Fieldwork Flashcards
How do we measure temperature?
Temperature data is measured using thermometers. Often meteorologists and weather stations will place thermometers inside a Stevenson Screen, along with other weather equipment, in order to create the best environment for accurate and consistent measurements.
What is a Stevenson Screen?
A Stevenson screen is a shelter used to protect meteorological instruments from external disturbances such as precipitation, direct heating from the sun, strong winds and animals, while still allowing air to freely circulate.
How is a Stevenson Screen positioned above the ground and why?
A Stevenson screen is raised above the ground (usually about 1.25m), so as to avoid intense temperatures at ground level, is painted white to reflect heat radiation, and has louvred sides to allow for sufficient ventilation.
What are some considerations when choosing the best site for a Stevenson Screen?
Care should be taken in choosing the right site to install a Stevenson screen. Some considerations when choosing the best site for a screen:
Over level ground
Not sheltered by buildings, trees, etc. so that it is freely exposed to the sun and wind
The temperature sensor must be shielded from direct sunshine and precipitation
Positioned over grass, but not compacted soil, tarmac or concrete, as these media absorb and radiate solar energy strongly, which can significantly affect readings
What is often installed in Stevenson screens?
Maximum and minimum thermometers are often installed in Stevenson screens.
What can the maximum and minimum thermometers that are installed in Stevenson screens record?
They are able to record the maximum and minimum temperatures reached over a period of time (e.g. 24 hours), and also show the current temperature. In a Six’s Thermometer (Fig. 3), the maximum and minimum temperatures can be recorded by reading off the base of the metal indices on both sides of the thermometer. After the extremes have been recorded, the thermometer must be reset in order to record a new set of temperatures.
So basically, if you don’t understand the above description, just remember this simple rule:
- to read the maximum temperature: read off the bottom of the metal indice at the right side (possibly represented as a small black rectangle box in colour)
- to read the minimum temperature: read off the bottom of the metal indice at the left side (possibly represented as a small black rectangle box in colour)
- to read the current temperature (read off both sides) the top side of the grey part.
A simple mercury thermometer may be used to collect individual temperature readings as well.
What are some precautions for using the maximum and minimum thermometer, also known as a Six’s thermometer/a simple mercury thermometer?
Precautions
Hold the thermometer upright and take reading from eye level, to prevent parallax error
The bulb of the thermometer should not touch your hands or any surface/object
Keep the thermometer some distance away from the ground or your body, in order not to affect the readings
What is humidity?
Humidity
Humidity refers to the amount of water vapour present in a particular volume of air.
Geographers often examine a specific measure of humidity. What is the specific measure of humidity?
Relative humidity
How is relative humidity expressed as?
Relative humidity is expressed in %, and refers to the ratio of the actual amount of water vapour in the air compared to the maximum amount of water vapour that a volume of air can hold at a specific temperature.
How do we measure relative humidity?
A hygrometer is used to measure relative humidity. It contains 2 thermometers, 1 dry bulb and 1 wet bulb thermometer. The dry bulb thermometer is at the left side of the hygrometer while the wet bulb thermometer is at the right side of the hygrometer.
Describe how the hygrometer looks like?
At the left side of the hygrometer, it contains the dry bulb thermometer while at the right side of the hygrometer, it contains wet bulb thermometer. Both thermometers contains mercury. The wet bulb thermometer is wrapped by muslin at the bottom. The bottom of the muslin is dipped in water.
how is humidity measured using a hygrometer?
The dry bulb thermometer functions as a regular thermometer and measures the normal temperature of the air.
The wet bulb thermometer has a wet piece of gauze or muslin wrapped around its bulb, with one part dipped in a dish/bottle of water, keeping it constantly wet. As water evaporates from the gauze/muslin, it cools the bulb, causing the temperature of the wet bulb thermometer to fall.
How much water is evaporated depends on the humidity of the air. The lower the relative humidity, the more water evaporates, creating a stronger cooling effect on the wet bulb thermometer and lowering the temperature. The difference between the dry and wet bulb temperatures is called the wet bulb depression.
Relative humidity is calculated from the dry and wet bulb temperature readings using a relative humidity table.
How do we measure rainfall?
A rain gauge is used to measure rainfall in millimetres, for a specific area over a period of time. Some rain gauges record rainfall automatically (see Fig. 7), while others have to be emptied every day by an observer (see Fig. 8).
What are some of the precautions of using a rain gauge?
The rain gauge should be firmly and securely installed in the ground, in order to prevent it tipping
The rain gauge should be installed on flat terrain
Objects which might disturb airflow or block rainfall should be located a considerable distance away from the rain gauge
How do we measure cloud cover?
Cloud cover refers to the he amount of the sky obscured by clouds, which is measured in units called oktas. Each okta represents one eighth of the sky covered by cloud.
An easy way to measure cloud cover at home is to cut out eight equal size rectangles from a piece of cardstock/heavy weight paper. Take this outside, hold it up to the sky and count how many of the rectangles are filled with clouds. Each rectangle is 1 oktas.
How do you know the cloud cover based on the number of oktas?
no clouds—> clear sky
1/8th sky cover —> (few)
2/8th sky cover (scattered)
3/8th sky cover
4/8th sky cover
5/8th sky cover
6/8th sky cover (Broken)
7/8th sky cover (Overcast)
Sky obscured from view (e.g. by smoke or fog)
What is air pressure?
Air pressure refers to the weight of air pressing down on all things on earth, and is measured in millibars (mb). While the average air pressure at sea level is 1013mb, different factors can cause air pressure to increase or decrease at specific locations.
How do we measure air pressure?
Air pressure is measured using a barometer. Originally, mercury barometers (Fig. 11) were used to measure air pressure. Aneroid barometers were invented in 1844, using a sealed metal chamber which would expand and contract depending on the atmospheric pressure. They would have a circular display with a moving hand that would indicate the current atmospheric pressure (Fig. 12).
While mercury barometers are still used today, aneroid barometers are more commonly used for fieldwork due to their convenience and portability.
Are both the direction and strength of the wind measured using the same or different instruments?
They are measured using different instruments.
How is wind direction measured?
Wind direction is measured using a wind sock or wind vane.
- Wind Sock
A piece of cloth attached to a pole
Measures the general direction of the wind
Often used along bridges and air fields - Wind vane
Has four compass points which are stationary and a freely-moving arrow
As the wind blows, the arrow (or vane) moves and points towards the direction that the wind is coming from
The wind vane consists of - pointer
- compass points
-vertical shaft
Data collected on the wind direction is often recorded in a ________.
Wind rose.
What does the wind rose indicate?
The wind rose indicates what direction the wind is coming from over a period of time. For example, in Fig. 16, the direction of the prevailing wind for a day is recorded in the wind rose over an entire month, with the date indicated in each square.
So if you want to find out the prevailing wind for the month—> you look at the direction with the most number of boxes.
The number in the centre of the wind rose tells us the number of days in the month without wind.
