which in turn determines the temperature of the air above. The receipt of solar radiation drives evaporation, so long as there is water available. Heating of the air determines its stability, which affects cloud development and precipitation. Unequal heating of the Earth's surface creates pressure gradients that result in wind. So you see, just about all the characteristics of climate can be traced back to the receipt of solar radiation.

Air masses

Air masses as an element of climate subsumes the characteristics of temperature, humidity, and stability. Location relative to source regions of air masses in part determines the variation of the day-to-day weather and long-term climate of a place. For instance, the stormy climate of the midlatitudes is a product of lying in the boundary zone of greatly contrasting air masses called the polar front.

Pressure systems

Pressure systems have a direct impact on the precipitation characteristics of different climate regions. In general, places dominated by low pressure tend to be moist, while those dominated by high pressure are dry. The seasonality of precipitation is affected by the seasonal movement of global and regional pressure systems. Climates located at 10o to 15o of latitude experience a significant wet period when dominated by the Intertropical Convergence Zone and a dry period when the Subtropical High moves into this region. Likewise, the climate of Asia is impacted by the annual fluctuation of wind direction due to the monsoon. Pressure dominance also affects the receipt of solar radiation. Places dominated by high pressure tend to lack cloud cover and hence receive significant amounts of sunshine, especially in the low latitudes. 

Ocean Currents

Ocean currents greatly affect the temperature and precipitation of a climate. Those climates bordering cold currents tend to be drier as the cold ocean water helps stabilize the air and inhibit cloud formation and precipitation. Air traveling over cold ocean currents lose energy to the water and thus moderate the temperature of nearby coastal locations. Air masses traveling over warm ocean currents promote instability and precipitation. Additionally, the warm ocean water keeps air temperatures somewhat warmer than locations just inland from the coast during the winter.


Topography affects climate in a variety of ways. The orientation of mountains to the prevailing wind affects precipitation. Windward slopes, those facing into the wind, experience more precipitation due to orographic uplift of the air. Leeward sides of mountains are in the rain shadow and thus receive less precipitation. Air temperatures are affected by slope and orientation as slopes facing into the Sun will be warmer than those facing away. Temperature also decreases as one moves toward higher elevations. Mountains have nearly the same affect as latitude does on climate. On tall mountains a zonation of climate occurs as you move towards higher elevation.

Cherie and Nicole :)


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